Sunday, June 26, 2011

The General Knowledge Show: How Coins are Made

Contributing AuthorAnastasia Meredith Oh

Coins are made of copper and nickel because they are cheap.

A mint can made commemorative coins out of silver. This silver goes into a casting furnace to be melted at 2100 Degrees Farenheight, and when it comes out the silver is shaped into a long bar 5 inches wide, and 1.5 inches thick. This long bar is then cut into a 30 inch long bar before it is placed in a roughing mill.

The mill uses 9 tonnes of force to push the silver bar between 2 rollers which squeeze the bar so it becomes thinner. To make the bar 0.5 inches thick it may be passed into the mill up to 12 times. After this the bar still needs to be reduced in height, so it goes into a finishing mill to be made into the final height needed for the coin, which varies depending on the design and denomination.

A blanking machine stamps out blank coins from these strips of silver. The excess silver from around the circular holes, called cissel is taken to be re-melted in the casting furnace.

The blank coins are then fed into the rimming machine, which has a spinning wheel to press a raised edge or rim onto each coin. The coins are placed into a tub of water, cleaning fluid and steel beads which polish and smooth the blanks because the steel beads act as an abrasive agent.
When the blanks are removed from the tub they are separated from the beads using a sifter before being hand dried with a towel which ensures there will be no water marks on the coins.
The silver can become very brittle as it is being worked, so brittle that if somebody hit it, it could shatter. To prevent this, the silver is frequently placed into an annealing furnace.

The design is made from photos and artwork and is designed on the computer. When the mint approves this design it is engraved onto a large plaster disk with a 10.5inch diameter. This is a negative design, before they cast a positive mould, on which the artists adds some more detail. From this a positive mould is made from rubber mould. A negative mould is made of epoxy, before a pentagram reduces the design by 1.5 times its size onto a brass disk. This reduction can take 36 hours!

The brass model is then fine tuned by an engraver using a microscope, and the lettering is put on. After this it is back in the pentagram, where a stainless steel matrix. The matrix is then turned into a negative dye.
The dyes are installed into a press; there are 2 dyes per press, as both sides of the coins needs to be designed. The coin is then stamped 2 times to ensure high quality. Circulation coins are just struck once.