In contrast to the modern computerised production processes, the essence of coin making is entirely reliant on the honoured skills of master craftsmen. The die-sinkers at the South African Mint combine their artistic flair in interpreting designs, with the strict discipline that is required in a highly advanced technological environment.
The first step in the creation of a master die is three-dimensional modelling of the design in plasticine, followed by refinement in fine plaster.
An acrylic model of the completed artwork is reduced and cut in steel by precision 3D reducing machines. The South African Mint also uses computer controlled engraving machines for high volume production items.
Cut steel dies are turned into master dies from which the production dies are reproduced
Production - Numismatic Coins
The numismatic proof coin manufacturing process is quite different from circulation coining. At the SA Mint, circulation coin manufacturing is an automated, high-speed process whereas the numismatic proof coin manufacturing process is very slow and labour intensive. South African numismatic proof coins are manufactured from the following materials:
Precious metal proof coins are manufactured according to the following process phases:
Bronze plated steel
Sterling silver (925Ag)
22 ct Gold
24 ct Gold (999.9Au)
The metal is cast in a vertical continuous induction furnace and the crucible and casting dies are manufactured from very high-grade graphite.
A starter bar is placed in the die, which acts as a plug during the smelting process. The molten metal attaches to the starter bar, initiating the casting process. As soon as the correct temperature is reached, the starter bar is slowly withdrawn from the die. The molten metal solidifies in the die and as it is pulled out of the die the molten metal again flows into the die hence, the name continuous casting.
The classification of a successful smelt depends on:
The surface finish of the strip
The solidification pattern
The absence of surface cracks on the drawn bar.
After the casting process, the drawn bar is washed and then rolled to the correct thickness. This is done on a rolling mill. In this process, the drawn bar is rolled into a strip. By passing the strip between the two rollers, the strip length is increased as the thickness is decreased. Two operators control this process manually, one placing the strip into the mill and the other removing the strip from the mill. It is very important that the strip is handled with extreme care so as to prevent surface contamination as well as avoid scratches to the strip surface. When the correct thickness is achieved, the strip is washed and inspected for any surface errors. The strip is now ready to be blanked.
Blanking is the process whereby round discs are cut from the strip. The blanking machine, fitted with a punch and a cutter, according to the specific diameter required, is operated manually by one operator. It is very important that the strip is handled with extreme care so that the surface is not scratched and the maximum yield of blanks is acquired. After the blanks are cut, they are individually inspected, the inspection criteria being, weight, diameter, and visual acceptance. Blanks are placed in an annealing furnace to relieve stress. The furnace is an open-ended steel belt furnace, equipped with heating and cooling zones that are protected by a continuous flow of inert gas. After stress relieving, the blanks are placed in a heated acid solution to eliminate any surface contamination. The blanks are now ready to be polished.
Polishing is the process whereby the surface of each blank is polished to a very high lustre and an excellent surface finish is acquired. The blanks are polished in an automated centrifugal finisher. The blanks are automatically removed and dried and the lustre and surface finish are inspected. If the blanks are acceptable they are individually packed in a special tray. The blanks are now ready for the coining process.
In the coining process the blank is embossed with two dies simultaneously, one for the obverse (front) coin face and the other for the reverse (back) coin face. A serrated collar restricts the sideway flow of the material. Coining is undertaken with multi-stroke knuckle presses. Depending on the diameter and detail of the coin, the strokes can vary from three to eleven strokes per coin. The coining operator individually selects each blank for the coining process. The operator places the blank in the machine and the machine is activated. After the coin is struck, the operator removes the coin and inspects it under a magnifying glass. If the coin is accepted, it is placed in a special storage tray. After each coin is struck, the dies are cleaned with a special soft cloth to remove any residue from the die faces. The proof coins are now ready to be packed according to specific packaging requirements. It is important to note that this is the last phase in the manufacturing process of proof coins and from this point on nothing can be done to the coin to improve the quality.
Packing is the last phase before a coin is ready to be sold. This process is also a manual process during which the packers do a final inspection of the coins as well as the actual packaging.
In this way, the most beautiful coins in the world are manufactured.