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Preparation of Working Dies

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    Posted: 04 Nov 2006 at 1:05pm

Preparation of Working Dies

The first step in producing a coin is for an artist to design the coin and the Secretary of the Treasury approves it. Next, a United States Mint sculptor-engraver prepares a plastilene (modeling wax) model in bas-relief from the approved sketch. Sculptor-engravers must keep in mind the depth of relief that is suitable for producing coins. This model is generally between three and twelve times larger than the size of the finished coin.

A plaster of Paris negative is then cast from the plastilene model, with great care being used to incorporate the necessary details and to refining the design. They make a plaster positive after suitable preparation of the negative model. The sculptor then submits the plaster positive or photographs of it to United States Mint officials and other interested parties.

When final approval is received, another negative is prepared. They use this to make a copper electrotype or galvano. The negative plaster cast is dried thoroughly and treated with hot beeswax and powdered copper. Finally, they attach a conductor and suspend the treated model in a copper electroplating tank. This process deposits a layer of copper that is at least 1/16th of an inch thick on the negative model. The resulting copper shell, called a galvano, is separated from the plaster and trimmed. After eliminating all defects from the galvano, the engravers add lead to the back to strengthen it.

They then mount the completed galvano on a Janvier transfer engraving machine. This machine cuts the design in a soft tool steel blank that is the exact size of the finished coin. This produces a positive replica called a hub. The hub is then heat-treated to harden it. It is then placed on a hydraulic press to prepare a master die, which must then harden. Using a cold forging process, they extract a working hub. United States Mint employees carefully store the original hub to insure against loss of the original reduction. The working hub is what produces the dies that strike the actual coins.

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