I was doing some research into a coin I'd acquired some time ago, and to get a better understanding how how it came to exist I wanted to get a really clear understanding of the exact production process that coins go through. This knowledge is particularly important, especially if you're a collector of pattern or proof coins.
I've put together the following notes based on what I could find on the internet - I took bits and pieces from a few different sources. If you can clarify or correct any of the information included, do let me know.
The process leading up to coins being struck is as follows:
- The coin design is selected & approved;
- A sculptor creates a plaster model of the coin's design. The artist will work from a drawn / sketched or computerized image to produce the plaster mould. Due to the difficulty a sculptor has in working with a small surface area, the plaster model is always significantly larger than the coin's actual size, and must be reduced to actual size.;
- The plaster model is coated with rubber and / or epoxy to produce a galvano. The rubber mould / epoxy galvano is used because the original plaster design would be badly damaged by the image reduction process. Epoxy is a hard form of plastic that allows for sharp design detail to be transferred.;
- A Janvier Transfer Reducing Machine is used to reduce the image on the epoxy galvano onto a metal master hub. Once the master hub has been produced, the design is the same size as it will be on the coin itself. In order to ensure that design is transferred accurately, a hub or die will be annealed and re-struck at least twice, if not numerous times before it is finally ready. Dies and hubs are annealed at each step of the process, this ensures that the metal is able to accept the design being struck onto it without being damaged;
- The metal of the master hub is hardened;
- A small number of metal master dies are produced using the master hub;
- These master dies are used to make working hubs;
- The working hubs are used to make working dies;
- The working dies are used to strike coins.
Prior to, or at the same time as this process is taking place, the following steps happen:
- Metals are melted and combined to produce ingots in the alloy required;
- The ingot is rolled into a sheet;
- Blank discs (“blanks” or “planchets”) in the correct size are punched from the rolled sheets;
- The blanks are edged and / or annealed to prepare them for being struck;
- The working dies are used to strike coins from the annealed blanks.
Information on the production of blanks was taken from the Japan Mint website.
A Galvano, is a copper shell emoved from the inside of a negative plaster model. (the negative plaster was made using a positive image made from clay) The copper was electroplated to the plaster negative to be at least 1/16th of an inch thick. The "galvano" was removed from the plaster and would be a positive image of a coin. Lead was then added to the back of the galvano to add support before the galvano was placed on the Janvier reducing lathe to make a master hub.