History of Bimetallic Coins
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, although the English Rose Farthing 1625-1649, during the reign of Charles I, had a brass wedge inserted into the copper as an anti-forgery device.
Throughout the 19th and 20th Centuries many tokens and medallions have been struck by various countries, but the first bi-metallic coin to be widely used in modern times was the 500 Lira issued by the Italian government in 1982. Today, more than 117 countries have issued bi-metallic coins and the number is growing. 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 big worry when these coins were introduced was that they would fall apart in use, but that hasn't happened. Hundreds of millions of them are in circulation around the world, and that problem just has not occurred. There are very few mint errors of missing rings or missing centers, such as the first year of the Canadian polar bear two-dollar coins which made the front page in World
Coin News.
Coatings and Plating
An early method of producing bimetallic coins was to put a coating of one metal onto a core of another metal. This was often done to disguise a debased coinage, eg by using a copper core and "silver dipping" it. Rome first issued the antoninianus as a silver coin of 2 denarii, but only worth 1½ denarii. This coin was gradually debased and reached its lowest during the reign of Gallienus (253-68) when it was a copper coin with a thin silver coating which quickly wore off. This was possibly achieved by dipping the copper blanks in a silver nitrate solution so that some of the surface copper would be electrolysed and go into solution and silver metal would be deposited on the blank.
The later silver coins of Henry VIII of England were gradually debased until the last issue (1544-47) "silver" coins were struck from an alloy of one part silver and two parts copper (ie .333 fine). To make these coins appear to contain more silver than they actually did, the blanks were probably either silver washed (as above) or they may have been soaked in a weak acid solution to dissolve some of the surface copper. However, with wear from circulation, the richer silver surface was worn away, particularly on the high spots such as the king's nose, and this led to his nickname of "old copper nose". A number of modern coins have also been plated, probably electroplated
Wedges
During the reign of Charles I of England, a new series of royal farthing tokens was issued. These coins were made from a rod of copper with a wedge of brass that had been hammered into a groove that had been cut into the rod. Blanks were then sliced off the rod and the farthings struck from these blanks. As these pieces had a rose on the reverse (instead of a harp that had appeared on all the earlier issues of royal farthing tokens) they are usually referred to as rose farthing tokens. They were issued from 1636 until 1644 (when they were discontinued during the civil war). Thus these were bimetallic coins of copper with a brass wedge.
Plugs
After the restoration of Charles II of England, a major reform of the currency took place. This included the issue of copper halfpennies and farthings of full weight. However the price of tin had fallen placing the tin mines of Cornwall in crisis and the king, as the holder of the Duchy of Cornwall, was losing money. In order to help the tin industry (and the king's finances), it was decided to strike all halfpennies and farthings from tin. To help prevent counterfeiting, a square copper plug was inserted in the centre of the blanks prior to striking. Thus these were bimetallic coins of tin with a copper plug and they were issued from 1684 until 1692 during the reigns of Charles II, James II, and William and Mary.
Patterns with Rings, Plugs, etc
England also struck a number of bimetallic patterns. During the Commonwealth and the Protectorate (in the 1650s), and again during the reign of Charles II (particularly around 1665 and 1676), a significant number of patterns (both official and private) for base metal coinage were bimetallic. These pieces were struck from combinations such as copper with an inner brass ring, copper obverse and brass reverse, brass with a copper centre, tin with an inner brass ring, and tin with an inner copper ring.
Again in William and Mary's reign, trial pieces for a proposed coinage in "mixed metals" were struck (dated 1692) from copper with a tin centre and copper with a thin ring of brass let into either the obverse or reverse. However, in 1694, England reverted to copper for its base metal coinage. England appears to have then abandoned their bimetallic coinage experiments until 1844 when Joseph Moore (a Birmingham die sinker) produced his "model" pennies and halfpence. These were struck from copper with a white metal centre. They were a private proposal to replace the heavy copper coinage then in circulation.
Cladding
The U.S. 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 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
Outer Rings
Another type of bimetallic coin is where there is a large core of one metal with a ring (of another metal) around the core. This type of bimetallic coin has been increasing in use over the past decade.
Clad Centre with an Outer Ring
Another type of "bimetallic" coin is where there is a large clad core of three metals with a ring (of another metal) around the core. Although this is not actually a bimetallic coin, they appear to be, and are probably being considered (and collected) as bimetallic coins.
Production of bimetallic coins
The image to the below 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.
If two or more sheets or strips of metal are laid on top of each other with a sheet of high explosive above and below them, and the explosive is detonated, the sheets or strips of metal are welded together and appear (from the edge) to look like a sandwich. The resultant metal sandwich is then referred to as a "clad" metal and it can be worked in similar ways to other metals, including the various processes to make coins.
The Federal Republic of Germany (ie West Germany) used several clad metal combinations for their coinage. Bronze clad steel was used for 1 pfennig (1948-49) and 2 pfennig (1967-95) coins. Brass clad steel was used for 5 pfennig (1950-95) and 10 pfennig (1949) coins. Cupro-nickel clad nickel was used for 2 deutsche mark (1969-95) and 5 deutsche mark (1975-95) coins. Russia (after the break-up of the USSR) used brass clad steel for their 1992 1 rouble and 5 rouble coins. Due to the rising price of silver in the mid to late 1960's, the United States of America decided to stop using silver in their circulation coins and, from 1965, use a clad metal instead. They chose outer layers of cupro-nickel with a core of copper. Kennedy half-dollar coins (1965-70) and proof Eisenhower dollars were struck from a clad metal of two different silver alloys that had outer layers of .800 fine silver and a core of .209 fine silver giving a net content of .400 fine silver.
Few images of Bimetallic coins :
Images of Tri metallic coins :
