Friday, January 11, 2013

The Process of Proof Coin Production at Shenyang Mint

Translator's note: This post appeared a few years back. I am not sure whether Shenyang Mint is still following the same process in minting proof coins. But understanding the process is helpful when we discuss various topics relating to early modern Chinese coins.


Proof coins are our high tech masterpieces. Dies are made from clean, high quality steel. The relief part on the die is blasted with very fine zirconium sands, to achieve a frosted surface – technically known as "sand blasting." Other parts then go through the process of intensive polishing.

Manual polishing of the die field is done under microscopes. It takes about 2 to 3 hours. To achieve the final mirror effect, the mirror surface is polished with diamond grinding paste of different grits (from coarse to fine) on a small wooden stick (normally of soft birch wood). Finally a soft felt piece is attached to a drill bit for the finishing polish.

The whole die surface is then covered with transparent tapes. A fine knife is used to remove the transparent tape on the relief area, to expose the relief part. This area is sand blasted once again, for a more even frosted surface and sharp outlines of the relief.

After polishing, the die is coated with chrome or titanium nitride, to protect against damage from minute impurities, and to prolong the life cycle of the dies. This will reduce the number of re-polishing efforts on dies.

Proof coins require manual, piece-by-piece stamping in a specially designed environment with qualified air conditions. The risk of contamination on the coin surface is mitigated by wearing lab dress and gloves, pinning up the hair, adding a felt cover on the floor, wiping the surface of dies with a soft piece of cloth, and blowing the die continuously with air.

At 360 ton pressure, a single manual stamping machine can produce 500 proof coins every day.

Die colloars are used on the stamping machine to limit the radial flow of the metal. The size and shape of die sleeves depend on the size and shape of the final product.

The operator wears gloves, and puts the planchet into the stamping position on the stamping machine with a clamp. Stamping is repeated four times. The number of the stamping operation is based on the size and height of the relief. Before stamping the next planchet, the surface of the dies is cleaned with alcohol, to remove dust.

The operator will inspect every coin after finishing stamping. The Quality Assurance Department will follow up with random inspection on every lot of products. If one coin is found to be disqualified, the whole lot will be inspected.

Finally, the surface of proof coins made from bronze, copper-nickel alloy, aluminum bronze and standard pure silver will be spray with paint, to prevent oxidation. The coins are inspected once more at the same time.

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