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An Experiment in Manufacturing Blanks and Striking Coins*
Published online by Cambridge University Press: 08 February 2017
Abstract
In the second half of the 6th century BC four South Italian Greek colonial cities – Sybaris, Croton, Metapontum and Caulonia – were minting silver-copper alloy coins, all in the incuse fabric, with the same weight standard of c. 8gm. These incuse coins were to remain in production at Croton and Metapontum for the next 100 years.
Coins hoards indicate that these four cities began minting their coinage at the outset as very fine, artistic – even exquisite – objects of fine crafts-manship. Each coin was thin (1mm), broad (30mm) and of a consistently uniform weight and diameter, and each coin was struck between dies of exceptional quality. During subsequent decades the diameter of the coinage was progressively reduced.
At Monash University in 1980 we conducted experiments in coin manufacturing to determine how the minters at Croton in the 6th century produced these thin, incuse coins from only a small amount (8gm) of silver alloy, how they maintained a consistent weight standard across a century of minting, and why they progressively reduced the size of their coins during this period.
It is well-known that the manufacturing processes of objects made from metal alloys can be revealed by examining their crystal structures.
In our experiments in manufacturing broad, thin ‘Monash coins’, we examined the crystal structures at various stages throughout the process. To do this we made coin blanks of various diameters, all made from 8gm of silver-copper alloy. These blanks were subjected to hardness tests and photographs were made of the alloy’s crystal microstructures. ‘Coins’ were then minted by striking blanks between two manufactured replica dies, and their microstructures were compared with the microstructures of a genuine Croton incuse coin fragment.
This is the first time these results have been published.
- Type
- Research Article
- Information
- Copyright
- © The Australasian Society for Classical Studies 2017
Footnotes
*
I would like to acknowledge Mr Gary See Toh Lock for his work in carrying out the experiments with Ag4Cu and producing the micrographic structures for analysis at Monash University in 1980, and thank him for allowing me the use of his images in my PhD thesis, which are reproduced here in Figs. 6-13 and 15-20, and for his original tables from which Graphs 1 and 2 were derived. I also thank Dr Dick Jago of the Metals Department of Monash University (1980) for assisting me in the interpretation of the microstructures, and the late Dr Colin Kraay of the Ashmolean Museum for his advice and assistance in providing the Croton coin fragment for analysis in 1980.
References
1 This paper is a revised version of Chapter 4 in my PhD thesis: R.J. Williams, ‘The Incuse Coinage of Croton, 6th and 5th Centuries BC’, Monash University (1983), which made a detailed study of the entire extant Croton coinage. Much of the interpretation of the Croton incuses in this paper is derived from that study.
2 ‘Incuse’ is the term used to describe a coin that has a raised image (in relief) on the obverse and an almost identical image impressed or incused into the reverse. For an introduction to the incuse coinage of South Italy, see N.K. Rutter (ed.), Historia Numorum Italy (London 2001). The incuse coinage may have been derived from the deliberate and resourceful process of ‘hubbing’ reverse dies from pre-cut obverses. To hub a reverse die from an obverse die, the image is first cut into the obverse die face, then an uncut softened reverse die is driven into the image on the obverse die, so transferring an identical image embossed (raised) onto the reverse die. This reverse die then produces coins with an incused image. With the Croton (and the other South Italian) incuses, inscriptions and sub-symbols were then cut into the fields of both dies before they were hardened, so producing, at Croton, the incused tripod symbol on the reverse of the minted coins with the raised inscriptions and sub-symbols in both the obverse and reverse fields of successive coin issues. For further discussion on hubbing dies, see G.F. Hill, ‘Ancient Methods of Coining’, Numismatic Chronicle 5th Series Vol. 2 (1922) 1-42.
3 These coins depict the tripod with an added heron or crab sub-symbol (raised) in the field.
4 My study (Williams, n. 1) relied on individual coin images, together with weight and provenance, collected from numerous sources, including published volumes of Sylloge Nummorum Graecorum, articles, sales catalogues, and museum and university collections. Exact one-to-one images were printed to ensure meaningful comparisons could be made. All weight statistics in the study relied on the weights as recorded in the museum or published notes for each coin.
5 A standard deviation of 0.41 means that 667 Croton incuse coins (68% of the 981 coins) lie within the range 8.82gm (7.81gm+0.41gm) and 7.40gm (7.81gm–0.41gm). This is a reasonably low standard deviation, given that there are many in the collection that are corroded and worn, and indicates that most blanks were produced fairly close to the average weight. See http://libweb.surrey.ac.uk/library/skills/Number%20Skills%20Leicester/page_17.htm, for explanation and application of standard deviation.
6 The other incuse minters show similar consistent standards: Caulonia 7.91gm, standard deviation 0.30 from 194 coins; Sybaris 7.88gm, standard deviation 0.30 from 49 coins; and best of all, Metapontum 7.96gm, standard deviation 0.19 from 327 coins. Williams (n. 1), chap. 5, 125, n. 44.
7 The experiments were carried out by Gary See Toh Lock as part of his honors course Fourth Year Project in the Engineering Department at Monash University, under the supervision of Dr Dick Jago, in conjunction with my doctoral research on the coinage of Croton.
8 The coin was examined in an X-Ray diffraction machine. It bombarded the fragment with electrons, captured the particles so emitted, interpreted from which metals these particles had been emitted, and graphed the result. Later, a sample sliver of the coin was mounted in resin, and the edge polished down to a flat surface, etched and photographed under a photographic microscope. See Fig. 17 below.
9 For observations of this phenomenon, see http://www.isoflex.com/silver-ag.
10 This part of the experiment is of importance. C.F. Elam, ‘An investigation of the micro-structures of fifteen silver Greek coins (500 to 300 BC) and some forgeries’, Journal of the Institute of Metals 45 (1931) 111-16, notes that her microstructure images of a genuine 28mm Sybaris coin (?520 BC) indicate that it had been struck on an as-cast blank (p. 62). For further discussion, see End Note below.
11 At Monash University an electronic balance was used, accurate to 0.001gm, to weigh Ag4cu filings. The minters at Croton must have used a reasonably accurate weight balance to achieve their low standard deviations in blank weight.
12 During this ‘spreadability’ experiment, various furnace exit temperatures were tried. It was revealed that, if the alloy is poured at a temperature of 1100°C or higher (at least 50°C above the melting point), surface tension draws the pellet into a smaller, thicker spheroid shape before it solidifies, especially if the pellet weighs around 10gm (Graph 1, p. 31).
13 We conducted further experiments to test if an 8gm solidified Ag4Cu pellet would spread radially when struck between dies. It does not. Nor does applying pressure to the pellet between flat plates produce a flattened disc. The blanks must be manufactured to the desired coin size by some other method prior to striking into coin. Yet another test showed that striking coins ‘hot’ cannot be carried out in practice; the blank loses heat so rapidly that, for metallurgical purposes, it is ‘cold’ by the time it reaches the dies after being removed from the furnace.
14 It seems unlikely that the Croton minters had the technology to create a forge capable of melting large enough quantities of this alloy and to keep it in a molten state in a crucible long enough to be able to pour it into more than one mould at a time. Ag4Cu alloy melts at 1050°C. (By comparison, pure iron melts at c.1530°C, pure gold at 1063°C, lead at c. 330°C, and tin bronze of the type used for bronze-work in ancient Greece c. 800°C, and 99% pure silver melts at 961°C. See http://garelicksteel.com/pdfs/melting_melting_points_of_common_metals.pdf). Reasonably large quantities of molten tin bronze at 800°C were required to produce the impressive bronze cast artwork of the 6th and 5th centuries: see Carol C. Mattusch, Classical Bronzes: The Art and Craft of Greek and Roman Statuary (Cornell University Press 1996) 10-16. However, these large, hollow bronze figures were cast in sections in a ‘one-off’ casting operation and assembled later as the final piece of artwork. It would require considerably more continuous heat output from a forge to melt – and keep molten – sufficient quantities of Ag4Cu (melting point 1050°C) long enough to pour the metal into numerous small moulds for continuous coin blank manufacture.
15 These two coins are from a late series of small tripod incuses (27 coins extant) from Terina, founded by Croton in c. 480 BC, containing the letters TE in the field. The series has a standard deviation of 0.16. In our experiment we made 32 pellets from a split-mould, with standard deviation of 0.11. Hill (n. 2) 6 refers to the same phenomenon (using split-moulds to produce pellets) in some other coins of Magna Graecia.
16 For simple production methods used in making charcoal, see: L. Horn, ‘Fuel for the Metalworker: The Role of Charcoal in Ancient Metallurgy’, Expedition (Fall 1982) 9-11.
17 Placing the pellet with its flat upper surface face down on the anvil prevents the pellet moving laterally under hammering.
18 A few other examples of edge-cracking on blanks are evident in the extant Croton coinage, but mostly the coin-makers were careful in determining the right time to stop hammering the blank. Often, however, coins show signs of having their edges nicked during circulation to test authenticity, particularly if the coin appears oversize or worn.
19 HV is the abbreviation of the term ‘Vickers Hardness’, and refers to a standard measurement, based on the size of the imprint made by a standard pyramid-pointed tool when pressed into a metal sample by a standard force, in this case 5 kg. HV allows comparisons of hardness to be made between different metals (http://www.instron.com.au/wa/applications/test_types/hardness/ vickers.aspx).
20 ‘Annealing’ is a heating process that softens metal by altering its microstructure.
21 The most recent analysis of genuine incuse coins has been made by Kenneth Sheedy et al., ‘Strange Objects and Strange Explanations: Understanding the Incuse Coinages of South Italy by Non-Destructive Neutron Diffraction and Tomography’, Unpublished paper (2015).
22 The same observations were made by Elam (n. 10) 60-3.
23 When re-striking a half-minted coin between dies the coin must be relocated exactly with the dies, and the obverse and reverse dies must relocate exactly with each other, or else a double image will appear on the coin after the strike.
24 All the earliest 30mm incuses from Croton (Delphic tripod), Metapontum (ear of wheat), Sybaris (bull), and Caulonia (Apollo advancing right) were minted with the similar images on obverse and reverse carefully aligned. Later, when coin thickness increased, few dies were aligned.
25 Kraay, p. 170.
26 For a discussion on the purpose and use of coinage in the late 6th century BC, see R.M. Cook, ‘Speculation on the Origin of Coinage’, Historia 7 (1953) 257-62.
27 Elam (n. 10) 62.