Minting Techniques

From almost their first appearance, coins have called upon two technologies: one to produce blanks of defined size, composition and weight (planchets), and two to create moulds (dies) that would hold in negative form the required designs. Until late into the Middle Ages the procedure was the same: a planchet would be placed between the two dies, and struck with a heavy hammer, creating a positive image from the lower and the upper dies. The dies were commonly of bronze, and the dumpier planchets could be heated to soften the metal prior to striking the coin. {1-2}

The first coins came from Lydia, and were no more than irregularly shaped lumps of locally available electrum stamped with the issuing authority's guarantee of weight, purity, and value. Coinage spread rapidly, however, and by 500 BC most of the important Greek cities were turning out gold, silver and electrum denominations for treasuries, payment of mercenaries, tax and overseas trade. Bronze coins suitable as small change for everyday use came a little later. {1-2}

ancient minting techniqueAll steps were done by hand, and the Greeks took great pride in their workmanship. The metal was first refined to the right composition, re-melted and cast in moulds of the correct shape and weight for striking. Some moulds were for individual planchets. Others were branching strips of planchets broken apart after striking. Still others had bevelled or serrated edges. {1-2}

In the striking process, each planchet would be held in a pair of tongs, heated to just below its melting point, and placed between two dies. The observe die was fixed in an anvil, the reverse die was placed on top of the planchet, and the whole assembly firmly struck to stamp out the piece with a single blow. (Planchets in later coins, the thinner pieces of 'hammered coinage', were not necessarily heated, or created with a single blow.) Because each was produced individually, there were coin variations due to varying pressures of the strike, the temperature of the planchets, and die alignments, though hinged dies were later employed to keep the obverse and reverse dies in perfect alignment. Despite the rudimentary technological and the sheer labour involved, mints were able to turn out great volumes of good quality coins. The work was closely supervised, and miss-struck efforts returned to planchets for re-striking. {1-2}

The Greeks made their coins into small works of art, and employed talented individuals known as celators to engrave intricate coin designs onto bronze dies. Some celators travelled from city to city offering their services, and a celebrated few were allowed to add signatures to their handiwork. After engraving, the dies were hardened over intense fires, and were able to strike at least ten thousand coins without damage or significant wear. So continued these minting methods, practically unchanged from Greek and Roman times, through to 17th century Europe. {1-2}


The early Roman coinage — the aes rude — was not struck, however, but cast, as were some Celtic coins and perhaps a few Roman Britain issues. {3}

In contrast, the small denominations of east Asia (cash coins) were invariably cast, through two and a half millennia, until modern milling methods were introduced in the late nineteenth century.

The first moulds were individually carved into clay or soapstone, and the coins were therefore all a little different. Consistency had arrived by Han times, however, when a bronze master coin was employed. Matters were further improved in the sixth century when mother coins (mu qian) were first engraved in soft materials like tin, and then used to make hollow impressions in fine, damp sand mixed with a little clay and sprinkled with charcoal or coal dust. Within rectangular trays the impressions were arranged in 'coin trees' — assemblies of moulds joined by interconnecting channels allowing the molten metal to run freely from impression to impression. An upper and lower tray were bound together in a stout frame, and up to fifteen frames would be assembled for the casting process. The resulting coins were snapped off the tree when cool, and any imperfections remove by stringing the coins on a long square rod and filing the edges smooth. A final polish was given the coins in tubs of fine sand or chaff. The metal was commonly bronze but copper, iron or lead were also used, or some mixture of them. {4-6}

Quality was further improved in the eighteenth century when 'ancestor coins' (zu qian) were used to ensure consistency of the mother coins. From an ancestor coin, which was carefully engraved, polished and guarded, several thousand mother coins were cast, each in turn serving to cast thousands of circulating cash. {4-6}

Chinese cash entered Japan through trade and circulated widely, supplemented by small quantities of domestically produced cash from the late seventh century. Not until the sixteenth century were Japanese coins produced in any quantity, however, and these, like the Chinese cash, were round with a square central hole, though accompanied by various types of standardized silver and gold bars to serve as precious metal currency. Such cash coins continued to be cast until the 1860s, when the Shogunate was abolished and Japan embarked on rapid modernisation. {7-9}

Coins in Vietnam and Korea also followed the Chinese cash model, and elements of this pattern appeared in the Hindu kingdoms of Indonesian archipelago. The baht silver coinage of Siam was quite different, however, as was the rudimentary coinage of several trading empires of south-east Asia. {7-9}

The Screw Press

Around 1550, a new approach appeared in Europe. Roller mills turned out metal sheets or strips of the required thickness. Metal punches cut these into near-perfect blanks, and a screw-press imposed the required design. Until mechanised with water or steam power, the process was fairly laborious, however, and it was not until a century later that resistance from original coin makers was overcome and the practice became general. {10}

The Coin Press

Industrialisation introduced many techniques, the most useful being the Uhlhorn coin press of 1830, in which a lever rather than a screw press exerted the necessary pressure. The Thonnelier press, similar but driven by steam and then electricity, became the basis of today's coin production techniques that allow hundreds of coins to be produced per minute, all to exacting standards of uniformity. {10-11}

Modern Design and Minting

Today's coins are manufactured under tight control and security at every stage of the process. Design originates in a pencil sketch, often supplemented with photographs and other suggestions regarding the pattern. Once approved, the pattern is transferred into a modelling wax mould up to five times the size of the eventual coin. From this is made a plaster cast, and any last minute corrections or improvements added. From the plaster cast is made a rubber resin mould and then an epoxy resin mould. A reducing machine (pantograph) mechanically reduces the resin mould to a die of the correct natural coin size, and by repeated transference and reduction a steel master die is cut. From the master die several 'coining dies' will be produced. {11}

Mints today commonly buy in long strips of metal of the required composition, quality and thickness, which are then fed into a 'blanking press', which punches out round discs called blanks. The blanks are a little larger than the eventual coins because their rough edges have to be removed by further treatment, which includes inspection, acid treatment and annealing so that the blanks are both of the required hardness but will also take the die impressions properly. A collar prevents the metal from spreading, and also adds a legend or decoration to the coin's edge, deterring counterfeits. Rigorous quality checks are made at each stage of the process, and computers track the productivity of each press operator, any repairs needed, the quantities of coins struck per press, plus statistics on the installation, movement and destruction of the dies. Modern minting machines can produce 250 to 800 coins a minute. {10-11}

Bimetal Coins

Bimetal coins, common in antiquity, were generally ways of debasing the coinage. The Roman antonininus was a copper coin given a thin silver coating that soon wore off. The later silver coins of Henry VIII of England were struck from an alloy of one part silver and two parts copper: they had to be given a surface finish, either a silver coating or an acid treatment to remove the surface copper. Farthings of Charles I of England were struck from copper with a wedge of brass. Later British (1684-92) coinages struck halfpennies and farthings in tin with a square plug of copper. Late in the eighteenth century, the U.S. Mint experimented with a bi-metallic cent to keep the size of the coin manageable and meet the requirements of the Coinage Act of April 2, 1792. Bimetal coins where a large core in one metal is surrounded by a ring in another metal feature in the Euro and other modern coinages. {10-11}

Great aesthetic intelligence goes into modern coin design, but critics and collectors have generally found something wanting in their perfect workmanship. The nickel alloys are too hard to take a moulded and deeply incised impression, and the mirror brilliance soon wears off into an unattractive glitter. {12}

References and Further Reading

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