I found the following information from the Royal Australian Mint and the photographs from cruziscoins.com interesting and informative regarding the making of proof coins:
"Proof coins are the masterpieces of our craftsmanship. Clean, good quality steel is used to make proof dies. The die impression is grit-blasted with very fine zirconium sand to give the surface a frosted finish - the common term for this is sand-blasting. The surrounding background is then highly polished.
Hand polishing of the sections of the die face is performed under a microscope and takes two to three hours. To achieve a mirror-like finish, different grades of diamond lapping paste are applied to the surface and polished with wooden sticks (usually soft birch wood). This is followed by polishing with a soft felt pad attached to a dental drill.
The entire face is covered with clear tape and then, using a fine scalpel, the tape is cut away leaving the design exposed. This area is sand blasted again and given an even frosted appearance and a clean sharp profile around the impression.
After polishing, the die is chrome-plated or titanium nitride coated to resist damage from minute foreign particles and to extend the die life. This in turn reduces the number of times the die is refurbished.
Proof coins are manually and individually coined in a specially designed and air conditioned environment. Laboratory coats, hair ties, gloves, sticky mats on the floor, lint free material to wipe dies, and fans continually blowing air across the die in the coining presses are some of the measures taken to minimise the risk of blemishes on the coins.
On each manually operated press, using forces of up to 360 tonnes, we can produce about 500 proof coins per day.
Within the coining press a collar is used to restrain the blank. The size and shape of the collar determines the shape of the finished product.
Operators, with gloves and/or tongs, place the blank onto the coining press and strike it up to 4 times. The number of strikes is dependent on the size and the depth of relief. The die is wiped with alcohol, to remove any dust, before the next coin is struck.
The operator inspects each coin and then Quality Control randomly examines coins within every batch. If one fault is found the entire batch is thoroughly examined.
Finally, proof coins made from bronze, cupro nickel, aluminium bronze and sterling silver are sprayed with a nitro-cellulose lacquer to protect against oxidation. The coins are inspected once more."
(
http://www.cruzis-coins.com/Kangaroos/index.html)(
http://www.ramint.gov.au/making_coi...e=proof.cfm)
Posted 09/20/2006 12:03 pm
