|Most of us have seen at least one bi-metallic coin up for auction, such as the Canadian $2 with the Polar Bear, but for the most part, many U.S. collectors have ignored them. I purchased one several years ago, placed it with my small grouping of Canadian coins and promptly forgot about it, until recently I learned that the basic idea of bi-metallic coins is not a new one.|
|What is considered by many as one of the earliest strikings of bi-metallic prototypes dates back to 1730, when a silver token with a center copper plug was struck in Cologne, Germany. However, during the reign of Charles I, the English Rose Farthing 1625-1649, had a brass wedge inserted into the copper as an anti-forgery device.|
|The US Mint experimented with a bi-metallic cent to keep the size of the coin manageable and meet the requirements of the Coinage Act of April 2, 1792. The first silver center cents were struck December 17 to 18, 1792. Each was made by hand at the U.S. Mint, workmen first making the copper blank and then punching out a small hole, and next inserting the silver plug, and finally striking the coins using the appropriate dies.|
|Many tokens and medallions have been struck by various countries throughout the 19th and 20th Centuries. But, the first bi-metallic coin to be widely used in modern times was the 500 Lira issued by the Italian government in 1982.
Bi-metallic coins are now produced by over 105 countries world wide, and the number seems to be growing all the time. These coins are minted in many different combinations of precious and base metals: yellow and white gold, gold and silver, silver and titanium, silver and nickel, non-magnetic stainless steel and aluminium bronze, and combinations of copper or brass and nickel, etc.
|The image to the right illustrates one method of joining the bi-metal blanks. The external ring is manufactured by a mulitple-die progressive tool, which pierces out the center hole prior to blanking from a strip. The raised outer edge of the blank, formed by “rimming” assists in reducing the coining pressure.|
|The inner, or “dump” is made very much like an ordinary coin blank, except for the special milling applied to the edge. When the two components are struck by the assembling press, the outer ring deforms to flow inside the milled indentations, providing efficient anti-twist locking and increasing the strength of the bond. This method is used by Krupp VDM, a leading German coin blank manufacturer. There are other ways of joining bi-metal blanks, with each manufacturer having their own preferred method.The force required to expel the inner ring in the Krupp method would utterly destroy the coin. Krupp reports the force needed to expel a 17 mm inner from a 25 mm outer ring would be 450 to 510 kPa, or a pressure of approximately 68 to 72 pounds per square inch.|