Coining (metalworking)
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Coining is a form of precision stamping in which a workpiece is subjected to a sufficiently high stress to induce plastic flow on the surface of the material. A beneficial feature is that in some metals, the plastic flow reduces surface grains size, work hardening the surface, while the material deeper in the part retains its toughness and ductility.
Coining is used to produce money (coins), medals, police and fire fighter's badges, buttons, precision-energy springs and precision parts with small or polished surface features.
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[edit] Coining as a manufacturing process
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Coining is a cold working process (similar to forging which takes place at elevated temperature) that uses a great deal of force to plastically deform a workpiece, so it conforms to a die. Coining can be done using a gear driven press, a mechanical press, or more commonly, a hydraulically actuated press. Coining typically requires higher tonnage presses than stamping, because the workpiece is plastically deformed and not actually cut, as in stamping.
Coining is used to manufacture parts for all industries and is commonly used when high relief or very fine features are required.
[edit] Coining tools for currency
A coin die is one of the two metallic pieces that are used to strike one side of a coin. A die contains an inverse version of the image to be struck on the coin. To imagine what the incuse version looks like, press a coin into clay or wax and look at the resulting inverted image. Modern dies made out of hardened steel are capable of producing many hundreds of thousands of coins before they are retired and defaced.
[edit] Ancient coin dies
Prior to the modern era, coin dies were manufactured individually by hand by artisans known as celators. In demanding times, such as the crisis of the Roman Empire in the 3rd century, dies were still used even when they became very worn or even when they cracked. The die that was on the hammer side, usually the reverse (back),[1] tended to wear out first.[2] The flans were usually hot prior to striking.[2] On some Roman provincial coins, some believe the tongs used to move the heated flan left permanent center indentations on the finished coins. Others attribute these marks to surfacing tools used as a part of flan preparation.
[edit] Medieval coin dies
Medieval coin dies were created in mints by guild members known as engravers.
The vast majority of medieval coins were cold struck; the planchets were not heated. While medieval coin dies were largely made of iron, some dies have been discovered with a small region at the face of the die which is made of steel. As technology and the economy changed over the course of the Middle Ages, so did the techniques used to create coin dies. While most ancient coin dies used engraving very heavily, early medieval coinage was dominated by dies created mostly from punches, which displace the metal of the die instead of removing it. There is evidence of medieval die cutters using engraving tools to lay out designs, and to create detailed punches. However, engraving on the face of the die did not become commonplace until the early Renaissance.
Very detailed records exist for the Venetian mint. In the late Middle Ages, the dies used to create tornesellos lasted as follows: "hammer" die, ~17,000 strikes; "anvil" die, 36,000 strikes. The mint made an average of 20,000 coins per day, so they were making one hammer die a day and one anvil die every other day! The "hammer" dies wore out quicker because they tended to be smaller and were hit directly with a hammer, leading to severe mushrooming on the tops.
[edit] Modern die production
First, an artist creates a large plaster model of the coin. The plaster model is then coated with rubber. The rubber mold is then used to make an epoxy galvano. All of this takes place on a scale of around eight inches. Next, a Janvier reducing lathe takes several days to reduce the image onto a steel master hub. The master hub is then heat treated to make it hard. The master hub is then used to make only a few master dies via hubbing, which involves pressing the master hub into a steel blank to impress the image into the die. The master die is then used to form as many working hubs as needed through the same process, and then the working hubs are put through the same process to form the working dies. These working dies are the actual dies which will strike coins. The process of transferring the hub to the die can be repeated as many times as necessary in order to form the number of dies needed to make the amount of coins required. The difference between a hub and a die is that the hub has a raised image and a die has an incuse image, so one forms the other.
When making working dies, the Mint has found that by using a lower amount of pressure in the hubbing press, they can prolong the life of the hubs and dies used. In between each hubbing, however, the die being made must be subjected to an annealing furnace to soften the steel, making it easier to push the image into the die. As the die is compressed in the hubbing press, the molecular structure of the steel changes. The large amount of pressure exerted on the steel forces the molecules of the steel to be compacted, making this hubbed die much stronger and denser. If, when the die is subjected to another hubbing, it is not lined up exactly with the hub, the result is a secondary image, or doubling. This is called hub doubling, and results in such spectacular coins as the famous 1955 doubled die cent.
Since coin production in the United States has exceeded 20 billion coins in some recent years, this means that a lot of dies must be manufactured as well.[citation needed]
[edit] The third side of the coin
On the edge of the US dime, quarter and half dollar, and many world coins there are ridges, similar to knurling, called reeds. Some older US coins, and many world coins have other designs on the edge of the coin. Sometimes these are simple designs like vines, more complex bar patterns or perhaps a phrase. These kinds of designs are imparted into the coin through a third die called a collar. The collar is the final size of the coin, and the planchet expands to fill the collar when struck. When the collar is missing, it results in a type of error called a broadstrike. A broadstruck coin is generally a bit flatter and quite a bit bigger around than the regular non-error coin of the same denomination.
[edit] Use
In modern presses, a die strikes approximately 120 coins a minute. This rapid coining causes wear on the dies. Nickel, one of the main metals used in today’s coins, is exceedingly hard and causes wear quickly. Copper has been used for centuries because of its malleability and the ease with which it makes coins. However, it too wears the dies when they are used for too long. An infamous example is the 1955 "poorman’s double die." This coin is sold as a replacement for the 1955 doubled die, but it is no more than Die Deterioration Doubling, caused by wear on the dies. When a coin is struck, the planchet is not heated. Although the planchet would be softer and more malleable, the extra time and expense would prove too great for the mint. Thus, the metal cold flows into the die under the high pressure.
Mistakes can happen at any stage of this manufacturing process, and these mistakes are something that certain collectors look for. Coin errors that occur on the die are generally more desirable than errors made at the time of the strike. For example, a doubled die, where a date or another device appears twice slightly offset, is often a highly desired error. Strike errors are generally unique, whereas all coins struck with an error die will have the same characteristic. This makes them more easily collectible. The most famous doubled die in the past hundred years is the 1955 doubled die Lincoln cent. These trade for hundreds of dollars because the error can easily be seen by a casual observer. Many doubled die errors require at least a jeweler's loupe to be seen. Doubling can occur at the hub stage as well. Some more recent errors are hub doubled. Most famously, there is a 1995 doubled die cent that is hub doubled.
[edit] Coin die represented in other coins
A coin die itself, has been the main motive for many collectors coins and medals. One of the most recent and famous one, is the Austrian 700 Years City of Hall in Tyrol coin, minted in January 29, 2003. The reverse side of the coin shows the Guldiner silver coin. However, the design is negative, representing a coin die, as a reference to Hall’s history as a significant centre for minting coins.
[edit] References
- ^ Hu, Di. "Experiments in Ancient Roman Coin Minting". http://www.archaeologystudent.com/coinarch.
- ^ a b Jere M. Wickens (1996). "The Production of Ancient Coins". http://www.lawrence.edu/dept/art/buerger/essays/production.html.
[edit] External links
- Designing and minting coins The creation of a die and the minting process
- US Treasury: Preparation of Working Dies
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