Magnetic properties of ancient coins

Journal

of Archaeological

Science

1983, 10, 4349

Magnetic Properties of Ancient Coins+ G. S. Hoye” The almost ubiquitous presence of small amounts of iron in ancient coins gives them remanent magnetic properties which can be measured easily on commercial magnetometers. The stability of the remanence suggests that ancient coins might retain information about the geomagnetic field at the time and location of manufacture and so he valuable in setting up archaeomagnetic dating curves. However, measurements show that the remanence directions tend to he too scattered for this purpose, although it is quite possible to distinguish magnetically the obverse and reverse faces of struck coins. Saturation remanence studies of coins and coin analogues indicate that the main carrier of the remanence in struck coins is precipitated fine-grained iron (rather than the oxides of iron) which is produced during the minting process. The intensity of the remanence generated depends upon the thermo-mechanical history of the flan prior to striking, which suggests that the magnetic properties of such coins are not simply related to the provenance of the ores used. Keywords: ANCIENT COINS; GREEK; ROMAN; MAGNETISM; REMANENCE DIRECTIONS; ENCE; REMANENCE CARRIERS; IRON DESTRUCTIVE TESTING.

ANALOGUES; ARCHAEOINTENSITY OF REMANPRECIPITATES; NON-

Introduction Iron occurs as an almost universal minor constituent in ancient non-ferrous metals (Caley, 1964). Chemical analyses of coins in particular give values for iron content ranging typically from 0.01 ‘A to 1%. The chance discovery that some coins are appreciably magnetic led to a recent investigation of the magnetic properties of ancient coinage (Tanner et al., 1979) and it was suggested on the basis of this study that the magnetic characteristics of ancient coins may have archaeological significance: they might, for example, give a clue to the origin of the ore used for the coins. Measurements using a commercial paleomagnetic magnetometer on a number of coins from the University of Alberta Classics Department coin collection confirmed that coins generally possrss a measurable remanence. This in turn raised the intriguing idea that they may have recorded information about the geomagnetic field at the time and place of manufacture. If this could be shown to be true then measurements on ancient coins would be extremely valuable in defining secular variation curves for archaeologically important regions. The +The first part of this article is based upon a paper presented by the author to the Twenty-first Symposium for Archaeometry held at the Brookhaven National Laboratory in May 1981.

0305-4403/83/010043

‘Institute of Earth and Planetary Physics, University of Alberta, Edmonton, Alberta, Canada T6G 23 1. 43 +07 SO3.00/0 @ 1983 Academic Press Inc. (London) Limited

44

G. S. HOYE

present work was undertaken to examine this possibility and to identify the principal carrier of the remanent magnetization. An explanation of archaeomagnetic principles, terminology and techniques may be found in Aitken (1974) or McElhinny (1979). Remanence Directions It is generally accepted that struck coins were minted with the flan placed essentially horizontally, and it was expected that measurement of the remanence vector might yield the inclination of the magnetic field at the date and location of manufacture. The declination of the field would not be recoverable because there is no way of knowing the original azimuthal orientation of the horizontal flan. However this limitation also applies to the already established use of baked bricks and pottery in archaeomagnetism.

H (Oe)

Figure 1. A.f. demagnetization of coin G6. The directional changes are shown on an equal area net. Declination D is arbitrary. Inclination I is measured from the plane of the coin. A.f. values (H) are 0, 25, 50, 100, 150, 200, 300, 400 and 600 Oe.

Measurements were made of the NRM (natural remanent magnetization) direction of 27 coins, 11 of which were then subjected to stepwise alternating field (af) demagnetization. The results of a typical demagnetization run on a Greek bronze coin are shown in Figure 1. The remanence is quite resistant to af demagnetization (median destructive field N 200 Oersteds) suggesting that the remanence should not have altered appreciably over archaeological time. Unfortunately there is no easily discernable endpoint or stable direction indicating a single-phase remanence. This behaviour was frequently encountered in the af demagnetization runs and makes it difficult to define a characteristic inclination. Table 1 lists the directional information for the Greek and Roman coins studied. Inclinations from coins of the same period are sometimes very different, and there appears to be no connection between date and inclination. Furthermore the values obtained are generally much too steep for the Mediterranean region. The inclinations are, however, invariably negative in the sign convention adopted, i.e. invariably directed outwards from the obverse. Since the geomagnetic field is directed downwards in the

MAGNETIC Table

PROPERTIES

1. Inclination

Coin Greek G2 G3 G4 GS G6 G7 G8 G9 GlO Gil Republican R4 R5 R6 R7 Imperial 24 25 26 44 46 49 50 60 95 103 113 139 140

OF ANCIENT

of remanence

Material

in ancient

COINS

45

coins

Date

Inclination

Endpoint

Bronze ,, 1, 1, ,, ., 1. ,, II ,,

220- 83 196-146 after 191 c. 100 2nd century 238-168 345-317 306-289 241-221 241-146

BC BC BC BC BC BC BC BC BC BC

-78 -53 -89 -78 -39 -88 -82 -79 -87 -48

100 50 150 100 50 50 NRM 150 NRM 50

Silver ,, ,, ,,

111-110 c. 84 c. 49 c. 49

BC BC BC BC

-77 -80 -82 -82

NRM 100 NRM >,

54- 66 AD 64- 66 AD 64AD 85 AD 90- 91 AD 92- 94 AD 92- 94 AD 104-111 AD 163 AD 178 AD 194 AD 273-275 AD 274-275 AD

-76 -78 -85 -72 -88 -80 -82 -18 -84 -86 -85 -60 -73

NRM

Copper-based ,, 0 3, ,. ,, ,. .> ,, ,1 ,, .. ,,

5; 100 NRM ,1 7, ,, ,. 9, 1, 1, ,,

The coin name is the University of Alberta Classics Department’s Catalogue Number. Numismatic details are taken from the catalogue. The uncertainty associated with a measurement of inclination is determined mainly by the difficulty in orientating the coin in the sample holder, and may amount to +4”. The endpoint, when given, is a subjective estimate of the demagnetizing field at which the remanence shows least directional change, indicating a stable direction. NRM in this column signifies that no a.f. demagnetization was carried out because earlier measurements on other coins made it apparent that this was of dubious value, and it seemed preferable to preserve any other magnetic information that might be residing in the coins.

northern hemisphere, this is taken to indicate that the coins were struck with the obverse die below. This conclusion is supported by independent measurements on a large number of coins (D. Tarling, pers. comm.). Surviving dies bear out this observation: the lower die generally provided the obverse impression. The lack of consistent inclinations in the coins, while demonstrating that coins are unlikely to be a fruitful source of archaeomagnetic information, suggests that the local field between the dies may have been distorted. This could occur if there were iron in the vicinity of the flan. Although most surviving dies are of bronze, and bronze dies continued to be used for centuries after the introduction of iron, there is evidence that some dies were made of iron and that iron die cases may sometimes have been used to protect the reverse die (Vermeule, 1954). It is possible that the present preponderance of bronze dies over iron might simply reflect the greater corrosion resistance of bronze. In any case, repeated striking of an

46

G. S. HOYE

6-

$ II 0

2

4 Applied field

6

6

IC,

(kOe)

Figure 2. (a), Build-up of isothermal remanent magnetization to saturation for coin G6. (b), Build-up curves for Fe doped-Cu coin analogues, normalized to 1% Fe by weight. Roman numerals by the curves denote subsequent treatment: I, as cast; II, annealed only; III, cold-worked 20% only; IV, annealed then cold-worked I8 ‘4.

iron die held vertically would tend to magnetize the die along steeply inclined field off its end would be registered by the flan. upper and lower faces of coins can be distinguished magnetically attitude of coins (including the “double obverse” coins) to information might be of value in sequential die studies.

its axis and the resultant The fact that the original should allow the original be determined, and this

Remanence Carriers The form of the iron (whether oxide or element) is not normally determined in chemical analyses and it was decided to see if magnetic measurements could clarify this question. The way in which saturation isothermal remanent magnetization (SIRM) is approached is sometimes a helpful indicator, and has been used to distinguish between magnetite (Fe304) and hematite (Fe,O,) remanence carriers in rocks. Figure 2(a) shows a typical

MAGNETIC

PROPERTIES

OF ANCIENT

COINS

47

SIRM curve for a bronze coin. The curve reaches saturation at 6 kOe (which is approxithe peak field used by Tanner et al., 1979) and shows no further change up to the maximum applied field of 10 kOe. Other bronze, copper-based, and silver coins (two of each type) showed the same behaviour and this is taken to indicate that the same type of carrier is present in all these coins. The shape of the curve is characteristic of neither magnetite (which saturates at -2 kOe) nor hematite (which does not saturate below 10 kOe) but it is characteristic of fine-grained iron, suggesting that the remanence carriers in all these coins are fine-grained iron particles. mately

Percentage

cold work

Figure 3. Effect of cumulative cold-working on the remanent moment of an annealed specimen. The remanence is normalized to 1 yO Fe by weight. The cold-work is quantified as the percentage reduction in thickness of the discshaped specimen.

Bitter & Kaufmann (1939) investigated the magnetic properties of iron precipitated from a solid solution of iron in copper. The solubility of Fe in Cu in the melt approaches 4 % but at ambient temperatures it falls to only 0.004 % (Hansen, 1958). On cooling from the molten state, excess Fe either precipitates as face-centred cubic y-iron or remains in metastable solution. Aging at raised temperatures allows the precipitates to develop. Gamma-iron is antiferromagnetic and should not contribute to the remanence, but coldworking an aged specimen causes the y-phase to transform to the body-centred cubic a-phase which is ferromagnetic. The induced magnetization of these ferromagnetic precipitates saturates at around 6 kOe. Metallurgical investigations of ancient coins are rare. One such study (Elan, 1931) showed that it may not have been uncommon for a cast flan to be hammered to size and then annealed before the final striking. It seems possible then, that the sequence of operations used in the manufacture of copper-based coins could result in the formation of ferromagnetic particles. To test this idea, analogue coins were made by first casting flans of copper containing approximately 1 ‘A Fe. The flans were gas-quenched from the melt in an inert atmosphere. They were then treated either by thermal aging, by cold-working in a press, or by thermal aging and subsequent cold-working. The approach to SIRM of the different specimens (Figure 2(b)) shows that a significantly greater remanence, saturating at 6 kOe, is obtained only for the annealed and cold-worked specimen. The close similarity in shape and point of saturation to the curve in Figure 2(a) suggests that the remanence in copper-based

G. S. HOYE

48

coins is indeed carried by Fe particles. The other three specimens (those with no treatment, or with aging or cold-working alone) display a much weaker behaviour typical of magnetite, and this is attributed to initial surface oxidation of the fine-grained iron used in the melt. The silver coins studied exhibit the same saturation behaviour as the copperbased coins, indicating Fe carriers of remanence, and this is interesting because Fe is practically insoluble in Ag at all temperatures so that the same precipitation phenomenon will be much less important. Another aged Cu-Fe specimen was subjected to successively greater amounts of coldworking which resulted in an increasing remanence as the cold-work increased, presumably as more y-Fe underwent the phase change to a-Fe (Figure 3). The direction of the remanence was the same, within the experimental uncertainties, as the direction of the field within the jaws of the hydraulic press. Thus it is to be expected that the intensity of the remanence of a coin will depend on the amount of cold working it has undergone, and this introduces a complication to the interpretation of the magnetic properties of coins in terms directly interesting to the archaeologist, because struck coins of different origins clearly exhibit different amounts of cold-work. However, the characteristic SIRM curve of struck coins does offer the possibility of distinguishing between cast and struck coins, and this may be valuable as a totally non-destructive test for some kinds of forgery. Conclusions The results of this preliminary study on ancient struck coins may be summarized as follows: (1) remanence measurements give directions which are too scattered to be used for archaeomagnetic dating or secular variation purposes; (2) the remanence direction does, however, indicate the original attitude of the coin, allowing the original upper and lower faces to be distinguished; (3) the ferromagnetic remanence appears to result from a phase change in fine particles of precipitated iron; (4) the intensity of the remanence depends on the thermo-mechanical history of the specimen, and this is a factor to be considered in the archaeometrical interpretation of the remanence of coins; (5) the characteristic SIRM curve of struck coins should offer a non-destructive way to discriminate between cast and struck coins.

Acknowledgements This work was financially supported by the National Science and Engineering Research Council of Canada. I thank the Museum Committee of the Classics Department of the University of Alberta for the loan of many of the coins in their coin collection. The Fe-Cu solid solution specimens were very kindly supplied by M. Wayman of the Department of Mineral Engineering of the University of Alberta. I thank M. E. Evans of the I.E.P.P. for his critical reading of the manuscript.

References Aitken, M. J. (1974). Physics and Archaeology, 2nd Edn. Oxford University Press. Bitter, F. & Kaufmann, A. R. (1939). Magnetic studies of solid solutions. I. Methods of observation and preliminary results on the precipitation of iron from copper. Physical Review 56, 1044-1051.

Caley, E. R. (1964). Analysis of Ancient Metals. New York: Pergamon Press. Elan, C. F. (1931). An investigation of the microstructure of fifteen silver Greek coins (500-300 B.C.) and some forgeries. Journal of the Institute of Metals 45, 57-69.

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PROPERTIES OF ANCIENT

COINS

49

Hansen, M. (1958). Constitution ofBinary Alloys, 2nd Edn. New York: McGraw-Hill. McElhinny, M. W. (1979). Paleomagnetism and Plate Tectonics, 1st paperback Edn. Cambridge University Press. Tanner, B. K., MacDowall, D. W., MacCormack, I. B. & Smith, R. L. (1979). Ferromagnetism in ancient copper-based coinage. Nature 280,46-48. Vermeule, C. C. (1954). Some Notes on Ancient Dies and Coining Methods. London: Spink & Son.