The chemical composition of Parthian coins should be just as interesting and significant to the numismatist as the chemical composition of other ancient coins, and perhaps more so, as the coins of the Parthian kings constitute the chief archaeological remains of their empire, whereas an abundance of other kinds of remains have survived most of the other chief empires of the past. Any information that may be gleaned from the chemical investigation of Parthian coins is not only a contribution to the obscure numismatic history of this empire, but may also be a contribution to its still more obscure economic history.
As a somewhat incidental part of a general investigation of the chemical composition of ancient objects, various Parthian coins have been analyzed in the author's laboratory at various times in the past fifteen years. Although the analyses are not very many in number they are fairly representative, and it seems worthwhile to summarize and publish them at this time.
If the amount of published information is a true index of what has been done, very little attention, indeed, has been previously paid to the composition of Parthian coins, and, as far as the author has been able to determine, no chemical analyses were made prior to those reported here. The present essay may be considered an original introduction to the subject, but, since it is only an introduction, the various conclusions and interpretations based on the analyses are not intended to be final. It is the hope of the author, however, that the present essay will serve as a sound foundation for any future investigations of the chemical composition of Parthian coins. The critical study of the relationship between the fineness and specific gravity of Parthian silver coins in the latter part of the essay should be of interest as indicating the reliability of estimations of the fineness of ancient silver coins in general from specific gravity measurements.
Prior to the analyses here reported, only sixteen Parthian coins appear to have been investigated chemically in any way, and all these were silver coins that were tested by fire assay for their silver and gold content only. The results of these assays are shown in Table I. Unfortunately, there is no way to check the correctness of the attributions of these coins. However, the results indicate that the earliest coins contain the highest proportion of silver, and that later coins, leaving out of consideration the one late tetradrachm, contain moderately high amounts. Thus there is no indication of any serious or progressive debasement in this series of Parthian drachms such as occurs in the series of denarii of the Roman Empire. These results also indicate that individual coins of the same rulers differ considerably from each other in fineness. The proportions of gold in the coins, though high from the standpoint of modern silver coinage practice,
|1||Arsaces I (?)||250–248 (?) b.c.||946|
|2||Mithradates I||171–138 (?) b.c.||923||2|
|3||Mithradates I||171–138 (?) b.c.||899||5|
|4||Mithradates I||171–138 (?) b.c.||892||2|
|5||Phraates II||138–128/127 b.c.||709||3|
|6||Artabanus II||88–77 b.c.||854||1|
|7||Artabanus II||88–77 b.c.||728||2|
|8||Tiradates II (?)||26 b.c.||611||2|
|9||Orodes II||4–6 (?) a.d.||798||2|
|10||Orodes II||4–6 (?) a.d.||622||3|
|13||Mithradates IV||130–147 (?) a.d.||749||4|
|14||Volagases III||185 a.d.||334||1|
|15||Volagases IV||191–207/208 a.d.||779||3|
|16||Artabanus V||213–227 (?) a.d.||746||4|
Notes to Table I
With the exception of most of the drachms of Orodes I, the coins for this investigation were purchased by the author from dealers here and abroad at various times. Nearly all the drachms of Orodes I came from a large hoard, part of which is now in the numismatic collection of Princeton University.
According to information kindly supplied by Dr. Louis C. West, Curator of Coins and Medals, Princeton University Library, this hoard, estimated to have contained about 600 coins, was dug up in a small village near Ahar, 75 miles northeast of Tabriz, Iran, and was unearthed by a native worker digging a foundation for a house in the village. The hoard was found at a depth of 8 to 10 feet in a pot of black earthenware, which was broken when the coins were found. The exact date of the discovery is not known with certainty, but it is believed to have been about November, 1923. Early in 1924 this entire hoard was brought to Dr. J. Christy Wilson, then of the American Mission, Tabriz, who purchased something less than half of it. The examples he selected were representative of all the different coins in the hoard. A high proportion of the coins were of Orodes I, the others being coins of near predecessors of Orodes. This selection of coins from the hoard was brought back to the United States by Dr. Wilson, who sold most of the coins to the Princeton University Library. The number thus sold is not known with certainty, but it was probably about 200. Another large part of this hoard was bought by a representative of the Near East Relief in Tabriz, and he sold it to a New York dealer, who in turn sold the coins to various collectors. This part of the hoard numbered over 200 coins. The remainder of the hoard was sold to various people in Iran. The largest intact lot of coins from the hoard, and apparently also the most representative one, is therefore at Princeton. According to figures given to the author about 18 years ago by Professor Shirley H. Weber, then of Princeton University, the lot at Princeton consisted at that time of 5 coins of Artabanus II, 2 of Phraates III, and 2 of either Phraates III or the Unknown King, all the remainder being coins of Orodes I, of which there were 178. It is probable that some of those of Orodes had already been sold as duplicates. At the request of Professor Weber this lot of coins was cleaned electrolytically by the author in order to remove the spots and patches of greenish corrosion products that were present on all of them. All the coins prior to Orodes I and 31 of the best of this ruler were then placed in the collection of the university and the remainder were classed as duplicates. Several months after these coins were cleaned the author determined the specific gravities of 144 of these duplicates, and 10 of the poorest ones were given to him for chemical analysis.
The British Museum Catalogue of the Coins of Parthia (London, 1903), and Head's Historia Numorum (London, 1911), were used as the principal authorities for the identification of all the coins that were analyzed, due consideration being given to the uncertainties that still exists as to the proper attribution and dating of certain of the coins.
The author analyzed 6 of the drachms of Orodes I that came from the hoard; all the other silver and bronze Parthian coins were analyzed under his direction by Mr. Charles D. Oviatt, at present Professor of Chemistry at Tarkio College, Tarkio, Missouri. The work of Mr. Oviatt was in part supported by a grant from the Graduate School of The Ohio State University. For purposes of comparison, analyses of a few other ancient coins were made by Mr. Wallace H. Deebel, a graduate student in chemistry at The Ohio State University, under the direction of the author.
Before being analyzed, the specific gravity of each of the silver coins was measured by the method of Archimedes. The coins were next filed smooth and the specific gravity of each blank was also measured by the same method. The blanks were then divided into samples of suitable size for analysis. The specific gravities of the bronze coins were not measured, though samples for analysis were prepared in the same way.
For the analysis of the silver coins, accurately weighed samples of about a gram were treated with nitric acid for the separation of the gold and tin from the other metals. The ignited and weighed residue from the nitric acid treatment was extracted with cold, dilute aqua regia to dissolve the gold, and the resultant solution was diluted and treated with either ferrous sulfate or oxalic acid to precipitate the gold. This gold was then collected on filter paper, ignited, and weighed. By subtracting the weight of the gold from the weight of the residue, the weight of stannic oxide was obtained, from which the weight of the tin was calculated. In some analyses, as a check, the weight of the stannic oxide was also measured directly. The filtrate from the separation of the gold and tin was treated with hydrochloric acid to precipitate silver as the chloride. The silver chloride was collected in a weighed filter crucible, and after drying and weighing, the weight of the added silver chloride was found, from which the weight of silver was calculated. The filtrate from the separation of the silver was treated with sulfuric acid, and the solution was evaporated until fumes of sulfur trioxide appeared. After cooling, the residue was treated with water, and the lead sulfate was collected in a filter crucible, dried, and weighed. Copper was determined by electrolysis in the filtrate from the separation of the lead, and from the small amount of lead dioxide deposited on the anode and the previous weight of lead sulfate, the total lead content was found. The filtrate from the separation of the lead and copper was evaporated to small volume and treated with ammonium hydroxide solution to precipitate the iron. The precipitate was collected on filter paper, and ignited and weighed in a crucible, and the iron content was calculated from the weight of the precipitate. In the filtrate from the separation of the iron, nickel was precipitated with dimethylglyoxime. The precipitate was collected in a glass filter crucible, dried, and weighed, and from the weight of this precipitate the nickel content was calculated. The filtrate from the separation of the nickel was treated with nitric acid to remove organic matter and examined for the presence of zinc by adding phosphate. Zinc was found in only one coin, and for this determination the precipitate of zinc ammonium phosphate was collected in a filter crucible, dried, and weighed, the amount of zinc being calculated from the weight of the precipitate. The coins were also examined for the presence of arsenic and sulfur, but only negative results were obtained.
The procedure for the analysis of the bronze coins was similar except that the steps for the determination of gold and silver were omitted, neither being present in appreciable amount in any of these coins. Sulfur was found to be absent, but arsenic was present in all but one. For the determination of the arsenic a sample was first dissolved in concentrated nitric acid, the solution was evaporated to dryness, and the residue was baked to decompose the nitrates. This baked residue was dissolved in concentrated hydrochloric acid, and the hydrochloric acid solution, after adding an excess of ferrous sulfate, was distilled. In the distillate, properly diluted, the arsenic was precipitated as arsenious sulfide with hydrogen sulfide. The precipitate was collected in a filter crucible, washed first with water, next with carbon disulfide, and finally with ethyl alcohol. It was then dried and weighed, and the amount of arsenic was calculated from the weight of the dried precipitate.
This outline of the analytical scheme, from which many manipulative details have been omitted for the sake of brevity, is intended mainly to indicate the nature of the methods so that their validity may be judged. Where sufficient material was available, duplicate determinations of each metal were made, and the results were averaged to give the figures shown in the several tables. The closeness of the duplicate determinations to each other, and the closeness of the summations to 100% as shown in these tables, is a good indication, at least, of the generally satisfactory nature of these analytical methods and of the experimental manipulations.
The results of the analyses of twenty-two drachms are shown in Table II. On comparing the percentages of silver given in this table with the figures for the fineness of Parthian drachms given in Table I some interesting similarities and differences are apparent. Both groups of results indicate that only in the early coins of this Parthian series is the silver content of the coins really high, and that in most later coins it falls considerably below this high standard. Though the figures of Table I indicate that it does not fall below 60 %, the new results of Table II show clearly that it may fall nearly as low as 40%.
|No.||Silver %||Gold %||Copper %||Tin %||Lead %||Iron %||Nickel %||Zinc %||Total %|
Attributions and Dates
These new results are in direct contradiction to some general statements that have been made in regard to the fineness of the Parthian silver coinage. For example, Burns 1 states that the high initial standard continued with little alteration down to the end of the Parthian Empire in 227 a.d. However, as far as the present results show, the issue of really base drachms was confined to the reign of a single ruler, Orodes I of the period 57–38/37 b.c. It will be seen that in four of the coins of this ruler that were analyzed the silver content is below 50%. Their average silver content is only 60.65%. This is in marked contrast to the high silver content of 90.57% in a coin (No. 4 of Table II) of an immediate predecessor of Orodes I and to the generally high silver content of the coins of all his predecessors. Evidently a marked debasement of the drachm occurred during the reign of this ruler. The fact that the silver content of the coins of Orodes I is spread over a considerable range is not only a sign of debasement but probably also a sign of progressive debasement during his reign. It is obvious that when no debasement occurs during the reign of a ruler his individual coins selected at random will not only be of high standard but will differ little from each other in fineness, but that if debasement of the coinage begins and continues during a reign such individual coins will differ considerably from each other in silver content. Some illustrative data are shown in Table III. This table is derived from Tables I and II, and shows the range of silver content and average silver content of all Parthian drachms of which two or more of a given ruler have now been assayed or analyzed. It is not claimed that these figures are very reliable since so few individual coins of each ruler have been investigated. The data based upon only two determinations are especially open to question. However, these are the only such figures possible at present, and they at least appear to give significant indications. It will be seen that the range in the percentages of silver in the five coins of Mithradates I is only 5%, whereas in the 13 coins of Orodes I it is nearly 34%. Then in the 3 coins of Gotarzes the range is again only 5 %, with the coins of two of the other rulers in intermediate positions. In the group as a whole a rough inverse relationship exists between range and fineness. Apparently the debasement of the coinage during the reign of Orodes I was followed by a considerable improvement during the reigns of the
|Ruler||Date||No. of Coins||Range in Silver Content %||Average Silver Content %|
|Mithradates I||171–138 (?) b.c.||5||5.0||91.7|
|Artabanus II||88–77 b.c.||2||12.6||79.1|
|Orodes I||57–38/37 b.c.||13||33.7||60.7|
|Orodes II||4–6 (?) a.d.||2||17.6||71.0|
|Mithradates IV||130–147 (?) a.d.||2||2.1||76.0|
The percentages of gold shown in Table II are in approximate agreement with the fineness figures of Table I. In the analyses of Table II the average percentage of gold is 0.33, and in the assays of Table I the average gold content in terms of percentage is 0.25. There is a greater discrepancy in the ratios of gold to silver in the results in the two tables, but this lack of agreement may be due to the difference in the methods of determining the gold. It seems likely that the present results are more accurate. As compared to those in modern silver coins, the proportions of gold in Parthian drachms are very high indeed, but such relatively high proportions of gold are characteristic of ancient silver in general. The gold in these Parthian coins was apparently present as a mere fortuitious impurity that accompanied the silver, and its proportion varied considerably in accordance with the source of the silver and the accidental variations in the metallurgical operations. It seems improbable that ancient metallurgists had any means of removing gold present as impurity in silver, and it is doubtful that they were even aware that their silver contained gold as an impurity.
As the figures of Table II show, copper is the main alloying component in the metal of Parthian drachms. That it was introduced into the coinage alloy as the pure metal is very improbable as will appear from a consideration of the proportions of tin and lead in these coins.
Though the percentages of tin are not high numerically, being above 3 % in only 2 coins, they are nevertheless very high for ancient silver. They are generally higher in the debased coins of Orodes I than in the other drachms that were analyzed, especially the earlier ones of high silver content. Tin, when not entirely absent, is usually present in ancient coinage silver to the extent of only a few hundredths or tenths of a percent. In a series of 16 ancient Greek silver coins analyzed by Bibra, 2 3 were found to contain a trace of tin, the others none, and in a series of 22 Roman Imperial silver coins, many of them debased, which were analyzed by this same investigator, tin either was absent or was present in a mere trace in 11, and in the others the highest proportion found was 0.71% and the average was only 0.13%. The analyses in Table IV show his results on coins having about the same range of silver content as the drachms of Orodes I. According to the analyses of Bibra, tin is likely to be entirely absent from coins of very high silver content. The first two analyses listed in Table V are illustrative of his results. This absence of tin appears to be confirmed by later analyses of such coins by Elam. 3 These analyses are the last four cited in Table V. It is not certain that this analyst actually sought for the presence of tin in these coins, but if it had been present there is small likelihood that it could have escaped notice. The absence of tin from all such coins is what might be expected from its usual absence from deposits of silver ores. In general, then, tin is not normally associated with the silver of ancient coinage alloys, and there is no reason to believe that the Parthian coinage alloys were exceptional in this respect. It seems very probable, therefore, that most of the tin in the Parthian alloys was introduced along with the copper.
Similarly, the percentages of lead shown in Table II, especially in the coins of Orodes I, are unusually high for ancient coinage silver, as may be seen by comparing these percentages with those shown in Tables IV and V. All these percentages are further compared in Table VI, where it will be seen to what degree the proportions of lead in the drachms of Orodes I are abnormally high. Evidently a fairly constant small proportion of lead is almost always present in ancient fine silver, apparently as a residue from the imperfect cupellation of argentiferous lead, but the proportions of lead in the debased drachms of Orodes I are so abnormally high that it seems necessary to conclude that only part of this lead was introduced into the alloy along with the silver and that the rest was introduced along with the copper.
|Silver %||Gold %||Copper %||Tin %||Lead %||Iron %||Nickel %|
|Silver %||Gold %||Copper %||Tin %||Lead %||Iron %||Nickel %|
|Group||Silver %||Lead %||Ratio of Lead to Silver|
|Parthian Drachms||Max. = 94.17||Max. = 0.92||Max. = 0.014|
|Prior to Orodes I||Min. = 67.88||Min. = 0.37||Min. = 0.004|
|Av. = 86.37||Av. = 0.69||Av. = 0.008|
|Drachms of Orodes I||Max. = 75.57||Max. = 2.65||Max. = 0.059|
|Min. = 41.84||Min. = 0.61||Min. = 0.011|
|Av. = 60.65||Av. = 1.38||Av. = 0.023|
|Greek and Roman||Max. = 73.96||Max. = 1.85||Max. = 0.025|
|Coins of Similar||Min. = 40.66||Min. = trace||Min. = 0.000|
|Fineness||Av. = 52.28||Av. = 0.63||Av. = 0.010|
|Greek Coins of Very||Max. = 99.48||Max. = 0.85||Max. = 0.009|
|High Fineness||Min. = 99.07||Min. = trace||Min. = 0.000|
|Av. = 99.22||Av. = 0.38||Av. = 0.004|
The small percentages of iron shown in the analyses of Table II are probably of little significance, as iron is almost a universal accidental impurity in ancient metals and alloys. However, as shown by the analyses of Table V, the iron content of ancient silver coins of very high fineness is usually very small, so that it might well be that the noticeably larger proportions found in these Parthian drachms were introduced into the alloys along with the copper rather than with the silver. It is still more likely that the small proportions of nickel shown in the analyses of Table II were introduced with the copper rather than with the silver. In these analyses nickel is invariably present in coins of very high copper content (over 30%) but absent from nearly 40% of the others. Furthermore, the analyses of Table V indicate that nickel is not normally associated with ancient silver, and this same lack of association is apparent from other analyses of ancient silver coins of high fineness. The zinc found in one coin in small proportion and the trace found in two others is in all probability a mere accidental impurity that was introduced along with the copper. Neither arsenic nor sulfur in weighable amounts was found in any of these coins.
The results of the analyses of 7 tetradrachms are shown in Table VII. The most striking difference between these results and the results of the analyses of the drachms shown in Table II is the much lower range of silver content of the tetradrachms. Unfortunately, no tetradrachms prior to the reign of Phraates IV were available for analysis, so that no comparison can yet be made between earlier tetradrachms and drachms as to silver content. It may be that early tetradrachms had a silver content similar to that of early drachms, but it is certain that later tetradrachms had a much lower silver content in general than drachms of the same period. As shown in Table VII, the proportions of silver in the tetradrachms that were analyzed, with the exception of the one tetradrachm of Phraates IV, are all below 50%, and in this one exception the proportion is just slightly over 50%. On the basis of these analyses the alloy used for late tetradrachms must be classed as billon. It is interesting by way of confirmation that the
|No.||Silver %||Gold %||Copper %||Tin %||Lead %||Iron %||Nickel %||Zinc %||Total %|
Attributions and Dates
very low silver content of 28.29% found by chemical analysis in a tetradrachm of Volagases III agrees fairly well with the result of the assay of one of this same ruler given in Table I. From the results in Table VII it seems evident that the silver content of the late Parthian tetradrachms decreased progressively with time, which does not appear to be true for the drachms of the same period.
The average proportion of gold found to be present in these tetradrachms is 0.21% as compared with the average of 0.33% for all the drachms of Table II and the same figure of 0.33% for the 5 latest drachms. However, since the gold is in all probability associated wholly with the silver it is better to make a comparison between the ratios of the proportions of gold to silver in the coins of the two denominations. The average of the ratios of gold to silver in the tetradrachms that were analyzed is 0.0050, in all the drachms, 0.0052, and in the 5 latest drachms, 0.0047. The closeness of these ratios shows that the original silver metal that entered into the alloys of the coins of the two denominations had about the same gold content on the average, and this indicates that it was of about the same quality.
Copper is the chief alloying component in the metal of Parthian tetradrachms just as it is in the metal of the drachms, but the lower proportions of tin and lead in the tetradrachms indicate that the copper was introduced into the alloy in a relatively pure state and not in the form of bronze. The average proportion of tin in the tetradrachms that were analyzed is only 0.21 % as compared to the average of 1.15% for all the drachms and 0.88% for the 5 latest drachms. The differences in the percentages of lead and in the ratios of lead to silver are shown in Table VIII, where it will be seen that both the percentages and the ratios are lower in the tetradrachms than in the drachms. The full significance of these figures is shown in detail later in the discussion of the theory of the debasement of Parthian silver coins. No significant difference exists in the proportions of iron in the tetradrachms and the drachms, and this is what would be expected from its presence as a mere accidental impurity. Though the proportions of nickel in the tetradrachms of higher fineness (Nos. 1–4 on Table VII) are similar to those in the drachms, the proportion of this metal in the other tetradrachms is much higher, particularly in the last two. This is further indication a that the nickel in Parthian silver coinage alloys is associated with the copper.
It will be noted from Table VII that the summations of the individual percentages obtained on the analysis of the tetradrachms are generally lower than the summations of the individual percentages for the drachms as shown in Table II. In fact, the summations of the analytical figures for three of the tetradrachms does not even reach 99%. This is because of the presence of certain nonmetallic elements, namely, chlorine and oxygen, that were not determined by the analyst. These were present in the form of certain corrosion products, principally silver chloride and cuprous oxide, distributed throughout the metal of these coins. It might well be expected that corrosion would have proceeded to a greater extent in the tetradrachms than in the drachms because of the general difference in fineness. However, this does not account for the fact that the summations for certain tetradrachms (Nos. 2 and 3 of Table VII) are much lower than the summations for certain drachms (Nos. 14 to 17 of Table II) of about the same fineness. The metal of the tetradrachms was visibly less homogeneous than that of the drachms, which would account for the greater degree of corrosion. This lack of homogeneity was shown also by the poorer agreement of the duplicate determinations in the course of the analysis of the tetradrachms. Possibly this observed lack of homogeneity was simply the result of a greater degree of corrosion, but it seems more probable that it was due to an original lack of homogeneity in the metal of the tetradrachms. Possibly this lack of homogeneity in the metal was due to a lower degree of technical skill exercised in minting the tetradrachms or because it was less easy to form homogeneous flans of large size. At any rate there is a considerable difference in the homogeneity of the coins of the two denominations, which at least suggests that they may have been struck at different mints.
|Group||Silver %||Lead %||Ratio of Lead to Silver|
|All Drachms||Max. = 94.17||Max. = 2.65||Max. = 0.059|
|Min. = 41.84||Min. = 0.37||Min. = 0.004|
|Av. = 67.61||Av. = 1.14||Av. = 0.019|
|Drachms After||Max. = 77.00||Max. = 1.41||Max. = 0.027|
|Orodes I||Min. = 52.05||Min. = 0.54||Min. = 0.007|
|Av. = 70.71||Av. = 0.86||Av. = 0.013|
|Tetradrachms||Max. = 52.24||Max. = 0.80||Max. = 0.018|
|Min. = 24.24||Min. = 0.19||Min. = 0.006|
|Av. = 39.47||Av. = 0.43||Av. = 0.011|
The results of the analysis of 12 bronze coins are shown in Table IX. These are apparently the first analyses of any kind of a Parthian bronze object that have been reported. It will be seen that the 2 earliest coins are very similar to each other in composition, and that the 2 coins of Orodes I are also very similar to each other. Larger differences exist between the compositions of the 2 coins of Sinatruces, but they are similar to each other in the proportions of lead they contain, and this clearly groups them together as distinct from the earlier and later coins. Of the 4 coins of Gotarzes, 3 are similar to each other in composition, and all of them are distinctly different from the earlier coins. The 2 very late coins of Artabanus V differ radically in composition. With the exception of these, the bronze coins issued in the same reign have a certain similarity in composition which would seem to indicate that some degree of control and standardization was exercised in the preparation of even the bronze coinage alloys. Possibly the coins of Artabanus V were struck under conditions that precluded any exercise of choice in the selection of the metal for the bronze coins. Because of this possibility, the composition of these coins will not be further considered in the discussion that follows.
|No.||Copper %||Tin %||Lead %||Silver %||Iron %||Nickel %||Arsenic %||Total %|
Attributions and Dates
Though these coins viewed as a whole are not very different in composition, except in lead content, this one difference is very marked. The relationships of the proportions of the main components of the alloys to each other are perhaps more readily apparent from the ratios of the percentages, shown in Table X, than from the percentages themselves. For the coins of Sinatruces, Orodes I, and Gotarzes these ratios were calculated from the average percentage figures for each group. It will be seen that the ratios of the components are very similar in the two earliest coins, and that in the series as a whole there is little difference in the ratio of tin content to copper content. The most striking and significant difference is the progressive increase, beginning with the coins of Sinatruces, in the ratios of lead content to copper content and of lead content to tin content. This same sort of chronological change in these ratios, with the ratio of tin content to copper content remaining relatively constant, has been previously observed in various series of Greek bronze coins, and has been explained as being the result of the remelting of worn bronze coins of previous issue with lead in order to obtain metal for the issue of new coins. 4 However, the lead content of these Parthian coins is generally lower than that of contemporaneous bronze coins issued elsewhere in the ancient world, even in localities near Parthia. This is illustrated by the analyses listed in Table XI of a series of coins struck in Syria. 5 In this one respect, at least, Parthian bronze coins have a composition that is distinctive.
|Period||Ratio of Tin to Copper||Ratio of Lead to Copper||Ratio of Lead to Tin|
|171–138 (?) b.c.||0.076||0.044||0.58|
|No.||Copper %||Tin %||Lead %||Iron %||Nickel %||Zinc %||Arsenic %||Sulfur %||Total %|
Attributions and Dates
In view of the apparently systematic chronological increase in the lead content of Parthian bronze coins, there is a distinct possibility that such coins now of uncertain or unknown attribution could be roughly dated by means of chemical analysis, and thus be ascribed to the reigns of certain rulers. In order to do this, however, it would be necessary to make many more analyses of coins of known attribution so as to provide a reliable scale of reference. Furthermore, it would probably not be sufficient to analyze one unknown specimen, but as many as possible so that a reliable average figure would be obtained for comparison with the established averages for already attributed coins.
The percentages of the various impurities listed in Table IX are similar to those generally found in ancient coinage bronze. The nickel content is noticeably higher than in most ancient coinage bronze of the same period, and this may be of some significance as a distinctive characteristic. Though there appears to be some systematic variation in the arsenic content from one reign to another, this is probably fortuitous, as the arsenic content of ancient coinage bronze, like the iron content, usually varies in an erratic manner, thus indicating that both are mere accidental impurities. Arsenic was not determined in the last 6 coins because their small weight did not provide a sufficient sample. The low summations of Nos. 9 and 11 must in part, at least, be ascribed to the presence of oxygen, as these coins were noticeably corroded internally.
|1||Burns, A. R., Money and Monetary Policy in Early Times (New York, 1927), p. 164.|
|2||Bibra, E. von, Ueber alte Eisen- und Silber-Funde (Nürnberg and Leipzig, 1873), pp. 37, 40, 41.|
|3||Elam, C. F., Journal of the Institute of Metals, XLV (1931), p. 60.|
|4||Caley, E. R., The Composition of Ancient Greek Bronze Coins (Philadelphia, 1939).|
|5||From Table XVIII, pp. 92–93, of the work cited in Reference 4.|
The analytical results of Table II show clearly that some, at least, of the drachms of Orodes I were debased. In the discussion of these analytical results it was shown that part of the lead and virtually all the tin, iron, and nickel were associated with the copper and that these metals were in all probability introduced into the coins along with the copper. Such a mixture in the proportions indicated by the analytical figures would constitute a bronze. Consequently, it may be inferred that the alloy for the debased drachms of Orodes I was manufactured by alloying silver of good quality with bronze. Moreover, the composition of this bronze could be calculated from the figures of Table II providing the composition of this silver were known. Though there seems to be no way to find the exact composition of this silver, certain likely assumptions as to its composition may be postulated. These are:
On the basis of each of these three assumptions the composition of of the bronze used in producing the alloy for a typical debased drachm of Orodes I (i.e. No. 13 of Table II) may then be calculated in the following way.
On Assumption A. The average silver and gold content of the fine silver coins of Table V is 99.32%. Gold is counted with the silver in all these calculations since the two are associated. The average lead content of these coins is 0.38%. Therefore the proportion of this lead in terms of per cent that would have entered into the alloy of Coin 13 by the use of such silver is given by the expression:
50.97 + 0.35 / 99.32 × 0.33 = 0.20%
This figure is then subtracted from the 2.34% of lead found by analysis in Coin 13 to give 2.14% as the amount of lead introduced along with the copper. Because of the high purity of the silver, the percentages of the other metals in Coin 13 remain unaffected, so that the proportions of the components of the bronze are given by the following percentages:
Copper = 43.97
Tin = 2.35
Lead = 2.14
Iron = 0.03
Nickel = 0.02
Total = 48.51
These figures are then prorated to 100% to give the composition of the bronze:
Copper = 43.97/48.51 × 100 = 90.64%
Tin = 2.35/48.51 × 100 = 4.85%
Lead = 2.14/48.51 × 100 = 4.41%
Iron = 0.03/48.51 × 100 = 0.06%
Nickel = 0.02/48.51 × 100 = 0.04%
Total = 100.00%
On Assumption B. The average figures for the analysis of Coin 1, 2, and 4 of Table II are as follows:
Silver = 92.53%
Gold = 0.23%
Copper = 6.40%
Tin = 0.14%
Lead = 0.62%
Iron = 0.04%
Nickel = 0.03%
In the same way as explained for the calculations on the basis of Assumption A, the percentage of lead to be subtracted from the given percentage found in Coin 13 is given by the expression:
50.97 + 0.35/92.53 + 0.23 × 0.62 = 0.34%
Similarly, the amount of tin that would have entered into the alloy of Coin 13 by the use of silver of the composition shown by the above average analysis is given by the expression:
50.97 + 0.35/92.53 + 0.23 × 0.14 = 0.08%
This figure is then subtracted from the 2.35% of tin found by analysis in Coin 13 to give 2.27% as the amount of tin introduced along with the copper. In the same way the following expressions give the percentages of copper, iron, and nickel, respectively, to be subtracted from the percentages found by analysis:
50.97 + 0.35/92.53 + 0.23 × 6.40 = 3.54%
50.97 + 0.35/92.53 + 0.23 × 0.04 = 0.02%
50.97 + 0.35/92.53 + 0.23 × 0.03 = 0.02%
When these percentages are subtracted from the percentages found by analysis in Coin 13, the proportions of the components of the bronze are given by the following percentages:
Copper = 40.43
Tin = 2.27
Lead = 2.00
Iron = 0.01
Nickel = none
Total = 44.71
In the same way as shown in the calculations under Assumption A, these figures are then prorated to 100% to give the composition of the bronze:
Copper =40.43/44.71 × 100 =90.43%
Tin =2.27/44.71 × 100 =5.08%
Lead =2.00/44.71 × 100 =4.47%
Iron =0.01/44.71 × 100 =0.02%
On Assumption C. From the figures for the analysis of Coin 4 and those of Coin 13 the calculations of the composition of the bronze are made in the same way as shown in the calculations for Assumption B. The proportions of the components of the bronze are given by the following percentages:
Copper = 39.25
Tin = 2.30
Lead = 1.98
Iron = 0.01
Nickel = 0.02
Total = 43.56
When these figures are prorated to 100%, the composition of the bronze is found to be as follows:
Copper = 90.10%
Tin = 5.28%
Lead = 4.55%
Iron = 0.02%
Nickel = 0.05%
Total = 100.00%
No allowance is made in these calculations for any preferential loss of the components of the bronze by oxidation or volatilization during the fusion of it with the silver. It seems likely, however, that the results of the calculations would not have differed materially if allowance had been made for the various small losses that could have occurred in these ways. The results of the above calculations are shown in Table XII along with the results of similar calculations for the four still more debased drachms. It will be seen that the three sets of figures for each coin are similar to each other, in the proportions of main components at least, regardless of which assumption is made as to the composition of the silver that was debased. Hence the exact composition of this silver is, after all, not a matter of great importance for estimating the composition of the bronze. In general, as shown by the closer absolute and relative correspondence of the figures based on the three assumptions, the greater the degree of debasement the less the importance of the exact composition of the
|Coin No.||Assumption||Copper %||Tin %||Lead %||Iron %||Nickel %|
|All||Av. = 91.81||5.34||2.76||0.04||0.05|
|16 and 17 only||Av. = 91.20||5.50||3.18||0.07||0.05|
The source of this bronze may have been earlier Parthian bronze coins. It seems significant that the average figures calculated for Coins 16 and 17 are similar to figures for the composition of the bronze coins of Mithradates I and Mithradates II given in Table IX. Bronze of the composition of the bronze coins of Orodes I, either in the form of the coins of this ruler or in the form of bulk metal, evidently could not have been used in producing his debased silver coins. Moreover, it is improbable on the basis of the analytical figures that bronze having the composition of the bronze coins of Sinatruces, or bronze coins of this ruler, could have been used. Only one principal qualitative discrepancy exists between the calculated composition of the bronze used for debasing the silver coins of Orodes I and the actual composition of the two early Parthian bronze coins. This is the presence of arsenic in these coins and its absence from the debased silver coins. However, it is entirely possible that the arsenic in the bronze coins was completely oxidized and volatilized on remelting and that as a consequence none was incorporated in the debased silver.
That bronze in the form of coins, rather than in any other form, was used in debasing silver for the production of drachms of Orodes I is probable. It is the usual practice in mints to obtain much or most of the metal for the issue of new coins by melting down earlier ones, especially if these are badly worn, and at the time of Orodes I it is almost certain that most of the bronze coins of Mithradates I and Mithradates II still in circulation were in poor condition. Furthermore, the bronze coins of these rulers are of larger diameter and greater weight than the bronze coins issued by later rulers, and this could have been an additional reason for withdrawing these particular coins from circulation and using them as a source of metal.
In Table XIII are shown the results of calculations of the composition of the debased silver that could have been produced by melting bronze of the average composition of the coins of Mithradates I and Mithradates II with silver of the composition of Coin 4 of Table II to produce alloys having the silver content of Coins 16 and 17 of Table II. In making these calculations it was assumed that all the arsenic was volatilized from the bronze, and an allowance was made for a loss of 10% of the tin and lead by preferential oxidation in the process of remelting and alloying. The degree of debasement for Coin 16 is 52.4% and for Coin 17, 53.8%. It will be seen that there is a substantial agreement between the actual and the theoretical figures. On the whole, therefore, it does not appear at all unlikely that the metal for the debased drachms of Orodes I was made by melting down silver coins of his immediate predecessor, or of more than one predecessor, with early Parthian bronze coins.
|Coin No.||Source of Figures||Silver %||Gold %||Copper %||Tin %||Lead %||Iron %||Nickel %|
In Table XIV are shown the results of calculations of the composition of the base alloy of drachms of Orodes I of relatively high fineness, or in other words of drachms that were not deliberately debased or that were much less debased. The serial numbers of the coins in this table correspond to those of Table II. Assumption A was the only one applicable to these calculations, as the other two assumptions led to impossible figures for the percentages of iron or nickel in some of these coins. It will be seen that the composition of the base alloy in these coins is distinctly different from that in the debased drachms (Table XII). The individual alloys that contain less than 2% tin cannot be classified as bronze at all, but rather as a very impure copper. In Table XV are shown the results of similar calculations of the composition of the base alloy of some drachms of rulers other than Orodes I. Here again only Assumption A was applicable. In half of these coins the base alloy has a composition similar to that of the base alloy in the debased drachms of Orodes I, but in the other half it is merely an impure copper similar to that of the drachms of Orodes I of relatively high fineness. In general, therefore, the composition of the base alloy of Parthian drachms is either a bronze of low tin content and lower lead content, or a very impure copper containing both tin and lead.
|Coin No.||Copper %||Tin %||Lead %||Iron %||Nickel %|
|Coin No.||Copper %||Tin %||Lead %||Iron %||Nickel %|
In Table XVI are shown the results of calculations of the composition of the base metal in the series of Parthian tetradrachms of Table VII. Assumption A was the only one applicable to these calculations, as the other two led to impossible figures for the percentages of various metals in some of the coins. It will be seen that the base metal is not a bronze but a relatively pure copper. As may also be seen by comparing these results with those in Tables XIV and XV, this copper is much purer than any of that used for the drachms. The composition of this copper is very similar to that in the Roman As of the same general period, as may be seen by comparing the figures of Table XVI with those of Table XVII, which contains representative analyses from a list previously published by the author. 6 The analyses in this table are of coins of the reigns of Augustus to Hadrian, inclusive.
|Coin No.||Copper %||Tin %||Lead %||Iron %||Nickel %|
|Max. = 99.61||0.97||1.27||0.15||0.47|
|Min. = 98.12||none||0.13||none||none|
|Av. = 98.93||0.34||0.50||0.08||0.15|
|Coin No.||Copper %||Tin %||Lead %||Iron %||Nickel %||Other Impurities %|
|Max. = 99.65||0.73||0.46||0.43||0.40||0.63|
|Min. = 97.62||0.01||none||trace||trace||none|
|Av. = 98.74||0.30||0.17||0.16||0.27||0.19|
|6||Caley, E. R., The Composition of Ancient Greek Bronze Coins p. 107.|
The degrees of fineness, expressed on the usual basis of parts per thousand, of all the Parthian drachms that have been assayed or analyzed, are shown in Table XVIII. Figures are given not only for the fineness in terms of silver content but also for the total fineness, which includes the small proportions of gold present in nearly all the coins. Since it is practically certain that all, or nearly all the gold, was introduced into the coinage alloy along with the silver, the intended fineness of the coins is in all probability their total fineness, and not that due to their silver content alone. However, since the proportions of gold are relatively so small, the figures for silver fineness and total fineness are close together and parallel throughout the series. Therefore, it is sufficient to discuss the fineness of these coins on the basis of their silver fineness alone, as any comparisons that may be made as the relative fineness of coins of different date lead to the same conclusions no matter which set of figures is used. Furthermore, this basis is better for comparisons with figures for fineness obtained from specific gravity measurements, a topic discussed in the last three sections of this essay, since the fineness estimated in this way can be expressed only in terms of silver. Also included in Table XVIII are the weights of the drachms that were assayed or analyzed, and the actual weights of silver contained in these coins as computed by multiplying the degrees of fineness by these weights. Unfortunately, the fineness of No. 1, as published by Rauch, is not accompanied by a statement of its weight, and the weight of 3 grams for No. 7, as published by Imhoof-Blumer, appears to be only a rough approximation. Probably none of the weights in the table can be relied on as being accurate original weights of these drachms, as all the specimens analyzed in the present investigation were worn at least to some degree, and the same holds, in all likelihood, for those assayed by the previous investigators. Hence these weights generally must be regarded as tending to be lower than they were originally, and the same holds for the figures for the silver content.
|No.||Ruler||Date||Silver Fineness||Total Fineness||Weight Grams||Silver Content Grams|
|1||Arsaces I (?)||250–248 (?) b.c.||946||946 (?)||-||-|
|2||Mithradates I||171–138 (?) b.c.||942||943||3.81||3.59|
|3||Mithradates I||171–138 (?) b.c.||929||932||3.29||3.06|
|4||Mithradates I||171–138 (?) b.c.||923||925||3.70||3.42|
|5||Mithradates I||171–138 (?) b.c.||899||904||3.90||3.51|
|6||Mithradates I||171–138 (?) b.c.||892||894||3.40||3.03|
|Mithradates I||171–138 (?) b.c.||Av. = 917||920||3.62||3.32|
|7||Phraates II||138–128/127 b.c.||709||712||—||—|
|8||Artabanus II||88–77 b.c.||854||855||3.90||3.33|
|9||Artabanus II||88–77 b.c.||728||730||3.80||2.77|
|Artabanus II||88–77 b.c.||Av. = 791||793||3.85||3.05|
|11||Phraates III (?)||70–57 b.c.||906||909||3.96||3.59|
|12||Orodes I||57–38/37 b.c.||756||759||3.96||2.99|
|13||Orodes I||57–38/37 b.c.||748||751||4.02||3.01|
|14||Orodes I||57–38/37 b.c.||744||748||3.98||2.96|
|15||Orodes I||57–38/37 b.c.||742||745||3.82||2.83|
|16||Orodes I||57–38/37 b.c.||698||702||3.92||2.74|
|17||Orodes I||57–38/37 b.c.||668||672||3.78||2.53|
|18||Orodes I||57–38/37 b.c.||652||655||3.85||2.51|
|19||Orodes I||57–38/37 b.c.||582||587||3.70||2.15|
|20||Orodes I||57–38/37 b.c.||510||514||3.57||1.82|
|21||Orodes I||57–38/37 b.c.||473||477||3.69||1.75|
|22||Orodes I||57–38/37 b.c.||464||466||3.84||1.78|
|23||Orodes I||57–38/37 b.c.||431||434||3.45||1.49|
|24||Orodes I||57–38/37 b.c.||418||421||3.75||1.57|
|Orodes I||57–38/37 b.c.||Av. = 607||610||3.79||2.32|
|25||Tiradates II (?)||26 b.c.||611||613||3.50||2.14|
|26||Orodes II||4–6 (?) a.d.||798||800||3.20||2.55|
|27||Orodes II||4–6 (?) a.d.||622||625||3.20||1.99|
|Orodes II||4–6 (?) a.d.||Av. = 710||713||3.20||2.27|
|28||Vardanes I||41/42–45 a.d.||743||746||3.65||2.71|
|Gotarzes||40/41–51 a.d.||Av. = 776||779||3.58||2.78|
|32||Volagases II||77/78–146/147 a.d.||733||737||3.74||2.74|
|33||Mithradates IV||130–147 (?) a.d.||770||775||3.28||2.53|
|34||Mithradates IV||130–147 (?) a.d.||749||753||3.10||2.32|
|Mithradates IV||130–147 (?) a.d.||Av.= 760||764||3.19||2.43|
|35||Volagases IV||191–207/208 a.d.||779||782||3.80||2.96|
|36||Volagases V||207/208–221/222 (?) a.d.||521||523||3.32||1.73|
|37||Artabanus V||213–227 (?) a.d.||746||750||3.20||2.39|
It is obvious from Table XVIII (Nos. 1 to 6 inclusive) that the earliest drachms are of higher fineness than later ones, and that they generally contain the highest weight of silver. On the other hand, the weights of these same drachms are often exceeded by those of many later drachms, though they are heavier on the average than drachms issued after the beginning of the Christian Era. Serious debasement is apparent only in the drachms of Orodes I, though it is still possible that some coins of later rulers were similarly debased. The one coin of Volagases V (No. 36) is of low fineness, and possibly the examination of others of this same ruler would show that his coins were also much debased. Wroth 7 observes that certain individual drachms of other rulers in the collection of the British Museum appear to be struck from silver of poor quality, e.g. one of Sinatruces, one of Phraates III, and one of Phraates IV. However, as far as these present results are indicative, the drachms of Orodes I were debased to a greater degree than those of any other Parthian ruler. The existence of this debasement is further established by the measurements of the specific gravity of a much larger number of his drachms listed and discussed in Section IX of this essay.
In general, Parthian drachms are of lower fineness than similar contemporaneous silver coins issued by countries to the west of Parthia. None of the Parthian drachms reach the very high degree of fineness often found in Greek drachms, in Roman Republican denarii, and in the earliest denarii of the Roman Empire. However, the drachms issued by Parthian rulers at or near the end of their empire are generally of higher fineness and silver content than contemporaneous Roman denarii. Since the Parthian drachms issued after the beginning of the Christian Era were initially of fairly good silver and remained rather constant in fineness and weight while the fineness and weight of the Roman denarii continually declined, the point was ultimately reached when the two became about equal, and finally the denarii became inferior. This relationship is evident on comparing the data in Tables XVIII and XIX.
The degrees of fineness, weights, and silver content by weight of all the Parthian tetradrachms that have been assayed or analyzed are shown in Table XX. All these data have been computed and arranged in the same way as for the drachms. Here again, the weights of the coins and the corresponding silver content by weight must be regarded as tending to be lower than they were originally, and probably more so than with the drachms, as some of these tetradrachms were appreciably worn. Probably also, a greater proportional loss of metal from corrosion had occurred because of the poor quality of the silver as compared with that of the drachms. Obviously, Parthian tetradrachms of the period covered by these analyses were struck from silver of very low fineness. From the standpoint of proper nomenclature the metal of these coins should be called billon rather than silver. In only one of these examples, the earliest, is the degree of fineness above 500. Furthermore, the fineness evidently declined markedly in the period from about the beginning of the Christian Era to about the time of the end of Parthian rule. As far as the present figures are indicative, this decline in fineness amounts roughly to a half from the beginning to the end of this period.
|Emperor||No. of Coins||Fineness||Weight Grams||Silver Content Grams|
|Augustus||4||Max. = 991||Max. = 3.88||Max. = 3.84|
|Min. = 990||Min. = 3.73||Min. = 3.70|
|Av. = 990||Av. = 3.82||Av. = 3.78|
|Vespasian||4||Max. = 886||Max. = 3.87||Max. 3.43|
|Min. = 798||Min. = 2.68||Min. = 2.14|
|Av. = 841||Av. = 3.26||Av. = 2.75|
|Hadrian||8||Max. = 915||Max. = 3.47||Max. = 2.85|
|Min. - 809||Min. = 2.72||Min. = 2.49|
|Av. = 848||Av. = 3.21||Av. = 2.72.|
|Commodus||10||Max. = 720||Max. = 3.56||Max. = 2.56|
|Min. = 671||Min. = 2.48||Min. = 1.53|
|Av. = 711||Av. = 2.90||Av. = 2.03|
|Septimius||10||Max. = 755||Max. = 3.82||Max. = 2.74|
|Severus||Min. = 431||Min. = 2.09||Min. = 0.90|
|Av. = 595||Av. = 3.07||Av. = 1.88|
Notes to Table XIX
This table was constructed from weights of coins and determinations of their fineness published by:
|No.||Ruler||Date||Silver Fineness||Total Fineness||Weight Grams||Silver Content Grams|
|1||Phraates IV||37–3/2 b.c.||522||524||14.38||7.51|
|Gotarzes||Av. = 438||441||11.54||5.05|
|5||Pacorus II||79/80 a.d.||398||401||11.12||4.43|
|6||Volagases III||185/186 a.d.||334||335||12.80||4.28|
|7||Volagases III||147/148–191 A.D.||283||285||10.04||2.84|
|Volagases III||Av. = 309||310||11.42||3.56|
|8||Volagases IV||198/199 a.d.||244||244||12.49||3.05|
With the exception of the Alexandrian tetradrachms, which must be regarded as a special class, very few assays or chemical analyses have been made of contemporaneous tetradrachms issued by countries to the west of Parthia. In view of this lack of information about their fineness and the few determinations that have been made of the fineness of Parthian tetradrachms themselves, any conclusions as to the fineness of the latter relative to these others must necessarily be tentative. Our present state of knowledge about the fineness of other contemporaneous tetradrachms, including some Alexandrian tetradrachms, is shown in Table XXI. When compared, the data of Tables XX and XXI indicate that Parthian tetradrachms are of lower fineness than contemporaneous late Syrian tetradrachms of the Roman period, and they appear to indicate that they are of considerably lower fineness than contemporaneous Egyptian tetradrachms of the Ptolemaic period. However, the figure for the one late Ptolemaic tetradrachm may not be at all representative. Many such coins have the appearance of base silver, and lower figures have been obtained for a few earlier ones in the series. For example, a tetradrachm of Ptolemy X analyzed in the author's laboratory had a fineness of only 510, and Giesecke 8 reports an analysis of one of Ptolemy Auletes with a fineness of only 336. However, the latter was a plated coin. On the whole, it does not seem possible at present to draw any definite conclusions as to the relative fineness of contemporaneous Parthian and Ptolemaic tetradrachms.
Notes to Table XXI
Alexandrian silver coins struck under Tiberius of lower weight than the tetradrachm have been found to be of higher fineness than the one tetradrachm listed in Table XXI. However, as is shown in Table XXII, the actual silver content by weight of such coins and the tetradrachm are about the same, and decidedly below that of Parthian tetradrachms of about the same period. Though the average fineness of the coins of Tiberius is about the same as that of roughly contemporaneous Parthian tetradrachms, the fineness of the one tetradrachm that has been analyzed is much lower, and the coins as a whole contain less silver. As will be seen from Tables XX and XXI, there can be no doubt that Alexandrian tetradrachms of the emperors after Tiberius are of lower fineness than contemporaneous Parthian tetradrachms.
The discrepancy in fineness and weight between Parthian drachms and tetradrachms of the period after the beginning of the Christian Era is considerable, as may be seen by comparing the relevant data in Tables XVIII and XX. Not only are the tetradrachms of lower fineness generally, but their fineness declines markedly, whereas that of the drachms remains relatively constant. This certainly shows a lack of any fixed relationship between the values of the two. It probably indicates also that the two denominations were not only struck at different mints but in different parts of the Parthian empire. 9
On the average, the weights of the tetradrachms are never as much as four times the weights of the drachms, as may be seen from Table XXIII. Some of this shortage in weight may be due to the
|Coin No.||Fineness||Weight Grams||Silver Content Grams|
|Av. = 438||Av. = 3.90|
Notes to Table XXII
|Ruler||Tetradrachms Grams||Drachms Grams||Tetradrachm to Drachm Ratio|
|Average of Averages||12.10||3.59||3.37|
Note to Table XXIII
The weights here gathered together are those given by L. C. West in his Gold and Silver Standards in the Roman Empire, NNM No. 94, passim.
baser silver of the tetradrachms with its consequent greater tendency to corrode and thus to lose relatively more weight, but it seems doubtful, that this would account for all this shortage in weight. If the actual amounts of silver by weight in drachms and tetradrachms, i.e. their intrinsic values, are compared, the discrepancy between their nominal values is seen to be much greater than suggested by the general discrepancy in weight. For example, the average silver content of the three drachms of Gotarzes listed in Table XVIII is 2.78 grams, whereas that of his three tetradrachms listed in Table XX is but 5.05 grams, which is a ratio of 1.00 to 1.82. If the average weights of the coins of Gotarzes given in Table XXIII are used as the basis for computation with the same fineness figures, the ratio becomes 1.00 to 1.95. On either basis the tetradrachms of Gotarzes contain a little less than twice as much silver as his drachms. In view of the possibility of a greater relative loss of weight from the tetradrachms for the reason above mentioned, it is not unlikely that his tetradrachms as struck contained just twice as much silver as his drachms. In other words, his so-called tetradrachms were actually didrachms on the basis of their intrinsic value. It seems probable that this was recognized at the time and that these coins of larger module actually passed as didrachms in trade. A similar satisfactory direct comparison of the intrinsic values of the tetradrachms and drachms of other rulers is not possible from the data here presented. However, these data indicate that the so-called tetradrachms of later rulers were never more than didrachms in intrinsic value, and it would seem that in the time of certain late rulers, Volagases IV for example, the intrinsic value of the so-called tetradrachms was about the same as that of the drachms. This whole question of the relative values of the tetradrachms and drachms of Parthia is worthy of a detailed investigation.
|7||Wroth, W., B. M. C. Parthia, passim.|
|8||Giesecke, W., Das Ptolemäergeld (Leipzig and Berlin, 1930), p. 93.|
|9||On the basis of archaeological and numismatic evidence McDowell (Coins from Seleucia on the Tigris (Ann Arbor, 1935), pp. 159–177) concludes that Parthian tetradrachms were struck only at Seleucia and that the drachms were struck elsewhere. The technical data therefore tend to confirm his conclusion. Professor Thomas Mabbott reminds me that the earlier Sasanian kings struck coins obviously descendants of the billon tetradrachms, now uncommon enough to suggest limited circulation.|
As stated in Section IV of this essay, the specific gravities of all these Parthian coins and the clean blanks of these coins were taken before they were analyzed. Also, as mentioned in Section III, the specific gravities were taken of many drachms of Orodes I that were not analyzed, namely, all the remaining duplicates in the collection at Princeton University. This was done with the hope not only of being able to estimate the average fineness and the variation in fineness of a considerable number of drachms of a single Parthian ruler but also of being able to estimate the relative fineness of some varieties of his coins. Furthermore, in the course of the investigation a few other ancient silver coins were analyzed chemically and the specific gravities of these coins and their blanks were taken, so that their fineness as determined by chemical analysis might be compared with that estimated from specific gravity. All this information has made possible a much needed critical evaluation of the validity of specific gravity measurements of ancient silver coins as an index of their fineness. There is considerable evidence that uncritical reliance has been placed on the specific gravity of such coins as a measure of their fineness. Ondrouch, 10 for example, lists the specific gravities of twenty Roman denarii from a hoard and then expresses the corresponding fineness of these coins through two decimal places in percentage. It may easily be shown both theoretically and experimentally that no basis exists for expressing the fineness estimated by this means to any such degree of accuracy, especially the fineness of ancient silver coins.
The estimation of the composition of an alloy from its specific gravity is only possible theoretically if it is composed of two metals and no more. Hence in any estimation of the fineness of an ancient silver coin from its specific gravity it must be assumed that the coin is composed of silver and copper alone, though in fact, as shown by chemical analyses, as for example, those given in Tables II and VII, such a coin may be composed of an alloy containing six or seven metals. Although most of these metals other than silver and copper are often impurities present in such small proportion that they have little effect on the specific gravity of a coin, sometimes the proportions of certain of these other metals may be high enough to have an appreciable effect on the specific gravity and thus invalidate the necessary assumption that the coin is composed of silver and copper alone. Another assumption that is necessary for any simple computation of fineness from specific gravity is that no change in volume occurs when the two component metals are alloyed together. Actually, changes in volume do occur, but the experiments of Karmarsch 11 indicate that they are not great enough nor erratic enough to invalidate this second assumption. For this reason it is possible to use the following ideal formula for computing the fineness of a silver-copper alloy from its observed specific gravity:
Fineness = S1 Sx — S1 S2 / S1 Sx — S2 Sx × 1000
Where, on the same temperature basis for each,
S1 is the specific gravity of pure silver.
S2 is the specific gravity of pure copper.
Sx is the specific gravity of a given alloy.
In applying this formula, the specific gravities of both pure silver and pure copper must obviously be known with sufficient precision. Unfortunately, it is difficult to establish precisely the specific gravity of either of these metals for the general application of this formula because their specific gravities vary considerably in accordance with the mechanical or thermal treatment to which they have been subjected. After a discussion of the various figures that have been reported for the specific gravity of pure silver, Mellor 12 concludes that 10.5 is the most representative value, which is to say that the specific gravity of pure silver can in a general sense be expressed only through the first decimal place. For the same reason the most representative value for the specific gravity of pure copper should be expressed only as 8.9. However, this low degree of precision is inadequate for the estimation of the fineness of ancient silver coins from specific gravity measurements, as it allows only 16 units of measurement, i.e. the
|Specific Gravity||Silver Fineness||Increment||Rounded Fineness||Increment|
|Specific Gravity||Silver Fineness||Increment||Rounded Fineness||Increment|
|Observed Specific Gravity||Fineness by Fire Assay||Fineness by the Formula of Karmarsch||Difference Error||Fineness by Theoretical Formula, I||Difference Error||Fineness by Theoretical Formula, II||Difference Error|
|10.36||920||927||+ 7||897||— 23||925||+ 5|
|10.32||899||903||+ 4||874||— 25||903||+ 4|
|10.31||901||897||— 4||869||— 32||897||— 4|
|10.30||898||890||— 8||863||— 35||892||— 6|
|10.30||894||890||— 4||863||— 30||892||— 2|
|10.30||893||890||— 3||863||— 31||892||— 1|
|10.29||897||884||— 13||858||— 39||886||— 11|
|10.25||872||860||— 12||835||— 37||864||— 8|
|10.20||828||830||+ 2||806||— 22||836||+ 8|
|10.18||813||818||+ 5||795||— 18||825||+ 12|
|10.17||817||812||— 5||789||— 28||820||+ 3|
|10.07||750||751||+ 1||731||— 19||762||+ 12|
|10.05||750||739||— 11||720||— 30||751||+ 1|
|10.05||747||739||— 8||720||— 27||751||+ 4|
|9.98||688||696||+ 8||678||— 10||710||+ 22|
|9.97||690||690||0||673||— 17||704||+ 14|
|9.93||663||666||+ 3||648||— 15||681||+ 18|
|9.92||664||660||— 4||642||— 22||675||+ 11|
|9.87||626||629||+ 3||612||— 14||645||+ 19|
|9.79||584||581||— 3||563||— 21||597||+ 13|
|9.77||574||569||— 5||550||— 24||584||+ 10|
|9.76||564||563||— 1||544||— 20||578||+ 14|
|9.76||563||563||0||544||— 19||578||+ 15|
|9.69||521||520||— 1||500||— 21||535||+ 14|
|9.68||512||514||+ 2||493||— 19||529||+ 17|
|9.65||497||496||— 1||474||— 23||510||+ 13|
|9.63||500||484||— 16||462||— 38||497||— 3|
|Av. = — 2.3||Av. = — 24.5||Av. = + 6.2|
It will be seen that the figures for fineness computed from specific gravity by means of an empirical formula which he employed agree rather well throughout the whole series with the actual figures obtained by fire assay. This formula, which was really derived from his own experimental data rather than from theoretical considerations, is as follows:
Fineness = L —8.833/0.0016474,
Where L is the observed specific gravity of a coin.
However, this empirical formula cannot be applied with such satisfactory results to the estimation of the fineness of coins composed of pure silver or nearly pure silver. Karmarsch found that coins struck from pure silver had a specific gravity of 10.547. When this figure is substituted in his formula the fineness is found to be 1040, or 40 degrees too high. Likewise, coins with a fineness of 994 were found to have a specific gravity of 10.537, and this gives a fineness of 1034 by his formula, or again 40 degrees above the actual fineness. At the other extreme of the range of composition of silver-copper alloys his formula leads to even higher positive errors. He found that coins struck from pure copper had a specific gravity of 8.956, which by his formula gives a fineness of 75 instead of 0. Hence the empirical formula of Karmarsch gives results of satisfactory accuracy only in a certain range of composition. On the other hand, the theoretical formula, on the basis of the specific gravities he found for pure silver and pure copper coins, gives very close results at the extreme ranges of composition but much poorer results, as is shown by the fifth and sixth columns of Table XXVI, for coins of intermediate composition. However, fairly satisfactory results for coins of intermediate composition are obtained by the theoretical formula if the specific gravities of silver and of copper are taken to be 10.50 and 8.90, respectively, as is shown by the last two columns of Table XXVI. Regardless of the different results obtained by these different methods of computation, the fineness of modern silver coins may evidently be estimated with satisfactory accuracy from their specific gravities by the use of one formula or another, at least when the coins are of large size, for it is important to note that the results of Karmarsch were obtained mostly on crowns and thalers. It now remains to be seen whether the fineness of ancient coins may be estimated with an equal degree of accuracy by this means.
From the data in Tables XXV and XXVI it is evident that the specific gravity of a coin must be determined accurately through the second decimal place if results of satisfactory accuracy are to be obtained, since each unit in the second decimal place is equal on the average to 6 units of fineness. In practice this is not difficult to do with large ancient coins, such as tetradrachms, but it may be very difficult to do with small ones, such as obols. The most convenient and the usual way to determine the specific gravity of a coin is by the method of Archimedes. In this method the coin is weighed accurately in air and then weighed again while it is suspended in water by means of a fine wire. By subtracting from this second weight the weight of the wire alone suspended in water, the weight of the coin in water is found. The difference between the weight of the coin in air and the weight of the coin in water divided into its weight in air gives the specific gravity of the coin. In very accurate work, corrections are made for the buoyant effect of the air on both the coin and the weights, and for the density of the water at the temperature of weighing, but these corrections are of little importance in the determination of the specific gravity of ancient coins, for, as will be shown, other sources of error greatly overshadow these small ones. Though the weight of the coin in air may be determined with a high degree of accuracy by means of a good balance and weights, this is not true for the weight of the coin in water because the surface film of water clings to the suspension wire in the process of weighing and so prevents the balance beam from swinging freely. This occurs, of course, both while the coin is being weighed in water and while the weight of the suspension wire alone is being measured. The effect of this is to introduce an uncertainty into the weight of the coin in water. In spite of all refinements, such as the use of a very fine suspension wire and the addition of a wetting agent to the water so as to reduce its surface tension, the weight of the coin in water beyond the third decimal place in grams is very uncertain, and usually there is an uncertainty of one unit in the third decimal place. In other words, there is usually an uncertainty of one milligram in one direction or the other in the weight of the coin in water. This may seem a small error, but actually it may have a considerable effect on the computed specific gravity of the coin, especially for small coins, since the relative magnitude of the effect increases as the weight of the coin decreases. This is illustrated by the examples given in Table XXVII. These hypothetical examples show for ideal silver coins of three common Greek denominations the effect of a weighing error of one milligram in one direction or the other on the computed specific gravity and corresponding fineness. Here the fineness was calculated from the specific gravity figures by the theoretical formula, taking 10.50 as the specific gravity of silver and 8.90 as the specific gravity of copper. In these examples it is assumed that the correct specific gravity is 10.00 and that the correct fineness is 722. Obviously, when a coin is as small as an obol the error from this source is so great that any attempt to determine its specific gravity by the method of Archimedes and its corresponding fineness may give very inaccurate results. For such very small coins it is better to use some alternate method of determining specific gravity, such as a method involving the use of a special pyknometer or weighing bottle. Though this will eliminate the error
|Denomination||Weight in Air Grams||Weight in Water Grams||Loss of Weight Grams||Specific Gravity||Fineness|
An important source of error in the estimation of the fineness of ancient silver coins from specific gravity measurements arises from the presence of corrosion products in or on the coins. Since such corrosion products have a much lower specific gravity than the metals from which they are formed, that of silver chloride, for example, being only about 5.5, their presence will cause the observed specific gravity of a coin to be lower than it should be, and, of course, the computed fineness also to be lower. The importance of this as a possible source of serious error is indicated by the data given in Table XXVIII. The experimental data upon which this table is based were obtained over a century ago by Brüel 14 when he analyzed a miscellaneous series of ancient silver coins and also determined their specific gravities. These coins were not any more closely identified than is shown in the note to the table. Fortunately, this analyst not only selected somewhat corroded coins for analysis but determined the proportion of the main corrosion product, silver chloride. Hence, his experimental data, obtained over a century ago, now becomes useful for the purpose of showing to what extent corrosion products vitiate estimations of fineness based on specific gravity. It will be seen that the fineness computed either by the formula of Karmarsch or by the theoretical formula agrees satisfactorily with the fineness determined by chemical analysis for only the first two coins, those of highest fineness, and that for all the others there is no agreement at all. In general, the discrepancy increases with decrease in fineness, though the relationship between the two is very approximate and erratic. Although the errors are generally less in the six coins that contain the lowest proportions of silver chloride than in the five that contain the highest proportions, large irregularities and inconsistencies occur within these groups. For example, No. 5 contains less silver chloride than No. 1, yet the error is much greater, and No. 8 contains much less than No. 7, yet the error is about the same. Though exact calculations of the effect of the presence of the various proportions of silver chloride on the specific gravities of these coinage alloys are not possible because of uncertainties about the accuracy of the determinations of the minor components of these alloys, approximate calculations show clearly enough that the presence of such proportions of silver chloride is not in itself sufficient to account completely for the observed low specific gravities.
|Coin No.||Specific Gravity||Fineness by Chemical Analysis||Fineness by Formula of Karmarsch||Difference Error||Fineness by Theoretical Formula||Difference Error||Silver Chloride Content %|
|2||10.43||980||969||— 11||965||— 15||0.31|
|3||10.12||925||781||— 144||790||— 135||0.76|
|4||9.85||835||617||— 218||635||— 200||0.54|
|5||9.74||799||551||— 248||565||— 234||0.40|
|6||9.63||900||484||— 416||495||— 405||0.63|
|7||9.57||876||447||— 429||460||— 416||5.77|
|8||9.52||859||417||— 442||425||— 434||1.86|
|9||9.50||765||405||— 360||415||— 350||6.21|
|10||9.46||854||381||— 473||390||— 464||8.48|
|11||9.02||763||114||— 649||85||— 678||13.04|
Notes to Table XXVIII
These coins may be attributed only to the following extent:
|1. Denarius of Tiberius.||Wt. = 3.24 grams.|
|2. Roman Republican Denarius.||Wt. = 3.10 grams.|
|3. Denarius of Domitian.||Wt. = 2.85 grams.|
|4. Denarius of Vespasian.||Wt. = 2.51 grams.|
|5. Denarius of Faustina.||Wt. = 2.53 grams.|
|6. Denarius of Vespasian.||Wt. = 2.43 grams.|
|7. Didrachm of Neapolis.||Wt. = 7.07 grams.|
|8. Denarius of Hadrian.||Wt. = 2.89 grams.|
|9. Denarius of Hadrian.||Wt. = 2.66 grams.|
|10. Drachm of Hyela.||Wt. = 3.95 grams.|
|11. Obol of Heraclea.||Wt. = 0.76 grams.|
Two noticeably corroded coins analyzed in the course of this investigation showed even greater discrepancies between the silver content estimated from specific gravity and that determined by analysis. One was a drachm of Mithradates IV that had a specific gravity of 9.12, which corresponds to a theoretical silver fineness of 158. The actual fineness by analysis was 770, so that the discrepancy amounts to 612 degrees of fineness. The cleaned blank of this coin had a specific gravity of 9.56, which corresponds to a theoretical fineness of 453. Still greater discrepancies were found on the examination of a denarius of Vespasian. This coin had a specific gravity of 9.04, which corresponds to a theoretical silver fineness of 102. But the fineness as determined by analysis was 891, so that the discrepancy amounts to 789 degrees of fineness. The cleaned blank of this coin had a specific gravity of 9.10, which corresponds to a fineness of 144, so that even on the blank the discrepancy amounts to 747 degrees of fineness. The difference in the distribution of the corrosion products in these two coins accounts for the fact that the specific gravity of the blank of one of them is much higher than that of the coin itself, whereas with the other there is not much difference. The coin of Mithradates IV was much corroded on the exterior but the blank was not visibly corroded, whereas the coin of Vespasian was visibly corroded throughout. Nevertheless, it was also apparent that the presence of the corrosion products did not in itself account entirely for the very low observed specific gravities of these coins and their blanks.
In general, it is obvious from all these data that the specific gravity of visibly corroded ancient silver coins is not a reliable index of their fineness, except possibly when the observed specific gravity is very high.
In Table XXIX are shown data on the specific gravity and fineness of a group of ancient silver coins in ordinary condition, that is without any appreciable amounts of corrosion products visible on their surfaces. Probably all these coins had been cleaned mechanically or chemically at one time or another.
It will be seen that here again the fineness derived from specific gravity agrees reasonably well with that determined by chemical analysis only for the first two coins, those of highest specific gravity. For two other coins, Nos. 3 and 4, the agreement is perhaps close enough for practical purposes, but for the others the agreement is decidedly poor, though not nearly so poor in general as for the corroded coins of similar lower fineness listed in Table XXVIII.
|Coin No.||Specific Gravity||Fineness by Chemical Analysis||Fineness by Formula of Karmarsch||Difference Error||Fineness by Theoretical Formula||Difference Error|
|1||10.53||987||1030||+ 43||1016||+ 29|
|2||10.33||944||909||— 35||908||— 36|
|3||10.32||964||903||— 61||903||— 61|
|4||10.29||942||884||— 58||886||— 56|
|5||10.26||948||866||— 82||870||— 78|
|6||10.09||906||763||— 143||774||— 132|
|7||10.08||875||756||— 119||768||— 107|
|8||9.93||929||666||— 263||681||— 248|
|9||9.92||733||660||— 73||675||— 58|
|10||9.85||769||617||— 152||633||— 136|
|11||9.75||744||557||— 187||572||— 172|
|12||9.66||743||502||— 241||516||— 227|
|13||9.63||668||484||— 184||497||— 171|
|14||9.59||679||460||— 219||472||— 207|
|15||9.45||522||374||— 148||382||— 140|
|16||9.34||521||308||— 213||309||— 212|
|Av. = —133||Av. = —126|
Key to the Identification, Weight, and Chemical Composition of the Coins Listed in Table XXIX.
1. Drachm of Alexander the Great, Usual Type. Wt. = 4.13 grams. The results of chemical analysis were as follows:
Silver = 98.65%
Gold = 0.37%
Copper = 0.22%
Tin = 0.06%
Lead = 0.72%
Iron = 0.03%
Total = 100.05%
2. Roman Republican Denarius. Obv. Head of Roma r. Rev. ROMA, dioscuri to r. Wt. = 3.25 grams. The results of chemical analysis were as follows:
Silver = 94.43%
Gold = 0.49%
Copper = 4.42%
Tin = 0.17%
Lead = 0.39%
Iron = 0.07%
Total = 99.97%
3. Persian Siglos, Usual Type. Wt. = 5.53 grams. The results of chemical analysis were as follows:
Silver = 96.38%
Gold = 0.10%
Copper = 2.67%
Tin = none
Lead = 0.82%
Iron = 0.03%
Total = 100.00%
4. Drachm of Mithradates I. Wt. = 3.81 grams. Analysis No. 1 in Table II.
Silver = 94.79%
Gold = 0.54%
Copper = 4.24%
Tin = 0.02%
Lead = 0.23%
Iron = none
Nickel = trace
Zinc = none
Total = 99.82%
6. Drachm of Phraates III (?). Wt. = 3.96 grams. Analysis No. 4 in Table II.
7. Tetradrachm of Ptolemy X. Obv. Head of Ptolemy to r. Rev. ΠTOΛEMAIOY BAΣIΛEΩΣ Standing eagle 1. LI and ΠA tol. and r.Wt. = 13.31 grams. The results of chemical analysis were as follows:
Silver = 87.49%
Gold = 0.39%
Copper = 10.24%
Tin = trace
Lead = 1.46%
Iron = 0.04%
Total = 99.62%
8. Drachm of Mithradates I. Wt. = 3.29 grams.
Analysis No. 2 in Table II.
9. Drachm of Volagases II. Wt. = 3.74 grams.
Analysis No. 20 in Table II.
10. Drachm of Gotarzes. Wt. = 3.65 grams..
Analysis No. 18 in Table II.
11. Drachm of Orodes I. Wt. = 3.98 grams.
Analysis No. 7 in Table II.
12. Drachm of Vardanes I. Wt. = 3.65 grams.
Analysis No. 19 in Table II.
13. Drachm of Orodes I. Wt. = 3.78 grams.
Analysis No. 10 in Table II.
14. Drachm of Sinatruces. Wt. = 3.92 grams.
Analysis No. 3 in Table II.
15. Tetradrachm of Phraates IV. Wt. = 14.34 grams.
Analysis No. 1 in Table VII.
16. Drachm of Volagases V. Wt. = 3.32 grams.
Analysis No. 22 in Table II.
The abnormally high specific gravity of No. 1 of Table XXIX and the corresponding derived fineness of over 1000 is evidently due to the presence of sufficient proportions of gold and lead, both of higher specific gravity than silver itself. The effect of their presence on the specific gravity of the coin may be computed from the analytical figures if the assumption is made that no change in volume occurred when the component metals were alloyed. By dividing each percentage by the corresponding specific gravity, the volume occupied by a given metal in a hundred grams of alloy is found. Then by adding these volumes together and dividing into this weight there is obtained what may be called the theoretical specific gravity of the coinage alloy. The details of this computation are as follows:
For silver, 98.65 / 10.5 = 9.395 cc.
For gold, 0.37 / 19.3 = 0.019 cc.
For copper, 0.22 / 8.9 = 0.025 cc.
For tin, = 0.06 / 7.3 = 0.008 cc.
For lead, 0.72 / 11.3 = 0.064 cc.
For iron, 0.03 / 7.9 = 0.004 cc.
Total = 9.515 cc.
100.05 / 9.515 = theoretical specific gravity.
Instead of using 100.05, the actual summation of the analytical figures, for this computation, it is perhaps better logically to prorate the analytical figures to a summation of exactly 100.00, though if this is done only the figure for silver is affected and the final division, 100.00 / 9.50 = 10.52, gives the same result. It will be seen that this result agrees very well with the observed specific gravity of 10.53, and this high figure is thus satisfactorily explained.
In Table XXX are shown data on the specific gravity and fineness of the cleaned blanks of the coins listed in Table XXIX. It will be seen that the agreement between the fineness estimated from specific gravity and that determined by chemical analysis is generally very much closer for these blanks than for the coins themselves. This shows that the metal on the surface of such coins must be abnormally low in specific gravity, probably because it is more or less porous. Such porosity is probably caused by the superficial corrosion of the metal followed by the leaching out of the products of corrosion either naturally or in the process of cleaning. The greater lack of agreement between the fineness estimated from specific gravity and that found by chemical analysis for the baser coins, as contrasted to those composed of nearly fine silver, is in accordance with this explanation because the surface of such coins is more likely to be corroded under natural conditions.
The specific gravity of the metal removed from the surface of a coin in preparing a blank for analysis may actually be computed from the specific gravity of the coin, the specific gravity of the blank, the weight of the coin, and the weight of the blank. The volumes of the coin and the blank are found by dividing the respective weights by the respective specific gravities, and the difference of the two volumes is the volume of the metal removed. The weight of the metal removed
|Coin No.||Specific Gravity||Fineness by Chemical Analysis||Fineness by Formula of Karmarsch||Difference Error||Fineness by Theoretical Formula||Difference Error|
|1||10.56||987||1046||+ 59||1032||+ 45|
|2||10.41||944||957||+ 13||952||+ 8|
|3||10.32||964||903||— 61||903||— 61|
|4||10.35||942||921||— 21||919||— 23|
|5||10.32||948||903||— 45||903||— 45|
|6||10.18||906||818||— 88||825||— 81|
|7||10.22||875||842||— 33||848||— 27|
|8||10.10||929||769||— 160||780||— 149|
|9||10.09||733||763||+ 30||774||+ 41|
|10||10.05||769||739||— 30||751||— 18|
|11||10.01||744||714||— 30||728||— 16|
|12||10.06||743||745||+ 2||757||+ 14|
|13||9.86||668||623||— 45||639||— 29|
|14||9.85||679||617||— 62||633||— 46|
|15||9.68||522||514||— 8||529||+ 7|
|16||9.63||521||484||— 37||497||— 24|
|Av. = — 32||Av. = — 25|
Sr = Wc — Wb / Wc / Sc — Wb / Sb
Where, Sr is the specific gravity of the metal removed
Sc is the specific gravity of the coin
Sb is the specific gravity of the blank
Wc is the weight of the coin in grams
Wb is the weight of the blank in grams
The specific gravity thus computed is, of course, only an average figure, as the metal removed is in all probability not homogeneous. It may consist of solid metal, but usually it is metal mixed with corrosion products, metal that is porous, or metal that is both porous and mixed with corrosion products. Furthermore, it may differ in specific gravity with distance from the surface of the coin.
The data necessary for the computation and the results of computation by the above formula for the coins listed in Table XXIX are shown in Table XXXI. It will be seen that the average specific gravity of the metal removed from each of the coins is, with a single exception, lower than that of the corresponding coin or blank. In No. 3 it is slightly higher, but this is caused by the unusually small
|Coin No.||Weight of Coin Grams||Weight of Blank Grams||Specific Gravity of Coin||Specific Gravity of Blank||Average Specific Gravity of Metal Removed|
In general, therefore, it is obvious that the specific gravity of ancient silver coins in ordinary condition is not a reliable index of their fineness, except when the observed specific gravity is very high. The data in Tables XXIX and XXX also show that for such coins the fineness computed by the theoretical formula leads to noticeably closer results than when computed by the empirical formula of Karmarsch.
Some surprising results are obtained when an attempt is made to estimate the fineness of ancient billon coins from specific gravity measurements. Very often the specific gravity of a given coin is much below that of pure copper, so that the indication is that the coin contains no silver at all. Examples are shown in Table XXXII. Even the specific gravity of the blanks of most of these coins is below that of
|No.||Specific Gravity of Whole Coin||Specific Gravity of Blank||Fineness|
Key to Identification, Weight, and Chemical Composition of Coins Listed in Table XXXII
Silver = 15.53%
Copper = 81.00%
Tin = 1.74%
Lead = 0.13%
Iron = 0.08%
Total = 98.48%
Silver = 22.53%
Copper = 75.29%
Tin = 0.46%
Lead = 0.20%
Iron = 0.05%
Total = 98.53%
pure copper, which shows that the porosity due to corrosion was much more than superficial. Especially striking are the low figures for the specific gravities of the coin and blank of the last example in the table. From its composition as found by analysis, the specific gravity of this coin or its blank, if it were of solid metal, should theoretically be about 9.22. If the observed very low figures are due entirely to porosity, the figure of 5.60 for the coin means that on the average about 39% of its volume consisted of empty cavities. The corresponding figure for the blank is about 26%. Of course, the observed very low figures may not be entirely the result of empty cavities, but may be caused in part by cavities partly or entirely filled with corrosion products. The presence of some corrosion products is indicated by the failure of the sum of the metals to add up to 100.00%. The difference between this figure and the actual summation of 98.53% is certainly in large part due to the presence of chlorine, oxygen, or other undetermined non-metals. 15 However, since the specific gravity of certain of the more likely corrosion products approaches or exceeds the observed figures, that of silver chloride being about 5.5 and that of cuprous oxide being about 6.0, for example, it is improbable that more than a small proportion of such partly or completely filled cavities could have been present in this coin or its blank.
In Table XXXIII are shown, for the blank of this coin and the blanks of the other coins listed in Table XXXII, data on the approximate percentages of non-metals, as indicated by the deficiencies in the summation of the metallic components, the specific gravities, both observed and theoretical, their differences, and the estimated apparent porosity for each blank, which may be defined as the percentage of minute cavities or pores by volume, on the assumption that these are empty. This is computed by dividing the difference between the observed and theoretical specific gravity by the theoretical specific gravity and multiplying by 100. These data are based on the blanks rather than on the coins themselves since the purpose is to show to what extent porosity may exist deep within the body of such coins, and furthermore, the calculations are more valid for the blanks because the analytical figures were obtained on them.
It will be seen that the observed and theoretical specific gravities of No. 1 in Table XXXIII are almost the same, which indicates that the blank of this coin was not porous. The low specific gravity of the coin itself must be ascribed to porous metal on its surface. Nos. 2 and 3 were slightly porous, but the minute cavities of No. 2 were probably more or less filled with corrosion products, whereas those of No. 3 must have been largely empty. Nos. 5, 6, and 7 were much more porous than Nos. 2 and 3 and contained much more corrosion products. The rather small figures for the deficiencies in summation shown in the table represent much higher percentages of corrosion products. Silver chloride and cuprous oxide have been observed to be the principal products of corrosion included in the metal of coins of this sort. If silver chloride is the only one present, a deficiency in summation of 1% represents about 4% of this compound, and if cuprous oxide is the only one, the same deficiency in summation represents about 9%. Generally both products are present in various proportions. If present in equal proportions, 1% deficiency represents about 6.5% of corrosion products, and 6.5 may be used as a rough factor to convert percentage of deficiency in summation into approximate percentage of corrosion products. It is obvious, therefore, that
|No.||Deficiency in Summation %||Observed Specific Gravity||Theoretical Specific Gravity||Difference in Specific Gravity||Apparent Porosity %|
The computed specific gravity of the metal removed from each of these tetradrachms in preparing the blanks is shown in Table XXXIV. As will be seen, it is always much lower than that of the corresponding coin or blank. These specific gravities are all more than one unit low and some are over three units low. Those for Nos. 4 and 8, both Alexandrian tetradrachms, are surprisingly low. In general, these results show that the surface metal of all these base tetradrachms was very porous.
|Coin No.||Weight of Coin Grams||Weight of Blank Grams||Specific Gravity of Coin||Specific Gravity of Blank||Average Specific Gravity of Metal Removed|
Another interesting example of very low specific gravity was observed in a tetradrachm of Ptolemy X, though this coin had a considerably higher silver content than No. 8 of Table XXXII. The results of chemical analysis were as follows:
Silver = 50.99%
Gold = 0.23%
Copper = 42.20%
Tin = 1.56%
Lead = 1.30%
Iron = 0.05%
Nickel = 0.03%
Zinc = 0.10%
Total = 96.46%
The specific gravity of this tetradrachm was only 5.68, and its apparent porosity was about 41%, or more than that of No. 8 of Table XXXII. When the attempt was made to determine the specific gravity of the blank of this coin in the usual way, an interesting and significant phenomenon was encountered in the form of a slow increase in its apparent weight while it was suspended in water. The weight observed when the coin was first suspended in the water was 6.082 grams, but after a few minutes the apparent weight was noticeably greater, and several such successive weighings were made in the hope that equilibrium would soon be reached. When this did not occur, both the observed weights and the times of weighing were recorded. Some results of these timed weighings are shown in Table XXXV. As there indicated, final equilibrium was not reached until over 24 hours had passed. This gradual increase of weight was evidently due to the slow seepage of water into the pores of the metal.
|Consecutive Weighing||Elapsed Time Minutes||Observed Weight of Blank and Wire in Water Grams||Cumulative Gain in Weight Grams|
To accelerate this process, the vessel of water containing the blank was subjected to reduced pressure between the fifth and sixth weighings listed in the table. This served to remove air from the pores of the metal and to allow the water to penetrate faster. After the seventh weighing the blank was allowed to stand overnight in the vessel of water before the weighings were resumed. The specific gravity of the blank based on the first observed weight and the weight of the blank in air before immersion in water was 5.85. Its specific gravity based on the final weight at equilibrium and this same weight of the blank was 7.76. However, when the blank was dried after immersion in water it was found to weigh 7.047 grams as contrasted to its weigh of 7.109 grams before immersion. In the actual experiment the blank was removed from the water and allowed to stand exposed to the air at room temperature for 15 hours, at the end of which time its weight was found to be 7.050 grams. Then it was dried for an hour in an oven at 120° C. and on cooling was found to weigh 7.047 grams, which an additional drying in the oven for an hour did not change appreciably. The difference of 0.062 gram between the weight of the dry blank before and after immersion must be ascribed to a small amount of soluble corrosion products or other soluble material leached from the metal by the water. The specific gravity of the blank based on its first observed weight in water and the weight of the leached and dried blank was 6.11, which is perhaps the figure to be compared with the specific gravities of the blanks of the other tetradrachms, for this represents the apparent specific gravity of the blank before water had penetrated appreciably into the pores of the metal. Its specific gravity based on its final weight in water at equilibrium and the same weight of the blank was 8.25. Thus, several quite different figures for the specific gravity of this blank are possible in accordance with the particular weights selected for computation. The specific gravity 8.25 is nearer to that of the alloy itself, which theoretically should be about 9.68. The figure 6.11 represents an apparent porosity of about 36.9% and the figure 8.25 represents an apparent porosity of about 14.8%. This means that about 71% of the cavities or pores in the metal of the blank were open to penetration by water and about 29% were closed. In terms of the weight of the leached and dried blank, the weight of water absorbed, as measured by its apparent gain in weight on immersion, amounted to 4.26%. An approximate check on this result was obtained by soaking the blank thoroughly in water, allowing it to stand in air until no more water was apparent on its surface, weighing it, drying it completely, and weighing it again. The water thus found amounted to 4.78%. In terms of volume the percentage of water absorbed is much higher. From the gain in apparent weight in water, the volume of water absorbed was 0.299 cc., and the external volume of the blank based on a specific gravity of 6.11 was 1.153 cc., so that the volume of water absorbed amounted to 25.9% of that of the blank. In other words, the apparently solid metal of the blank absorbed about a fourth of its own volume of water. This result agrees fairly well with the figure on the change in apparent porosity on absorption of water. All these results serve to demonstrate the reality of the existence of cavities or pores in the metal of a sufficiently corroded ancient billon coin. In this particular coin an extensive system of connecting interal cavities or pores was evidently opened to penetration by water when the surface metal was filed off. Water was not absorbed when the coin itself was immersed in water, apparently because the pores were closed at the surface of the coin, and this may have been a result of polishing. However, the metal on the surface was very porous, as computation on the basis of 5.68 for the specific gravity of the coin and 6.11 for the specific gravity of the blank gave a specific gravity of 3.70 for the metal removed in the preparation of the blank.
As shown by the preceding examples, ancient billon coins have often undergone extensive internal corrosion with the formation of cavities and pores that have greatly reduced their original weight and specific gravity. Undoubtedly, the same coins have also undergone considerable external corrosion with the loss of metal from the surface, but measurements of billon coins in various states of preservation indicate that the diameter and thickness of severely corroded coins, after removal of crusts and layers of external corrosion products, are not much different from what they were originally. In other words, the volume of such coins has not been much altered in spite of a very considerable reduction in weight. Therefore, it should be possible to estimate, at least approximately, the original weight of a cleaned coin of this sort from its present weight, its present specific gravity, and its theoretical specific gravity by means of the following formula:
Original Weight = Wp × St / Sp
Where, Wp is the present weight
Sp is the present specific gravity
St is the theoretical specific gravity.
Results of a computation of the original weights, by this formula, of the coins listed in Table XXXII and of the one coin of Ptolemy X just discussed, are shown in Table XXXVI. These estimated original weights are in approximate agreement with the known weights of the same types of coins in a fine state of preservation.
It might be supposed that this formula would not be of much practical use to the numismatist for estimating the original weight of a billon coin that is abnormally light in weight from internal corrosion, since the theoretical specific gravity required by the formula is derived in these examples from chemical analyses which involved destruction of the coins. However, no exact figures for the fineness and corresponding theoretical specific gravity are needed in order to estimate the original weight of such a coin with sufficient accuracy for most purposes, for, as shown in Table XXXVII, considerable
|No.||Observed Specific Gravity||Theoretical Specific Gravity||Present Weight Grams||Original Weight Grams|
|Observed Specific Gravity||Present Weight Grams||Theoretical Specific Gravity||Corresponding Fineness||Estimated Original Weight Grams|
Even though specific gravity measurements are not generally reliable for estimating the fineness of ancient billon coins, especially those that are corroded internally, they evidently may be very useful for obtaining other kinds of information, as has been demonstrated by some examples.
The fineness estimated from specific gravity is generally much closer to that determined by chemical analysis for electrolytically
|Coin No.||Specific Gravity||Silver Fineness Computed From Specific Gravity||Silver Fineness By Chemical Analysis||Difference In Fineness||Corrected Fineness From Specific Gravity||Corrected Difference In Fineness|
|1||10.00||722||748||— 26||772||+ 24|
|2||9.91||669||698||— 29||719||+ 21|
|3||9.86||639||742||— 103||689||— 53|
|4||9.75||572||652||— 80||622||— 30|
|5||9.73||560||582||— 22||610||+ 28|
|6||9.53||434||510||— 76||484||— 26|
|7||9.48||402||473||— 71||452||— 21|
|8||9.45||382||464||— 82||432||— 32|
|9||9.42||362||418||— 56||412||— 6|
|10||9.38||336||431||— 95||386||— 45|
|Av. = — 64||Av. = —16|
In Table XXXIX are shown the results obtained on the blanks of the same coins. Here it will be seen that the difference errors between the fineness estimated from specific gravity and that determined by
|Blank No.||Specific Gravity||Silver Fineness Computed From Specific Gravity||Silver Fineness By Chemical Analysis||Difference In Fineness|
|Av. = +24|
|Blank No.||Observed Specific Gravity||Theoretical Specific Gravity||Difference||Actual Fineness||Computed Fineness||Difference|
|1||10.07||10.09||+ 0.02||748||774||+ 26|
|2||9.97||9.99||+ 0.02||698||716||+ 18|
|3||10.06||10.08||+ 0.02||742||768||+ 26|
|4||9.92||9.90||— 0.02||652||663||+ 11|
|5||9.86||9.83||— 0.03||582||621||+ 39|
|6||9.78||9.67||— 0.11||510||523||+ 13|
|7||9.64||9.60||— 0.04||473||479||+ 6|
|8||9.56||9.52||— 0.04||464||430||— 34|
|9||9.52||9.50||— 0.02||418||415||— 3|
|10||9.57||9.52||— 0.05||431||430||— 1|
|Av. = — 0.03||Av. = + 10|
In Table XLI are shown the results of calculations of the specific gravity of the metal removed from the coins in preparing the blanks. By comparing these results with those given in Table XXXI it will be seen that the specific gravity of the surface metal of these electrolytically cleaned coins was higher and more uniform generally. In only 30% of these coins does the specific gravity of this metal fall below 9.00, whereas the proportion for those in ordinary condition is 50%. Moreover, in none of the electrolytically cleaned coins does it fall below 8.00. Still more significant, however, are the differences in the ratios of the specific gravity of the metal removed to that of the corresponding blank for the coins of the two lots. It is evident that this is generally lower for the coins of Table XXXI than for those of Table XLI. The actual average figures are 0.865 and 0.930, respectively. All this shows that the surface metal of these electrolytically cleaned coins was much less porous.
In general, the results of these experiments indicate that the specific gravity of the surface metal of silver coins cleaned by electrolytic reduction is generally higher than that of untreated coins or those cleaned by other methods. The reason for this appears to be that some of the cavities or pores in the metal on or near the surface are filled or partly filled with new metal derived from the reduction of the corrosion products on the surface of the coin. Since porosity of the surface metal is the chief cause of error in estimating the fineness of ancient silver coins by means of specific gravity, this explains why better results are usually obtained on coins that have been cleaned electrolytically.
From the average diameter, the weights of the coin and the blank, and the corresponding specific gravities, it was possible to calculate
|Coin No.||Weight of Coin Grams||Weight of Blank Grams||Specific Gravity of Coin||Specific Gravity of Blank||Average Specific Gravity of Metal Removed|
|Group||Coin No.||Average Thickness of Metal Removed mm.||Specific Gravity of Metal Removed||Ratio of Specific Gravity of Metal Removed to That of Blank|
|Av. 0.08||Av. = 8.69||Av. 0.878|
|Av. = 0.29||Av. = 9.40||Av. = 0.966|
|10||Ondrouch, V., Der römische Denarfund von Vyškovce aus der Frühkaiserzeit (Bratislava, 1934), p. 11.|
|11||Karmarsch, K., Dinglers polytechnisches Journal, CCIV (1877), pp. 565–573.|
|12||Mellor, J. W., A Comprehensive Treatise on Inorganic and Theoretical Chemistry (London, 1923), III, p. 323.|
|13||Karmarsch, K., Dinglers polytechnisches Journal, CCIV (1877), pp. 565–573.|
|14||Reported in Journal für praktische Chemie, XXX (1843), pp. 334–342.|
|15||In general, the difference between the actual summation of an analysis and the ideal summation of 100.00% is due either to the presence of undetermined components or to experimental error. In this analysis there were no undetermined metals in appreciable proportion, and the experimental error was probably very small since the summations of two separate careful analyses came to 98.52% and 98.53%.|
|16||Technical Studies in the Field of the Fine Arts, III (1935), pp. 123–132.|
The rather satisfactory agreement (Table XXXVIII) between the corrected fineness estimated from specific gravity and that found by chemical analysis for the 10 electrolytically cleaned drachms of Orodes I from the hoard indicated that the specific gravity measurements that were made on the remaining 134 coins of the part of the hoard that was available should be a fairly reliable index of their fineness, for all these had also been cleaned in the same way. Even if incorrect results were thus obtained on a few individual coins, the results as a whole should be valid because of the considerable number of coins measured. The observed weights and specific gravities are listed in Table XLIII in decreasing order of specific gravity and fineness. The 10 coins that were analyzed are also included in this tabulation, and are indicated by asterisks. As with the coins that were analyzed, the theoretical fineness calculated from the observed specific gravity was arbitrarily raised 50 degrees for each coin in order to obtain the estimated actual fineness. Furthermore, instead of giving the fineness figures to the nearest unit as calculated, they were all rounded off to the nearest 5 degrees, for the accuracy of the method is certainly no better than this, and the appearance of fictitious accuracy is thus avoided. Actually, however, even if these figures had neither been corrected nor rounded off, the obvious conclusions about the relative fineness and range of fineness in this group of coins would have been very nearly the same.
As is shown in Table XLIV, the averages of all the results for the weight, fineness, and silver content in Table XLIII are in fairly close agreement with those for the 13 drachms of Orodes I that were analyzed chemically. They do not agree so well with those of the 10 from the hoard that were analyzed, but this is because these 10 coins were not truly representative of the large group of coins from the hoard, for it is evident that a disproportionate number of coins of low fineness happened to be selected for analysis. This selection was not entirely accidental, as relatively poor coins were naturally chosen for
|Serial No.||Type||Weight Grams||Specific Gravity||Fineness by Theoretical Formula||Estimated Actual Fineness||Silver Content Grams|
|123||E or F||3.92||9.56||455||505||1.98|
|Measurement||The 13 Coins That Were Analyzed||The 10 Coins From the Hoard That Were Analyzed||All the Coins From the Hoard|
|Average Weight, Grams||3.79||3.76||3.84|
|Average Silver Content, Grams||2.32||2.17||2.38|
|Range in Fineness||338||330||445|
As might be expected, the range of fineness in the debased drachms of Orodes I found by the examination of all the specimens in the large group from the hoard is considerably greater than that found by the chemical analysis of all 13 specimens or the 10 from the hoard. This shows the importance of examining as large a number of coins of a given type as possible in order to find the entire range of variation in fineness, and also the importance of a method that will make possible the estimation of the fineness of a very large number of coins of a given type, or all the coins in a hoard, without destroying more than a few specimens by assay or chemical analysis.
The designation of the coins in Table XLIII by type is in accordance with the classification in B. M. C. Parthia, which is based on the nature and number of the adjunct symbols that appear in the field on the obverse of the drachms ascribed to Orodes I. Nearly all the coins in this portion of the hoard are of Type A, B, or C, and it is not unlikely that the same was true of the hoard as a whole. Only 3 coins are of other types, and 6 coins could not be certainly identified as to type because they were struck off center. There are 63 coins of Type A, 37 of Type B, and 35 of Type C, so that coins of the first type are predominant with the other two in about equal proportion. A glance at Table XLIII is sufficient to show that the coins of these three principal types differ considerably in degree and range of fineness. Data on the maximum, minimum, and average weight, fineness, and silver content according to type are shown in Table XLV. It will be seen that the maximum, minimum, and average weights of the coins of the three types are in the descending order, A, B, C. However, both the maximum and average figures differ so little that it seems doubtful that they are significant from the standpoint of weight standards. In general, low weight is associated with low fineness and in the group of coins as a whole regardless of type there is a good correlation between fineness and weight, as is shown in Table XLVI. This suggests strongly that the coins of all three types were issued on the same weight standard and that they lost weight to different degrees by corrosion because of differences in fineness. Furthermore, the differences in weight are small as compared to the differences in fineness. All this shows that these coins were not debased by lowering the
|Measurement||Type A||Type B||Type C|
|Maximum Weight, Grams||4.06||4.05||4.01|
|Minimum Weight, Grams||3.72||3.57||3.26|
|Average Weight, Grams||3.87||3.83||3.80|
|Maximum Silver Content, Grams||3.28||2.63||2.88|
|Minimum Silver Content, Grams||1.59||1.63||1.34|
|Average Silver Content, Grams||2.64||2.11||2.29|
|Fineness||Average Weight Grams|
|450 and Below||3.65|
The distribution of degree of fineness according to type is shown in Table XLVII for ranges or steps of both 100 degrees and 50 degrees. On Plate I the same data are shown graphically in terms of percentage of coins in each range of 50 degrees. Although it is not possible to treat these data by any strict statistical method because the numbers of the units of each type and in each category are too small, certain definite qualitative conclusions may certainly be drawn. It will be seen that a much higher proportion of the coin of Type A are in the higher ranges of fineness as compared to those of either Type B or Type C, and that, conversely, much higher proportions of the coins of these other two types are in the lower ranges. Nearly 40% of the coins of Type A are above 700 fine, none of Type B, and only about 17% of Type C. On the other hand, only about 3% of those of Type A are 500 fine or less, as contrasted to 27% of Type B and about 17% of Type C. However, there is considerable overlapping in the distribution of fineness, for over 50% of the coins of each of these types are between 700 and 500 fine. The median fineness is 695 for Type A, 555 for Type B, and 610 for Type C. There are also distinct differences in both the range and the pattern of the distribution of fineness. The range for Types A and C is about the same, but that of Type B is smaller, and the fineness of the coins of Type B is distributed in a much more regular manner.
The distribution of the silver content of the coins according to type is shown in Table XLVIII for ranges or steps of 0.40 gram and 0.20 gram. On Plate II the same data are shown graphically in terms of percentage of coins in each range of 0.20 gram. As might be expected, the distribution of silver content follows the same general pattern as the distribution of fineness, except that the pattern of the distribution of silver content is more regular for Type A but more irregular for Types B and C. The median silver content is 2.66 grams for Type A, 2.13 grams for Type B, and 2.26 grams for Type C.
However, the figures in Table XLVIII and the percentage distribution shown on Plate II are based on the present weights of the coins,
|Fineness||Number in Range|
|Type A||Type B||Type C|
|Silver Content||Number in Range|
|Grams||Type A||Type B||Type C|
Even more significant, perhaps, are the similar differences in the fineness and silver content of drachms of the three types with the same monogram or mintmark. In Table L are shown figures for the fineness and silver content of drachms of these types with the monogram , and in Table LI are shown the figures for those with the monogram . It will be seen that here again the drachms of Type A have the highest maximum and average fineness and the highest maximum and average silver content, and that those of Type C are next in order, with those of Type B last. However, the average fineness and silver content of the drachms of Types A and C with the monogram are so close that the differences may not be significant. The drachms of Type C have the highest minimum fineness, with those of Type A next, and those of Type B last. The range of fineness and silver content of the drachms of Type A is the greatest, with those of Type B next in order, and those of Type C last. It might be supposed that the different ranges shown in Table L are simply the result of the different number of coins of each type, but this does not seem to be true, for the order is the same in Table LI where the numbers are nearly the same. Drachms bearing other monograms or
|Silver Content||Number in Range|
|Grams||Type A||Type B||Type C|
To what extent the observed differences in fineness and silver content among the three principal types of drachms from the hoard are significant from the numismatic standpoint depends largely on whether the coins of the lot studied in this investigation are truly representative of the fineness of the coins of these three types in the entire hoard, whether those in the hoard were truly representative of the fineness of those in circulation, and whether the coins of the three types available to the hoarder were truly representative of the whole issue of these three types, or at least the issue up to the time the hoard was completed. It is known that the part of the hoard purchased by Dr. J. Christy Wilson contained a representative selection of the coins of the hoard, that most of this part was acquired by the
|Measurement||Type A||Type B||Type C|
|Number of Coins in Group||36||22||7|
|Range in Fineness||345||220||175|
|Maximum Silver Content, Grams||3.07||2.63||2.88|
|Minimum Silver Content, Grams||1.90||1.72||2.26|
|Average Silver Content, Grams||2.61||2.10||2.56|
|Range in Silver Content, Grams||1.17||0.91||0.62|
References in B. M. C. Parthia
|Measurement||Type A||Type B||Type C|
|Number of Coins in Group||8||6||6|
|Range in Fineness||280||190||120|
|Maximum Silver Content, Grams||3.28||2.43||2.91|
|Minimum Silver Content, Grams||2.04||1.64||2.22|
|Average Silver Content, Grams||2.80||1.99||2.58|
|Range in Silver Content, Grams||1.24||0.79||0.69|
References in B. M. C. Parthia
Princeton University Library, and that the 135 coins of the three types studied in this investigation constituted the major part of the lot at Princeton. However, 31 of the finest specimens of all types of the drachms of Orodes I were placed in the collection there and were not included in the present investigation. Because of their fine state of preservation, these selected coins are probably higher in fineness on the average than the remainder classed as duplicates. However, not all of these selected coins were of Types A, B, and C, and since the number of coins of these types thus excluded is small compared to the 135 that were examined, it is likely that the results here obtained would have been little changed if they had been included. Possibly the ranges of fineness and silver content would have been slightly extended upwards for each type, with a corresponding slight increase in the average fineness and silver content, but it is doubtful that there would have been any appreciable change in the relative fineness and silver content of the coins of the three types. On the whole, therefore, it is highly probable that the lot here studied is fairly representative of these coins in the hoard. Whether those in the hoard represent the entire range of fineness and the true average fineness of any or all these types is quite uncertain for a variety of reasons. The period during which the hoard was assembled may have coincided with the entire period of the issue of one of these types and not of the other two, or of two of them and not the remaining one. Moreover, the coins may have been collected at irregular intervals, that is, many more at one time than at another, although the rather regular percentage distribution of the fineness and corrected silver content of the coins of the three types seems to indicate the contrary. It is also possible that the hoarder preferentially selected coins of one or two of these types, so that their relative numbers in the lot here investigated, and hence in the hoard itself, bear no relationship to the relative abundance of the types available to the hoarder, or to the abundance of these types in general. However, selection on this basis does not seem probable. That there was any selection on the basis of fineness is very improbable, since the new or relatively new coins coming into the hands of the hoarder would have had the same superficial appearance regardless of differences in fineness. On the whole, it seems rather probable that the coins of the large lot here investigated are fairly representative of the relative fineness of the coins of these three types available to the hoarder.
In spite of the uncertainties just discussed, some definite conclusions of numismatic significance may be based on the technical data obtained on the coins of the lot from the hoard. It has already been demonstrated from the chemical analyses that these coins were debased. The specific gravity measurements indicate the same fact, for these show that about 15% of the coins as a whole are composed of billon, or about 3% of Type A, about 27% of Type B, and about 17% of Type C. The chemical analyses show that the coins were deliberately debased, and the wide range of fineness of all these coins and of the coins of each of these three types is also an indication of deliberate debasement, for it is very improbable that such a wide variation in the proportion of silver could have been caused by mere carelessness on the part of the coiners. According to the evidence at present available, ancient coiners in general were able to control the fineness of silver coins within rather narrow limits, and no lack of proper control is indicated for other Parthian issues. The debasement of these coins differs in one important respect from the debasement of other series of ancient silver coins about which we have sufficient information. The debasement of Roman Imperial denarii, Alexandrian tetradrachms, and Parthian tetradrachms followed a slow progressive course that extended over some two centuries, but the debasement of these Parthian drachms of Orodes I obviously occurred in a much shorter time. Since his whole coinage extended over a period of twenty years at the most, and consisted of a considerable number of classes or types that were evidently issued in some sort of systematic chronological order, the period of issue of each class or type must necessarily have been brief, and for some of them it may have been less than a year. This means that some extraordinary circumstances must have caused the severe and very rapid debasement of the drachms of each of the three types here considered.
In general, as is shown by various examples in the history of modern states, the rapid and severe debasement of a coinage is usually caused by the disruption of economic life that accompanies or follows intensive warfare, and there is no reason to suppose that the same cause and effect were not operative in ancient states such as Parthia. It is known that the first part of the reign of Orodes I was a time of great civil strife between Orodes and his brother Mithradates III. Indeed, according to McDowell, 17 the supreme power alternated between the two brothers. At the death of their father Phraates III, Mithradates succeeded to the throne in 57 b.c. but was soon deposed by Orodes. In the next year Mithradates seized the throne but after a short interval Orodes again became the supreme ruler, and, finally, on the death of his brother in 54 b.c. became the sole ruler. In the very next year the first serious military clashes began between the Parthian and Roman empires, for at that time Parthia was invaded by the army of the Roman Proconsul Crassus who was decisively defeated at the battle of Carrhae in 53 b.c. 18 This was soon followed by the Parthian invasion of Syria in 51–50 b.c. Thus the civil wars were followed by foreign wars, and this severe and prolonged warfare may have been in itself a sufficient primary cause of the debasement of the coinage.
The rate of issue of Parthian drachms, in other words the volume of these coins coming into circulation, appears from the available evidence to have been unusually high during the reign of Orodes I. McDowell 19 states that a slow steady increase in rate of issue occurred up to the reign of Phraates III, and that from about 70 b.c. to 38/37 b.c. the rate abruptly increased about threefold. During the reign of Phraates IV (37–3/2 b.c.) the rate dropped back to about what it had been prior to 70 B.C., and after this the rate of issue of drachms remained uniform at a still lower level. McDowell 20 attributes the unusually high rate of issue to greatly increased transit trade between Iran and Central Asia, India, and China. The closer control by Parthia under Orodes I of trade outlets to the Mediterranean may have further increased such commerce and the volume of coinage required. Nevertheless, the military events between 57 b.c. and 50 b.c. may have had the major influence on the volume of coinage required by the economic situation during the reign of Orodes I. It may well have been that the available supply of pure silver simply could not keep pace with the increased demand for coins during his reign. Since there was no reduction of the weight standard for the drachm, the only way by which the demand could then be met would be by the debasement of the coinage silver.
From McDowell's interpretation 21 of the significance of the legends on the reverse of the drachms of Mithradates III and Orodes, from his attribution of certain classes or types to one ruler or the other, from the legends on the drachms of Types A, B, and C of the lot here considered, and from catalogue descriptions of other drachms of these types, it would appear that they all belong to his fourth class for Orodes and were not issued before Orodes became sole ruler in 54 b.c. That the drachms of these three types were issued concurrently to any extent is doubtful in view of the presence of so many of the same mintmarks on the drachms of all three types. Though only a few of the same mintmarks occur in the drachms of all three types in this lot, the listings in various catalogues show clearly enough that a large proportion of the principal or more common mintmarks occur on all three types and that others appear on two of the types. Since the find spot of the hoard, which this lot represents, lies in the far northwestern corner of the region occupied by the Parthian Empire and since the hoard was evidently accumulated during a short period of time, it is hardly to be expected that this lot would contain drachms of all three types from all the numerous mints. The presence of mintmarks common to all three types is therefore indicative of their consecutive issue, for it seems very unlikely that the same mint would issue drachms of different types simultaneously. Nevertheless, because of slowness of communications, or some other cause, there may have been some overlapping of the periods of issue of the drachms of these types at the various mints considered as a whole.
It seems rather probable that the issue of the earliest of these types began in 54 b.c. or very shortly after, for this would seem to be the first issue of drachms after Orodes became sole ruler in that year. When the issue of the latest of the three types terminated is less easy to estimate, as information is lacking on the length of period of issue of any of these types. Possibly the different ranges of fineness or silver content of the drachms of the three types from two of the mints, shown in Tables L and LI, are a clue to the relative lengths of the periods of issue. If debasement occurred at about the same rate for each of these types, a greater range of fineness or silver content would indicate a longer period of issue. On this assumption, the drachms of Type A would appear to have been issued over a longer period than those of either Type B or Type C, and those of Type B over a longer period than those of Type C. The ranges of fineness and silver content of the drachms of the three types in the entire lot, shown by the data in Tables XLV, XLVII, XLVIII, and XLIX, would also appear to indicate that those of Type A were issued over a longer period than those of Types B or C. However, the same data also appear to indicate that those of Type C were issued over a longer period than those of Type B. The larger number (63) of drachms of Type A in the lot may also be indicative of a longer period of issue for the coins of this type, whereas the approximately equal numbers (37 and 35, respectively) of the drachms of Type B and Type C may be indicative of shorter and approximately equal periods of issue. However, these numbers are a valid index of the relative periods of issue only if the rate of issue of the drachms of each type was about equal and if the drachms of the lot are a truly representative sample of the numbers of drachms of these types that were issued. On the whole, it seems probable that the drachms of Type A were issued over a longer period than those of either of the other two types, and that the lengths of the periods of issue of the drachms of Types B and C relative to each other is uncertain. However, the periods of issue of both were probably short, and the total length of the periods of issue of the drachms of these two types may have been about the same as that for the drachms of Type A alone. Since there were still other classes or types of drachms of Orodes I that were evidently issued later in the reign of this ruler, the issue of the latest of the Types A, B, and C must have terminated considerably before the end of his reign. It may have been as early as 50 b.c. or as late as 40 b.c., though some intermediate date such as 45 b.c. is probably nearer the truth.
The order of the issue of Types A, B, and C cannot be established with certainty from the technical data. However, since a higher fineness and silver content is normally associated with an earlier time of issue when debasement occurs during the reign of a ruler, the drachms of Type A would clearly appear, from the data in Tables XLIII, XLV, XLVII, XLVIII, XLIX, L, and LI, to be the first in time of issue. No such clear distinction of order of issue for Types B and C is apparent from these same data. The higher average fineness and silver content of the drachms of Type C seems to be an indication that they were issued before those of Type B. On the other hand, the most debased drachms in the lot are of Type C, and this would appear to indicate the reverse order. The truth may be that the consecutive periods of issue of the drachms of these two types were so brief that no clear distinction as to order of issue should be expected from the data on their fineness or silver content. In general, the technical data tend to support the commonly accepted order.
There are so few examples of the other, and evidently later, types of drachms of Orodes I in this lot from the hoard that nothing certain can be concluded about the average fineness or range of fineness of drachms of these types. To those in Table XLIII definitely identified as to later type should be added at least some of the coins of questionable type, as certain of these were clearly of types other than A, B, or C, even though their exact type could not be more precisely established. As may be seen from this table, all the possible examples of drachms of later types are of medium to low fineness, and that the best one is exceeded in fineness by about 65% of the drachms of Type A. The indication is that the drachms of these types were at least as debased as the drachms of Types B or C, and probably more so. The very small proportion of later types of drachms of Orodes I in the lot, and presumably in the hoard itself, is an indication that the hoard was buried, or at least completed, shortly after the issue of drachms of Types A, B, and C had terminated and before any large number of drachms of later types had come into circulation, unless, indeed, the rate of issue of drachms of these later types was abnormally low. However, since the number of coins issued during a period of debasement tends to increase rather than decrease towards the end of the period, an abnormally low rate of issue does not seem at all probable. Therefore, it is likely that this hoard was completed before the end of the reign of Orodes I, perhaps by 40 b.c. at the latest. The very high proportion of drachms of Types A, B, and C, and the small proportion of earlier drachms indicates that the accumulation of the hoard was begun in the period when the drachms of these types were being issued, in other words not before about 54 b.c. Hence the longest possible period of time during which this hoard was accumulated would seem to be the 15 years from 54 b.c. to 40 b.c., inclusive. However, it is rather probable that the period was actually shorter.
The marked debasement of his coinage may explain why so many different types of drachms of Orodes I were issued even after he had gained sole control of Parthia. If the drachms of one particular type had met with full acceptance, need for a variety of types would not have arisen, especially since the innate conservatism of eastern peoples as regards types of coins that are preferred would have strongly favored the continuation of drachms of one fixed type. However, if the drachms of a particular type issued early in his reign were progressively debased during the period of their issue, and if the more debased drachms were detected, as might well happen after brief circulation, then public acceptance of the coins of this particular type would lessen. The obvious remedy would be to change the type noticeably but not radically and begin a new issue on a higher standard than these more debased coins, and perhaps with provision for redemption of the latter. If the drachms of this new type were in turn progressively debased, then the same remedy could be applied again, and be repeated through a series of types. The technical data, especially the marked overlapping of the ranges of fineness of Types A, B, and C, supports this theory.
|17||McDowell, R. H., Coins from Seleucia on the Tigris, pp. 215–216. McDowell designates this Orodes as Orodes II in view of the possibility that there was a previous Orodes, who was a son of Mithradates II, and who ruled briefly about 80 b.c.|
|18||Sykes, P. M., A History of Persia (London, 1915), I, pp. 373–380.|
|19||Op. cit., pp. 170–171.|
|20||Op. cit. p. 200.|
|21||Op. cit., pp. 213–214.|
This whole study of the validity and utility of specific gravity measurements for the estimation of the fineness of ancient silver coins, especially the method followed in the investigation of the fineness of the group of coins from the hoard, indicates the general procedure that should give the best results for the estimation of the average fineness and range of fineness of a large number of specimens of coins of a given type or series, or of a large number of coins from a hoard. In the first place, the reliability of the results that are obtained depends on the condition, fineness, and weight of the coins that are studied. No worth while results can be expected from coins that are badly corroded either externally or internally. All coins to be investigated by this procedure should be cleaned by electrolysis, except possibly coins of very high fineness, which apparently may be cleaned adequately by chemical or even mechanical methods. Occasionally lacquered coins may be encountered from which the lacquer must be removed by an appropriate solvent. Coins of very low fineness, i.e. billon coins of poor quality, cannot be expected to yield reliable results. Nor can reliable results be obtained on very small coins, such as the obol and its fractions, because of the technical difficulty of determining their specific gravity with sufficient accuracy.
The determinations of specific gravity should be made with apparatus, materials, and a manipulative technique that make possible such determinations accurately through the second decimal place. The necessary apparatus and materials are here listed.
The first step in finding the specific gravity of a coin is the determination of its weight in air accurately through the third decimal place in grams. This involves weighing to the fourth decimal place with sufficient care to establish the figure in the third place with entire certainty. The coin is then attached to one end of a fine suspension wire or is placed in a wire weighing cradle attached to such a wire. For this purpose the wire may be attached adequately by wrapping one turn tightly around the coin in one direction, crossing the wire with a single twist at the center of the coin, wrapping another single turn at right angles to the first, and attaching the short end of the wire to the long end with a single twist at the edge of the coin. Any excess of the short end is then broken or cut off. The application of more than single turns of wire is neither necessary nor desirable. A loop for attachment to the suspension hook above the left pan of the balance is then made in the long end of the wire at such a point that the coin will be suspended in the middle of the glass vessel for the water when the balance is at rest. Any excess of wire is broken or cut off. The coin and suspension arrangement are rinsed with acetone or ether by directing a stream of the solvent from a wash bottle or pipette at a point just below the suspension loop and allowing it to run down over the entire coin. Not more than 10 ml. is needed. The purpose of rinsing with such a solvent is to remove any grease or oil that originally may be on the coin or wire or that may be transferred to them while handling. Such grease or oil may prevent the wetting of the metal by the water and cause the formation of adherent air bubbles or films. As soon as the solvent has completely evaporated, the glass vessel is placed on the bridge, the coin is suspended from the hook, and distilled water, previously boiled and cooled to 25° C., is poured into the jar until the level of the liquid is so high that no part of the coin or the supporting wire immediately around it will reach the surface of the liquid when the balance beam swings. Any air bubbles present on the coin or submerged wire are removed by touching them with the end of a piece of wire. The weight of the coin and its suspension arrangement in water is then determined by taking the average of at least three weighings. In making these weighings the swing of the beam of the balance should be small, and the point of equilibrium should be approached from both directions, i.e., in one weighing, weights are selected such that their total weight is slightly more than is necessary to balance the coin and its suspension arrangement, and then weight is cautiously removed until equilibrium is reached, whereas in the next weighing the total weight is first slightly less, and weight is added until equilibrium is reached. Ordinarily, the final adjustment is made with the balance rider, or on some balances with a weight chain. The average of these weighings should be expressed through the third decimal place in grams. The coin is then removed and the weight of the empty suspension arrangement in water is determined in the same way. Care should be taken that the level of the water is at the same point on the fine suspension wire as when the coin was present. The original level of the water may be conveniently marked on the outside of the glass vessel by means of a wax pencil before the coin is removed. With a small coin no significant drop in water level occurs when it is removed, but with a large coin it is necessary to add a small volume of water to compensate for the drop in level.
To calculate the specific gravity, the average weight of the suspension arrangement in water is first subtracted from the average weight of the coin plus its suspension arrangement in water. This gives the weight of the coin alone in water. Then this weight is subtracted from the weight of the coin in air, and the result is divided into the weight of the coin in air. The result of this division is the specific gravity at 25° C. as compared to water at this temperature, and it should be expressed through the second decimal place.
The theoretical fineness, expressed to the nearest 5 units, corresponding to any specific gravity in the range likely to be encountered,
|Specific Gravity||Fineness||Specific Gravity||Fineness|