of Bi-metallic Coins
Data extracted from the Royal Australian Mint's publication, Mint Issue,
The Complete Encyclopedia of U.S. and Colonial Coins - Breen,
and the WBCC's Online Image Library
|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.||
1625-49 Rose Farthing
1792 Silver Center Cent
|The U.S. Mint
with a bi-metallic cent to keep the size of the coin manageable and
the requirements of the Coinage Act of April 2, 1792. The first
center cents were struck December 17 to 18, 1792. Each was made by hand
at the Mint, workmen first making the copper blank and then punching
a small hole, and next inserting the silver plug, and finally striking
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.|
or "dump" is made very much like an ordinary coin blank, except for the
milling applied to the edge. When the two components are struck
the assembling press, the outer ring deforms to flow inside the milled
providing efficient anti-twist locking and increasing the strength of
bond. This method is used by Krupp VDM, a leading German coin blank
manufacturer. There are other ways of joining bi-metal blanks, with
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.
|Be certain to
Worldwide Bi-metallic Collectors Club Image Library. They have
is very likely the most extensive collection of bi-metallic coin and
images on the Internet.
The coin shown to the left is a New Zealand one year 50 cent, proof bi-metallic coin with a aluminum-bronze center and a silver outer ring.
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