Evolution of coin striking processes: A mechanical survey. I. Hammer striking

Journal o f Mechanical Working Technology, 11 (1985) 37--52

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Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

EVOLUTION OF COIN STRIKING PROCESSES: A MECHANICAL SURVEY. I. HAMMER STRIKING

F. DELAMARE and P. MONTMITONNET Ecole des Mines de Paris, Centre de Mise en Forme des Matdriaux (CEMEF), E R A CNRS 83 7, Sophia Antipolis, 06565 Valbonne Cedex (France) (Received February 6, 1984; accepted June 20, 1984)

Industrial summary In a previous paper, the computation of the energy necessary to form a coin has been presented. In the present paper, it is applied to hammer striking. First, problems pertaining to monetary alloys (testing, evaluation of yield stress) are briefly reviewed,

along with blank forming techniques. Then, ancient coins (incuse archaic Greek coins, Vespasian tetradrachma, Ptolemy II's decadrachma) are taken as examples of coins of increasing size. In each case, the energy necessary to strike the coin is compared with the energy available from the hammer blow, which has previously been estimated. This method may suggest if striking at high temperature may have been necessary. Finally, some ways of decreasing the energy of striking are analyzed; some of their applications in ancient minting are given.

Introduction Although minting techniques are c o m m o n l y referred to as "Mechanical Arts" [1,2], few mechanical engineers have written about them. Indeed, most technical studies have been undertaken by metallurgists (e.g. [3,4]) whereas studies in manufacture have been carried out by historians or numismatists [5--7]. Only the mechanics of minting processes, and its historical and social contexts, will therefore be dealt with in this paper. Striking causes the metal to flow; hence, the present work is based upon the theory of plasticity, a part of c o n t i n u u m mechanics. However, striking must be studied in conjunction with the preceding minting operations. The striking of a coin is the last of the four steps of minting: (i) carving the dies; (ii) melting the m o n e t a r y alloy; (iii) forming the blank; and (iv) striking the coin, each step consisting of several operations. Carving of the dies will not be considered in this paper, although this step imposes the topography of the coin, and hence the strain to be undergone by the metal [8]. The preparation of the alloy and the estimation of its flow stress will be discussed. The determination of the geometry of the blank will then be described, along with hammer striking. A later paper will study screw-press striking, rolling and minting using several types of presses. 0378-3804/85/$03.30

© 1985 Elsevier Science Publishers B.V.

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The monetary alloy Problems relating to alloying The nature and description of the alloy are prescribed by the authorities, so its mechanical properties (yield stress, ductility, etc.) are thus similarly largely prescribed. Implementation on an industrial scale can prove difficult: from alloys of various descriptions, or even goldsmith's materials, an alloy close to the nominal description must be prepared, using casting, refining and testing techniques. Surprisingly, these techniques do not seem to have evolved much between the period of the first historical documents -- 2000 BC -- and the eighteenth century AD. For this latter date, much information about minting techniques used by the Monnaie de Paris may be found in technical works [1,2,9]. Cupellation, coupled with precision weighing (0.1 mg) was then used both to refine alloys on an industrial scale and to test them on a laboratory scale. The elimination of copper from a gold-rich alloy was, however, already being carried out in Egypt by the XXth century BC. Coupled with chloridation of silver, cupellation was in use in Greece to refine electrum -- a natural gold--silver alloy [10] -- and was described by Diodore of Sicily who observed it in Egypt. In the middle of the VIIth Century BC, the first coins, struck in Lydia, were not made of electrum but of a specially prepared alloy. The skill of the ancients for alloying is well known [11]. Testing techniques have long been in use. In Mari (Mesopotamia, around 2000 BC), a civil servant reports: "from the four minae (1 mina = 60 shekels; 1 shekel = 180 grains; 1 grain = 0.0444 g) m y Lord sent me, a technician carved o f f from the surface o f the four ingots four shekels (= 32 g) o f gold. I p u t it into the furnace to analyse what would be left. There was a loss o f a hundred grains (4.4 g) o u t o f four shekels. Since the loss is 25 grains per shekel (= 14%) and the goldsmith said "This is not red gold", the gold we received was not sterling gold. Now, what is left o f the gold I p u t into the furnace, I have sealed into a parcel with m y o w n seal, and had it brought away to m y Lord. Let m y Lord decide what he will" [12]. The method described was probably cupellation. Other tablets prove that quantities as small as 10 grains (0.44 g) or even 1/4 of a grain (= 10 mg) were weighed [12]. In the XIVth Century BC, Burraburias, king of Babylonia, complained before Pharaoh of the poor quality of gold from Egypt: 20 minae of alloy had yielded only 5 minae of gold! [13]. Later, Titus Livius reports that, when the questors in charge of precious metal testing tried the gold sent by Carthage as a tribute (200 BC), it "lost one fourth at melting" [14]. Though touchstone testing has been known since Antiquity [15], these texts suggest that cupellation was used. From Mari to the Monnaie de Paris, thirty-six centuries later, these texts show the same concern of Royal authorities to keep metal losses (which were unavoidable in the long series of forming operations) within strict

39

limits. As for the civil servants in charge of minting, they show the same care to avoid any charge of fraud. In Paris, this resulted in a succession of controls of the description of the monetary alloy - - a t the melting stage, after the casting of the sheets, and before striking. Later, coins were tested by the testers of the Cour des Monnaies. Discrepancy in the results led first to proving tests following a strictly codified procedure. If the description really was out of tolerance, the Directeur de la Monnaie was very heavily fined by the Cour des Monnaies.

Yield stress of monetary alloys Experience has shown the prominent part played by the hardness of the alloys in the striking processes: this is all the more so as the available energy is limited. Very soon, means of softening metals and alloys were found: the annealing of silver, and the annealing plus quenching of gold, to avoid precipitation hardening. In fact, the energy dissipated during striking is proportional to the yield stress o0 of the alloy, where o0 depends largely on strain @(work-hardening), strain-rate e , and temperature T. The laws of the dependance of o0 on @, e and T are well known for c o m m o n l y used metals and alloys (steels, aluminium alloys, copper, etc.), but little is known for monetary alloys. Further, hammer striking imposes very high strain-rates (up to 104 s -1) for which data are scarce. For computation, certain hypotheses will be made which will be described later. The Brinell hardness of gold--silver--copper alloys has been systematically measured by Sterner-Rainer [16]; three alloys only are dealt with here: pure silver, and silver alloys containing 10 and 28% of copper. The data of reference [16] have been re-interpreted using the theory of plasticity [17,18]. Figure 1 shows the work-hardening curves thus obtained at room tem.perature and low strain-rates (e = 0.1 s-l). Unfortunately, the influence of ~ and T has not been studied for silver alloys. The present estimation is f o u n d e d on data for a rather similar metal: copper. Temperatures have been corrected in the ratio of the absolute melting temperatures of the two metals. o o

,

MPa 400

~

,

.[ . . ~ /- "/

-'~g-1O°oCu ///~/~

200

._ 0

i

Ag ~ 2 8 % C u /

i

01

I

i

03

i

l

0.5

I

[

Fig. 1, Work-hardening curve of silver and silver--copper alloys for r o o m temperature a n d low strain-rates. (after [ 16 ], interpreted o n the basis of [ 17,18 ]).

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The yield stress of pure (99.99%) copper increases by some 10% when e increases from 0.1 s -1 to 103 s -1 [19,20]. Alloying copper emphasizes this trend: for a 2% Zn alloy, a0 increases by 20% between 0.1 and 103 s -1, whilst for a 10% Zn alloy the increase is 25% [19]. Extrapolating hightemperature data is somewhat harder as the influence of e is stronger. At a strain of 20%, the yield stress of 99.99% copper is multiplied by 7 if e increases from 0.1 to 103 s -~. Applying these trends to silver introduces further error, and only general trends may be given. Figure 2 displays the estimated variation of o0 with T and ~ for silver and a 10% copper--silver alloy. The mean overall strain for ancient coins is generally about 0.2: the mean yield stress during striking thus corresponds approximately to ~ = 0.1. Two strain rates are considered: e = 0.1 and 103 s -I. The percentage increases of a0 with either T or $ are those measured for Cu and Cu + 10% Zn in reference [19]. t

l

'

-Ag+10~ Co MPa ,e.~.~ %

20C -

3J

iO~s-~l v~. | ~ v/

• ,';'-. v ' ~ =

o.I • to 3

~'g I

0

1

o. t \ e . _ _ _ _ I I I Q|

200 400 600 T°C

Fig. 2. E s t i m a t e d i n f l u e n c e o f t h e s t r a i n r a t e e a n d t e m p e r a t u r e T o n t h e f l o w stress o 0 o f silver a n d a 10% c o p p e r - - s i l v e r alloy. A f t e r [ 1 9 , 2 0 ] . • Ag, 0.1 s - l ; • Ag, 103 s - l ; • Ag + 10% Cu, 0.1 s - ~ ; , Ag + 10% Cu, 103 s - l ; s t r a i n ~ = 0.1.

Problems pertaining to blanks The study of the blank is the first step of the study of striking: it fixes the initial condition for the geometry, the structure of the metal and the work-hardening state.

Forming of blanks Ancient blanks were cast directly at the desired weight of the coin, using single or multiple moulds. Cylindrical, lens-shaped or spherical blanks were cast. Blanks could also be cut from a cast cylindrical bar [21]. The metal was then in a completely annealed state. In the Middle Ages, the alloy was cast into sheets, then cold-hammered, and then circular blanks were cut off (Fig. 3). Later, until 1645, a more complex process -- or was it just more carefully described? -- was used in France. The alloy was cast into thick sheets ("lames") which were hothammered ("battre la chaude"). Several operations were required to obtain blanks of a given weight.

41 "When the sheets were "spread" almost to the thickness of the coins to be struck, the "pr~vot" or the "lieutenant des ouvriers" took them and dealt them out among the workmen to be cut into pieces nearly of the size of the coins; this was called "to cut squares" ("couper carreaux"). Then, the "squares" were annealed, spread with a flattener ("flattoir"); then the angles were cut off with bench-shears ("cisoires"), which was called "to adjust squares" ("ajuster carreaux"); so they were made of the right weight, by weighing them with "d~n~raux" while they were "adjusted": this was "to approach squares" ("approcher carreaux"); then the angles of the squares were bent to be rounded, which was called "to warm squares" ("r~chauffer carreaux"): they were

Fig. 3. The young Emperor Maximilian I of Austria visiting a German minting workshop. Wood engraving by H. Burgkmair, beginning of XVIth century. One can see the melting of alloy, hammer flattening of a sheet, clipping of blanks and hammer striking. The master weighs and controls the coins (A. Weil collection).

42 held with pincers called "estanques" on an anvil, so that with a few strokes of a hammer called "warmer" ("rechauffoir") on the edge of the squares, the angles were bent and smoothed, so that they were of the same shape as coins; this was called flatten ("flattir") [2]. Then the blanks were made" [2]. They were treated to "color gold blanks" and "whiten silver blanks". A first process consisted in annealing in a reverberatory furnace, followed by etching with a solution of nitric acid. A second process, avoiding metal loss, replaced etching by boiling in water followed by sprinkling with sand and rubbing in rags. Thus, in most cases, the metal was annealed before hammer striking.

Calculation of blank geometry from the shape of the coin In some cases, a careful examination of a coin betrays the shape of the blank or even its dimensions. For instance the presence of diametral marks on the edge demonstrates the use of cast spherical blanks. Striking on broad blanks shows the initial surface topography of the blank (for instance hammer traces) and its thickness [8]. However, few such examples are known, so that it seemed important to derive a rational m e t h o d for the estimation of the strain undergone by the metal, though only the final coin geometry is known. Such a m e t h o d has been described [8]: from a known cylindrical blank, the minimal strain necessary to form the design may be calculated. Conversely, if only the final geometry is known, it is possible by assuming that the blank was cylindrical, to calculate -- in a few iterations -- the blank thickness which, by this same minimal strain, would yield the coin geometry. It may be called the "ideal blank" for this coin: if the blank were thinner, a coin of the same thickness would have an incomplete design, if thicker, it would need a greater strain to reach the same final state, which would waste energy. Real coins were probably struck from blanks close to the "ideal": this has been verified in the case of a Byzantine gold coin [8]. Hammer striking Hammer striking is the simplest way of making a coin by plastic deformation, and it appeared in Lydia (Asia Minor) under the reign of Ardys (652-615 BC) and was still used in Sudan between 1908 and 1914 [22]. The process is simple: a man, sitting (Fig. 3) or standing (Fig. 4) in front of a wooden block supporting the fixed die ( " h e a d s " or obverse) strikes with a hammer the free die ("tails" or reverse) which he holds in the hand, and flattens the blank. It seems that, generally, three or four hammer strokes were needed. If, after that, the design was not completely formed, the coin was replaced between the dies and struck again until perfect [2]. No lubrication of the blank is even mentioned, which seems strange for this kind of process.

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Estimation o f the available mechanical energy Hammer striking is an impact metal-forming process: as far as kinetic energy is concerned, the speed is of m or e significance than the mass. Hence, very high strain rates are achieved (up to 104 s -1) which increases the yield stress o0 (see Fig. 2).

Fig. 4. A minter featured on a capital of St. Georges de Bocherville Abbey (France) XIth century: the "Moneyer" pin indicating the size is 1 cm wide. The question arises as to how much energy a single h a m m e r stroke can provide. Suppose the ha m m e r falls f r om one meter. If the fall were free, its speed would be 4.5 m/s. Taking into account the e f f o r t of the workman, the speed may reach between 7 and 10 m/s. Thus, if the h a m m e r weighs 1 kg, its kinetic energy will range from 25 to 50 Joules; if it weighs 1.5 kg, f r o m 37.5 to 75 J. This energy must now be transmitted to the free die. If the die is much heavier than the hammer, the energy transmitted will be low, since the h a m m e r r eb o u n ds on the die. If the die is m uch lighter than the hammer, it absorbs part of the energy in deforming plastically: these cumulative deformations of the top of the die lead to the f o r m a t i o n of " b a r b s " (Fig. 5). The t h e o r y of elastic impact shows that the o u t p u t is highest when the mass of the h a m m e r equals that of the die. In reality, in similar processes such as forging, the w o r k m a n chooses the ratio of the masses of the workpiece and h a m m e r so that the h a m m e r rebounds slightly, making the w ork easier. Thus, an o u t p u t of 80% seems reasonable. The available energy therefore ranges f r om 20 Joules for a normal stroke to 75 Joules for the strongest stroke. 30 Joules seems t o be a reasonable level for the mean available mechanical energy (Wa).

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Fig. 5. A pair of dies from York, used in 1354 to strike pennies (middle) and half goats (on both sides). After [6]. Note the barbs at the top of the tail.

Comparison of available and dissipated energy The e n e r g y W* necessary to strike three t y p e s o f silver coins o f increasing v o l u m e will n o w be c o m p a r e d t o the previously e s t i m a t e d u p p e r - b o u n d o f 30 Joules per h a m m e r stroke, Wa.

Incuse square punching (Fig. 6) A m o n g the archaic, globular or pebble-shaped G r e e k coins are some with a r a t h e r d e e p , p u n c h e d square m a r k [ 2 5 ] . These coins are " p u n c h e d " r a t h e r t h a n " s t r u c k " , since o n l y a small part o f the area o f the blank is c o n c e r n e d . T h o u g h t h e strain is high (deep i n d e n t a t i o n ) , t h e d e f o r m e d v o l u m e is low: the dissipated e n e r g y is thus also r a t h e r low. T a k e a 99% silver coin. Driving a 5 X 5 m m square p u n c h 2 m m d e e p gives rise t o a strain = 0.50. T h e flow stress of silver for t h e m e a n strain (~ = 0.25) is 170 MPa (Figs. 1 and 2). Calculation shows t h a t the dissipated energy is W* = 14 J, f o r which a single stroke at a m b i e n t t e m p e r a t u r e is sufficient.

Vespasian's tetradrachma These r a t h e r h e a v y coins, struck in Cyprus f r o m 76 t o 80 A.D. (Fig. 7), have a m e a n weight o f 11.7 g [ 2 6 ] . T h e m e a n diameter, as m e a s u r e d o n

.~ig.6. Chio's silver starer (author's collection). Obverse: sphinx and amphora; reverse: incuse square (545--500 BC).

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Fig. 7. Vespasian's tetradrachma struck in Cyprus (76--80 AD). Obverse: Vespasian. Reverse: Aphrodite's temple in Paphos. Moulding from the Berlin Mflnzkabinett (Courtesy of B. Helly). coins f r o m the Cabinet des M~dailles (Paris) is 24 mm. The mean field thickness, from the design, is 2.3 mm. The mean height of the design is 0.5 mm on the obverse (Vespasian 's head), and 0.1 m m on the reverse (Aphrodite's temple in Paphos); the height of the legend is 0.1 mm. A threedimensional p r o f i l o m e t r y leads to a r e f i n e m e n t of the measurement of the volumes o f the design and the legend (Fig. 8). The total volume of the coin is 1.1 cm 3, which corresponds -- for its mean weight -- to the density drawn f r om the com posi t i on [27], which is: 85.2% Ag + 12.5% Cu + 1.8% Pb + 0.4% Au. These data seem preferable to those o f reference [28], owing to the analytical m et hods used. No diametral marks have been f o u n d on the edges of the coins from the Cabinet des M~dailles, so the blank was p r o b a b l y approximately cylindrical. C o m p u t a t i o n [8] shows that the minimal strain to obtain this coin is ~ = 0.23 f r o m a 2.9 mm thick blank. The flow stress of the alloy corresponding to ~ = 0.11 and high strain rate is a0 = 270 MPa (Fig. 2). An upper-bound calculation, which is generally a rather precise m e t h o d , gives W* = 65 J for forming at r o o m temperature. Note that t he lower-bound m e t h o d , which is generally far less precise, gives a minimal value for the energy of 51 J. This coin may be struck with two strokes, at room temperature. None o f the 103 coins studied in reference [26] shows a double stroke (coins are f r o m the Cabinet des M~dailles; mouldings of coins from the Ashmolean Museum, Oxford; MacClean; Hunterian College; British Museum;

Pig. 8. Relief measurement of the obverse of another Vespasian's tetradrachma from ~yprus: (a) three dimensional profilometry; (b) contour lines of the relief.

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American Numismatic Society of New York; Berlin Mfinzkabinett; Cyprus Museum; treasury of Latakia). Note that the striking force (expressed as the ratio of power to velocity) increases from 7 to 10 metric tons during the stroke (70 to 100 kN). The minter replaced a 100 kN press!

Ptolemy IInd's decadrachma bearing the effigy o f Arsinoe H Here is an example of a heavy silver coin (Fig. 9), for which the blanks seem to have been cylindrical. The chosen coin, reference no. Z 2884/120 in the Cabinet des M~dailles and 461 7 in reference [29], has average dimensions with regard to the coins examined. It was struck in 263 BC. Its weight is 33.87 g and its diameter is 35.5/36.8 mm (elliptical shape). The mean thickness outside the design is 3.0 mm. The design mean thickness on the obverse (Arsinoe's head) is 0.6 ram, and on the reverse (cornucopia) 0.1 mm. The legend height is about 1.0 mm. The total volume is 3.3 cm 3. The composition of the alloy is unknown, but is assumed to be the same as that of the previous coin. The c o m p u t e d minimal strain # is 0.21, starting from a 3.7 mm thick blank. The yield stress of the alloy, at room temperature and high strain rate, will be around 270 MPa (Fig. 2). The upper-bound method gives W* = 184 J, the lower-bound gives 147 J; the striking force, for a single blow, reaches 25 tons (245 kN).

A

B

Fig. 9. Ptolemy II's silver decadrachma bearing the effigy of Arsinoe II (263 B.C.). Specimen no. Z2884/120 of the Cabinet des M~dailles, Paris. Obverse: Arsinoe II. Reverse: cornucopia (Courtesy of the Biblioth~que Nationale, Paris).

49 If the coin had been struck at room temperature, it would have needed at least six strokes; however, none of the 19 coins of the Cabinet des M~dailles betrays a defect due to double striking. In order to find the minimum temperature at which the coin could be struck in one powerful stroke, it can be seen from Fig. 2 that, as W* is proportional to e0, W* falls to 60 J at 700°C.

Optimization o f striking Technical problems must have arisen from the small amount of energy available per stroke. Many solutions to this limitation have been used, some of which tend to increase the available energy Wa, whilst the others aim at decreasing the energy W* necessary to strike the coin. Increasing stroke energy Wa The simplest way to increase Wa is to produce it in several strokes, which requires a certain skill to avoid double strokes and also lowers the productivity. Another solution consists of using a heavier hammer and dropping it from a greater height to obtain a greater striking force. It seems more efficient, however, to optimize the hammer--die impact by appropriate choice of their masses, i.e. by optimizing either the mass of the hammer or that of the free die. It is difficult to modify the hammer mass, b u t it is quite easy to increase the free die mass. It can be observed that ancient dies are often housed in an iron block. An example is the die used to strike Augustus' denier, presently in the Cabinet des M~dailles in Paris [23]. Made o u t of a 21% tin bronze this die weighs a b o u t 60 g, but it is housed in a steel block which provides a total mass of a b o u t 0.7 kg. This technique provides three possible advantages: (i) a considerable saving of the costly metal of the die itself; (ii) it enables hooping of the die. The compressive stresses at the surface of the die may then prevent cracks opening, and thus increase tool life. However, radiography of Augustus' die [24] shows that in this case, hooping is negligible; (iii) optimization of the o u t p u t energy of the impact and of the c o m f o r t of the worker. The first two points aim at decreasing tool costs; the last one both at increasing the striking energy and decreasing the effort of the worker. Reduction o f forming energy W* W* consists of three terms [8] : W* = V.e (h/a, figure geometry), e0 (~, e, T) + Wc (o0, h/a, figure geometry) + Wf ( ~ , o0, h/a, figure geometry). The first two terms give the energy dissipated by the plastic flow of metal, and increase with deformed volume V, strain ~ and yield stress o0. Other

50 important parameters are the blank "slenderness" (ratio of height to diameter), the figure geometry and the striking temperature T. The last term is the friction energy, depending on the friction coefficient ~ , the slenderness and the figure geometry. W* is proportional to a0. This equation explains the following technical solutions used by minters throughout the centuries. Note that the first two solutions did not alter the shape -- i.e. the type -- of the coin, so the decision was probably taken by the technician alone. The last four, however, lead to strong geometrical changes and probably needed the agreement of the minting authorities. (1). Lowering of the yield stress of the minting alloys, secured by (i) cold striking of an alloy softened by heat treatment (quenching of gold alloys, annealing of silver alloys), or (ii) hot striking. (2). Lowering of the friction coefficient ~ . Lubrication is never mentioned. However, whilst it seems necessary in certain cases, particularly for very flat coins, it disfavours the vertical flow of metal which forms the figure. (3). Optimization of die design. A large figure in the centre of the coin and a reduced legend near the edge lower the three terms of W*. Ancient Greek coinage provides an example of such a technique. (4). Striking of more slender coins. V being kept constant, the thickness is increased while the diameter is reduced; thus, the strain is lower and friction energy decreases. Considerable saving of energy may be achieved by this method. Compare the geometry of gold solidi of the Byzantine Empire during the VIIth Century struck in Constantinople and Carthage. The former are reasonably flat, with W* ranging from 19 to 32 Joules depending on the type. At the same time, solidi of the same alloy and the same volume struck in Carthage [30] need an energy decreasing from 18 to 3 J as their shape becomes more and more globular. (5). Decreasing the volume struck. Only the central part of the coin is really struck. The dot border diameter/coin diameter ratio decreases, but the dot border radius must be kept large enough not to alter the legibility of the legend. Hence, the coin radius increases, whilst the coin becomes progressively flatter. There is a lower limit to the thickness: for too-flat coins, the slenderness ratio becomes so small that W* increases tremendously. Striking only the center of the blank also creates high circumferential tensile stresses in the unstruck part. Depending on the thickness and on the ductility of the alloy, either radial cracks appear in the external part or the coin takes on a concave shape: this is the primary reason for the concavity of Byzantine "scyphates". The regular shape of the real scyphates is then probably obtained by changing the shape of the dies to avoid the angle otherwise present at the boundary between struck and unstruck parts. (6). From forging to deep-drawing. When monetary alloy is scarce, the flattening of coins reaches a limit: the thickness is so small that not only is W* enormous, but also there is not enough metal left to form the figures both on the obverse and the reverse. The solution then used entails replacing forging by deep drawing. The coin thickness is constant along the diameter;

51

the figure stands o u t in relief in the obverse and is incused on the reverse. A considerable energy saving is achieved." This is the principle of the striking of " b r a c t e a t e s " in Germany during the XIIth and XIIIth centuries. Conclusions Hammer striking provides a rather limited a m o u n t of energy and, also, the high strain-rates involved disfavour the optimal use of this energy. It has been shown that the o u t p u t may be optimized. Minting operations are very complex processes and operations performed before striking play a prominent part in the production of a satisfying coin: the quality of the relief depends partly on the quality of the sculpture of the die. The strain to be imposed on the metal, and hence problems pertaining to ductility, are closely related to the m e t h o d used to form the blank. Contrary to popular belief, minting techniques do not seem to have evolved to any significant degree: for example, no fundamental improvement in the refining or the testing of monetary alloys has been noted between the beginning of the second millenium BC in the Middle East and the XVIIIth century AD in Europe. Blank-forming and hammer-striking techniques have probably not changed much, either. Differences imposed by economic conditions seem more relevant. Minting techniques have oscillated between two limits: (i) very complex processes, needing a lot of skilled workers to make coinage of high quality as seen in Rome, during the Empire, or in French texts of the XVIIIth century [1,2,9]; (ii) summary processes, with few workers producing poor quality coins: the description of the minting workshop of All Dinar, Sultan of Darfur (Sudan) in 1908 [22] illustrates this. As hammer striking is easy and cheap to operate, it makes counterfeiting very easy. Moreover, as coins are often not perfectly circular, and as weight tolerances are not always precisely respected, clipping is quite easy. Thus hammer striking particularly favours alterations of coinage, which minting authorities cannot tolerate. This is the reason w h y new mechanical devices were looked for which would strike coins of perfect quality; and then along came screw presses. References 1 D. Diderot and J. D'Alembert, Encyclop~die ou Dictionnaire Raisonn~ des Sciences, des Arts et des M~tiers. Article: Monnayage. Paris, 1751. 2 C.J. Panckoucke, Encyclop~die MSthodique. T.V., article: Art du monnayage. Paris, 1788. 3 C.F. Elam, J. Inst. of Met., 45 (1931) 57. 4 M. Barral, K. Gruel, J, Barralis and F. Widemann, Quelques ~lSments de rnStallurgie mon~taire gauloise. Journ~es de Pal6om~tallurgie de Compi~gne, University of Compi~gne, France, F e b r u a r y 22--23, 1983.

52 5 T. Hackens, Terminologie et techniques de fabrication. In: Numismatique Antique, Probl~mes et M~thodes. Annales de l'Est, Nancy--Louvain, 44 (1975) 3. 6 P. Grierson, Numismatics. Oxford University Press, 1975. 7 D. Sellwood, Minting. In: D. Strong and D. Brown (Eds.), Roman crafts. Duckworth, 1976. 8 P. Montmitonnet and F. Delaware, J. Mech. Work Tech., 10 (1984) 253--271. 9 Abot de Bazinghen, Trait~ des Monnaies et de la Juridiction de la Cour des Monnaies. Paris, 1764. 10 R. Halleux, Le Probl~me des M~taux dans la Science Antique. Les Belles Lettres, Paris, 1974. 11 R. Halleux, Les Alchimistes Grecs. Les Belles Lettres, T.I. Paris, 1981. 12 J.M. Durand, ARMT VIII, 89 et le travail du m~tai ~ MarL In: M.A.R.I. 2, Recherche sur les civilisations, Paris, (1983) 123. 13 C.H.V. Sutherland, Gold, its Beauty, Power and Allure. Thames and Hudson, London, 1959. 14 Titus-Livius (59 B.C.--17 A.D.), Ab Urbe Condita Libri. XXXII. 15 A. Riche, Monnaies, M~dailles et Bijoux. Ballli~re, Paris, 1889. 16 L. Sterner-Rainer, Z. Metallkunde, 18 (5) (1926) 143. 17 D. Tabor, Rev. Phys. Tech. 1 (1970) 145. 18 K.L. Johnson, J. Mech. Phys. Solids, 19 (1970) 115. 19 H. Suzuki, S. Hashizume, Y. Yabuki, Y. Ichihara, S. Nakajima and K. Kenmochi, Studies on the flow stress of metals and alloys. Report of Inst. of Ind. Sci. Tokyo Univ., 1968. 20 G. Regazzoni, F. Monthelllet, R. Dormeval and M. Stelly, Deformation of polycrystals: mechanisms and microstructures. In: N. Hansen. A. Horsewell, T. Leffers and H. Lilholt (Eds.), Proc. of 2nd Ris@ Int. Syrup. on Metallurgy and Materials Science, Roskilde, Denmark, 1981. 21 E.R. Caiey and W.M. Deebel, Ohio J. of Sc., 55 (1955) 44. 22 A.J. Arkell, Numismatic Chronicle, 5~ s~rie, XIX (1939), or Sudan Notes and Records, XXIII (1940) 151. 23 M. Dhenin, Bull. Soc. Fr. Numiswatique, XXXII (4) (1977) 185. 24 M. Dhenin, F. Drilhon and C. Lahanier, Annaies du Labo. de Recherches des Mus~es de France, 1977, p. 42. 25 P. Naster, Le carr~ creux en numismatique grecque. In: Numismatique antique: Probl~mes et m~thodes. Annaies de l'Est, Nancy--Louvain, 44 (1975) 17. 26 B. Helly, PACT, 5 (1981) 106. 27 J.H. Evers, De Geuzenpenning, (1970) 29. 28 D.R. Walker, The metrology of the Roman Imperial coinage, Part I. B.A.R. Sup. Series, 5 (1976) 131. 29 J.N. Svoronos: Ptolemaic Coinage. Athens 1904, Vol. II. 30 C. Morrisson, J.N. Barrandon and J. Poirier. Evolution comper~e de la monnaie d'or Constantinople et dans les provinces d'Afrique et de Sicile. Jahrbuch der 3sterreichischen Byzantinistik, 33 (1983) 267.