Coin production techniques and machines

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From the Myntkabinettet in Stockholm, a Greek minter. For many centuries, hammering was the favourite technique to make a coin. It was a simple and effective method. It just took a hammer and two dies: one mounted on an anvil, the other on a small bar. The minter would strike by holding the dies just right and striking hard on the centre of the bar. The disadvantage of this method was that production was slow and imprecise.

Peter

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The Romans were fond of technology. Their contribution to minting technology is the pincer die, a hinged set of dies, suitable for smaller and thinner coins. The first two pictures show a Roman pincer die, open and closed, from the collection of the Cabinet des Médailles of the Bibliothèque Nationale in Paris. The hinge would make sure that the coin would be struck well centered on both sides. The Byzantine version shown makes it easier to keep the pincer closed while waiting for the hammer.

There is no further mention of this invention in numismatic folklore. Of course, it could have been lost in the early middle ages, like other Roman technologies. However, I think it survived. There is no agreement on how medieval bracteates were produced, but I think this instrument would be a good candidate.

Note that the Byzantine pincer could be worked with one or two minters. There is large (but not total) agreement that the Romans would strike heavy coins with three minters, one putting dies and flans in place and two striking by turns. As one could strike while the other was lifting the hammer, production could be increased considerably.

Peter

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Between 800 and 1000, the world's biggest copper mine opened: Stora Kopparberg in Sweden. The mine produced so much copper, that for centuries, its production dominated the European markets. The Swedish kings tried to control the copper price by regulating the amount of copper they would sell, but this left them with unimaginable stocks of copper.

From 1644 to 1776, the Swedes would resort to striking huge copper plate coins. These pieces were much too large to be hammered. The Swedish solution was to strike a coin-like emblem in the four corners. They developed a heavy contraption to do this mechanically. The first illustration shows how the heavy plate was stamped. Next is a 1 daler plate coin (official weight 1970 grams), heavy and awkward, but at least it can be handled by one person. The largest is a ten daler piece 1644. The illustrations show the whole piece and the stamps used. Only the smallest values circulated widely. The larger values were widely used as ships' ballast, that could be sold wherever the market price was higher than in Stockholm.

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No technology story is complete without Leonardo da Vinci and minting is no exception. Leonardo came up with a mechanical planchet cutter, using the same principle as the guillotine, much later: the weighted cutter was hoisted up by a cable rolled up on a spoked wheel and over a pulley. At the top of the machine, the weight pulled a handle, freeing the cutter. The cutter cuts the planchet and is stopped by a heavy spring, ready to be pulled up again.

As usual, the machine was built only much later. Around 1960, IBM subsidized the construction of a series of Vincian machines, including the planchet cutter. It worked perfectly well. It is not clear to me if the machine would also have worked with the technology available in Leonardo's time. After all, the guillotine was notorious for getting stuck on its way down. The first illustration shows Da Vinci's drawings and notes, the second the machine as built in the 20th century.

Peter

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Rolling was a known technique, used to shape metal into plates. It was also used to roll coin images on coins. The technique was used mostly where water could be used as a source of energy. Rolling mills used strips of metal, that were fed between two rolls with the coin dies. The "printed" strip was used to cut coins from. It was necessary to have excellent co-ordination between the rolls, but also between the strip and the die cutter. This may explain why the technique was not widely used, especially as long as labour was cheap.

The first illustration shows a pair of rolls, the second a "printed" strip. The third and fourth show an error from bad co-ordination between strip and die-cutter.

Peter

[Prosit]

Rolled.... but not a real coin.

Dale

[andyg]

Here is a more recent version of a rolled coin....

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I guess every coin colector knows this picture. It was used to instruct emperor Maximilian about the coinage process. The point of showing it here is that our lonely Greek minter had become surrounded by people of other trades as well as a control infrastructure.

At ten o'clock in the picture is the furnace, where metal was molten. In practice, it was not in the mint, as the heat would have made a confined place unsuitable for working. The unspeakables working with the oven are not shown. They were poor and pretty naked and therefore of no interest to an emperor. The molten metal was rolled to plates. One of these is being worked in the centre. A trained craftsman is hammering flans out of the plates. The rest of the plate will be re-molten.

The planchets will go to the man at 8 o'clock. He will check them on weight and roundness with his scissors-like instrument, actually a measuring tool. The approved planchets will go to the minter at 4 o'clock. Apart from his clothes, he's not very different from his Greek counterpart above. However, note the child, putting flans on the lower die. The boy has a dangerous job. Move too quickly and the flan will not be well positioned on the die. Since the minter is paid by the piece, that will not make him happy - and the boy is paid by the minter. Move too slowly and fingers may be lost to the dies and hammer. Consider that 10 to 12 working hours a day were found normal, also for children.

The coins struck would end up in the bucket at 5 o'clock, but a few would be set apart for random checking by putting them into the locked box at 6 o'clock. Usually, the box could be opened only by a set of keys. The mint master would have one of them, the representative of the ruler (proof master) another. Sometimes the assayer would have a third. In cases of fraud or corruption, the mint master was able to buy off the proof master or tamper with the mint box. An often occurring form of corruption would be that the mint master would have a large number of coins weighed together for approval, then separate out the relatively heavy coins for his friends, who would clip and sweat them and return them for re-melting.

Finally, the coins would be sold by the mint master at 12 o'clock, probably to a merchant. The mint master was likely to be a private person who had bought the right to mint for a certain period.Mint organizations differed, but some other mint staff not shown in the picture are likely to be an overseer of the ruler, choosing the coins to be put into the locked case for random checking, an assayer, and an engraver.

In the next installments, we will see how the problems of fraud and corruption were solved and the functions shown and not shown on the picture were mechanized.

Peter

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Machines need a source of energy. Up to the second half of the 18th century, that source of energy was human, as in the picture above of the Swedish plate money mint. Some mints were able to use water power. Wind power was known, but I have never heard of its use for coining. It was impractical to design machines that no source of power would drive. An important improvement was made in the Paris mint, with the horse-driven treadmill (first picture). The superior power of 4HP meant that heavier machines could used, or that more machines could be driven with the same power (second picture). The time for coining machins had arrived.

Peter

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Maybe the first real machine in the Mint was the "Handle", a device that would bore planchets out of plates, mechanizing the job of the man in the centre of the picture made for emperor Maximillian. Diderot was so impressed with the device that he put an exploded picture of it in his encyclopedia. The plate was held between the beehive shaped part (the knife) and the drop-shaped part above it. By turning the handle, the drop-shape would force the knife through the plate, cutting a planchet. The screw movement for exerting precision force was an idea whose time had come.

Peter

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Cranks were known in Roman times. The knowledge wasn't lost in the Middle Ages, because the crank was an essential part of the crossbow. Yet, it took centuries before the crank came to minting. The invention is usually credited to an Augsburg silver smith, Marx Schwab. However, metalworking machines driven with cranks were already in use in the silver mines of the Harz.

Schwab's merit is perhaps that he made a machine that would actually work for longer periods, using the correct trade-offs of thickness of plate and precision of parts. This came to the attention of Charles de Marillac, French ambassador to the free city. Marillac went into secret negotiations with Schwab, who was code-named "Chevalier de Saint-Sépulchre". A top-secret mission, consisting of the intendant (minister of finance and Marillac's brother) and François Guilhem, director of the Lyon mint, went to Augsburg to buy the equipment. Guilhem was dead set against it, foreshadowing two centuries of opposition against machines in all mints. He was replaced by an engineer, Aubin Olivier. The transaction was concluded and the machines shipped to Paris before an envoy from Charles V arrived in Augsburg. Meanwhie, Aubin Olivier had learned how to work the machines. The first mechanized mint was set up on the Ile de la Cité in Paris, using water power.

The new mint operated at full capacity from 1551 to 1554. The cost of the mint were higher than expected and the opposition of the minter's guild was venomous. Mint master Marillac was accused of fraud, the cour des monnaies decided the new mint could only make medals, top staff was promoted away. However, what killed the mint was the succession of the mentally weak Charles IX in 1559, who considered himself an accomplished old-fashioned minter.

This scenario was repeated in other mints, only the names of the actors changed. The main problem was probably that the machines could not yet be made with enough precision, so they broke down often, which would explain the higher cost. As technical knowledge improved over the next 100 years, the screw press proved unstoppable. No amount of guild power could reason with the beautiful, well struck, round coins of mechanized mints.

The illustration is again from Diderot. At the time the screw press had become commonplace. Note that the boy is still there, feeding planchets into the machine, but there are now four minters.

Peter

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Presses and handles mechanized processes known before. The edge mill introduced a new process: minting the edge of a coin. The coin was placed between two "rulers" with an edge die and rolled until the edge was completely covered with decoration. The small parts shown are to adjust the mill for different coin sizes. The effect was twofold: the edge of the coin would be upset, forming a protection of the faces and an aid to form piles. In addition, the edge decoration was an effective protection against clipping. Once more, the illustration is from Diderot's encyclopedia.

Peter

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Not all screw presses were as big as all that. The beauty of the screw press was that it could be made in all sizes, of course with consequences for the number of minters working the press and the thickness of the coins. This explains why private minters could take over in Britain with well struck halfpenny and farthing tokens. Token makers worked for a commission, or sometimes for their own account, producing generic tokens and tokens with political subjects. One is even known to throw halfpennies out of his window while working, to attract attention.

The picture is not very good, but the best I could find. In reality, the dies would not have been so far apart and the minter would have been unable to exert as much force as on the picture without falling over. What is important is the table-top model of the press.

Peter

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Another technical revolution was brewing, though. The steam engine provided a new source of power. The first steam presses looked like screw presses, except that they had a different source of energy. Boulton and Watt employed them enthusiastically for making coins, buttons, decorations, tableware and all kinds of metal parts and trinkets. That meant they could strike heavier, more intricate coins, as exemplified by he 1794 series for Madras, to be followed by the 1797 series for Britain. The Tower mint was threatened with irrelevancy if it couldn't break the resistance of the minters.

Slowly, the Tower Mint filled up with steam-driven machines. An American visitor wrote: "At that time there was in the coining room a row of screw coining presses similar to those in our home Mint save that they were driven by steam power ... The top of the screw still carried its heavily weighted balance lever, from the momentum of which the coin impression was made; the weighted lever end striking a wooden spring block was thrown back by the recoil, operating the dies for thrusting out the piece coined and inserting a fresh planchet". The minter finally disappeared, to be succeeded by an engineer, continuously oiling the clanking contraptions. A report from a visitor to the Tower mint says that it was impossible to be heard within the Mint. Boulton and Watt lost their competitive edge and sold their coin presses to Ralph Heaton.

Peter

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Heaton (illustrated) would have lost out to the Tower mint if it hadn't been for a German engineer, Dietrich Uhlhorn. He devised the knuckle lever coin press. Think of the movement your leg makes when inflating a rubber bed with a simple foot pump.  Heaton bought the rights for Britain and once again had a technological edge on the Tower mint, which was once again slow to react. The Uhlhorn press and its successor, the French Tonnelier press, conquered the minting world.

The second illustration shows the first Uhlhorn in the Staatliche Museum in Berlin. The sketch gives a clear view of the mechanism. Later Uhlhorns have a distinctive heavy horseshoe shape. From here on, the story of the coin press gets boring. Electricity brought even more power, which was used for always higher speed and more precision. Today's presses are just blinking fast metal boxes, the mechanisms almost completely hidden, with a maintenance schedule no Victorian engineer would believe possible.

Peter