Coinage metals

The coinage metals comprise those metallic chemical elements and alloys which have been used to mint coins. Historically, most coinage metals are from the three nonradioactive members of group 11 of the periodic table: copper, silver and gold. Copper is usually augmented with tin or other metals to form bronze. Gold, silver and bronze or copper were the principal coinage metals of the ancient world, the medieval period and into the late modern period when the diversity of coinage metals increased. Coins are often made from more than one metal, either using alloys, coatings (cladding/plating) or bimetallic configurations. While coins are primarily made from metal, some non-metallic materials have also been used.

History
Early coinage made from metal came into use during the Axial Age in the Greek world, in northern India, and in China, as coins became a widespread embodiment of money. Bronze, gold, silver and electrum (a naturally occurring pale yellow mixture of gold and silver that was further alloyed with silver and copper) were used. Silver coins from about 700 BC, are known from Aegina Island. Early electrum coins from Ephesus, Lydia date from about 650 BC. Ancient India in 6th century BC, was also one of the earliest issuers of coins in the world.

The gold Croeseids, issued in Lydia, were the first true gold coins with a standardized purity for general circulation. The gold and silver Croeseids formed the world's first bimetallic monetary system, c. 550 BC. The Persian daric was also an early gold coin which, along with a similar silver coin, the siglos, (from Ancient Greek σίγλος, Hebrew שֶׁקֶל (shékel)) represented the bimetallic monetary standard of the Achaemenid Persian Empire. These coins were also very well known in the Persian and Sassanids era, most notably, in Susa and in Ctesiphon.

Precious metals were used historically in commodity money and are found in bullion coins and some collectable coins. Coins functioning as fiat money are now made from a larger variety of base metals.

Multiple metals
Coins may be composed of multiple metals using alloys, coatings, or bimetallic forms. Coin alloys include bronze, electrum and cupronickel. Plating, cladding or other coating methods are used to form an outer layer of metal and are typically used to replace a more expensive metal while retaining the former appearance. For example, United States cents since 1982 are zinc with copper-plating, and thus retain their prior copper look while having a less expensive composition. Coatings may also be used as a form of debasement in commodity money. Bimetallic coins are used for their distinctive appearance and generally have an outer ring of one metal or alloy surrounding a center of contrasting metal.

Requirements for a coinage metal
Coins that are intended for circulation may circulate for decades and thus must have excellent resistance to wear and corrosion. Achieving this goal typically necessitates the use of base metal alloys. In addition, some metals, such as manganese, are unsuitable as they are too hard to take an impression well or are apt to wear out stamping machines at the mint.

When minting coins, especially low denomination coins, there is a risk that the value of metal within a coin is greater than the face value, leading to negative seigniorage. This leads to the possibility of smelters taking coins and melting them down for the scrap value of the metal. Pre-1992 British pennies were made of 97% copper; but as of 2008, based on the price of copper, the value of a penny from this period is 1.5 new-pence. Modern British pennies are now made of copper-plated steel.

Cupronickel, a base metal alloy with varying proportions of copper and nickel, was introduced as a cheaper alternative for silver in coinage. Cupronickel, most commonly 75% copper, 25% nickel, has a silver color, is hard wearing and has excellent striking properties, essential for the design of the coin to be pressed accurately and quickly during manufacture. However, in the 21st century with the prices of both copper and nickel rising, it has become common to experiment with various alloys of steel, often stainless steel as an even cheaper alternative. For example, in India some coins have been made from a stainless steel that contains 82% iron, 18% chromium, and many other countries that have minted coins that contain metals now worth nearly the coin face-value, are experimenting with various steel alloys. Italy had earlier experimented with acmonital, a stainless steel alloy, for its coins.

A number of more exotic metals have been used to make demonstration or fantasy coins which have not been used to make monetized coins for a nation-state. Some of these elements would make excellent coins in theory (e.g. zirconium). More expensive metals that are intrinsically valuable as commodities are less practical as coinage due to their cost, but could be used for bullion coins.

Chemical elements used in circulating coins
In 1992, twenty-four chemical elements used in world coinage were documented by Jay and Marieli Roe in an award-winning exhibit and publication: aluminum, antimony, carbon, cobalt, copper, gold, hafnium, iron, lead, magnesium, molybdenum, nickel, niobium, palladium, platinum, rhenium, silver, tantalum, tin, titanium, tungsten, vanadium, zinc and zirconium. Chromium and manganese, however, were not mentioned, even though both elements had been used in common circulation coins (Canada wartime V nickels and US wartime Jefferson nickels, respectively) long before the article's publication.

Non-circulating
Chemical elements used in non-circulating commemorative, demo, bullion or fantasy coins, medals, patterns, and trial strikes:


 * Cadmium: 1828 medal made by G. Loos for the marriage of Heinrich von Dechen, "of Silesian cadmium".
 * Cobalt: 2005 Cameroon 750 CFA francs struck in cobalt-plated iron.
 * Hafnium: Fred Zinkann demo coin.
 * Iridium: 2013 $1/25$ oz 10 franc bullion coin issued by Rwanda as part of "Noble Five" precious metals set.
 * Molybdenum: Demo coin, Fred Zinkann. 2008 1 tr oz coins by Coins By Design, Murray Buckner (mintage 250).
 * Niobium: Austria has issued a number of bimetallic 25 euro coins with a niobium center.
 * Palladium: First issued 1966 by Sierra Leone. Also presentation sets from Tonga and bullion coins of various countries.
 * Rhenium: Fred Zinkann fantasy pieces, Pope Matthew Triple Ducat and Malvinas 5 Australes
 * Rhodium: 2014 $1/25$ oz 10 franc bullion coin issued by Rwanda as part of "Noble Six" precious metals set. Also Cohen Mint bullion round.
 * Ruthenium: 1967 $1/2$ Hau from Tonga was 98% palladium and 2% ruthenium.
 * Selenium: 1862 medal in UK Science Museum, commemorating Berzelius, discoverer of the element.
 * Silicon: Privately struck US quarter patterns dated 1964 (Pollock-5380) in nickel-silicon alloy.
 * Tantalum: Used in a bimetallic silver-tantalum coin from Kazakhstan.
 * Tellurium: 1896 Hungarian mining medal. Reproductions exist from 1975.
 * Titanium: First issued 1999 by Gibraltar. Austria has made bimetallic silver/titanium commemoratives.
 * Tungsten: While Tungsten alloys are too hard for practical use, a few private demos have been struck for experimentation, e.g. Fred Zinkann US half eagle patterns.
 * Uranium: Two types of a German medal of native uranium.
 * Vanadium: 2011 1 Troy ounce coins by Coins By Design, Murray Buckner (mintage 20).
 * Zirconium: 2012 1 Troy ounce coins, including 50 black & 50 Rainbow, by Coins By Design, Murray Buckner (mintage 500).

Element Series
Beginning in 2006, Dave Hamric (Metallium) has been attempting to strike "coins" (technically tokens or medals, about the size of a US cent) of every stable chemical element. He has struck tokens of the following elements, apparently not only metals: aluminium, antimony, barium (reactive, sealed in glass capsule), beryllium, bismuth, boron (mixed with binder, sealed in resin cast), cadmium, calcium (reactive, sealed in glass capsule), carbon (mixed with binder, sealed in resin cast), cerium (reactive, sealed in glass capsule), chromium, cobalt, copper, dysprosium, erbium, europium (reactive, sealed in glass capsule), gadolinium, gallium, gold, hafnium, holmium, indium, iridium, iron, lanthanum (reactive, sealed in glass capsule), lead, lutetium, magnesium, mercury (sealed in resin cast, containing the expected coin-weight of liquid mercury), molybdenum, neodymium (reactive, sealed in glass capsule), nickel, niobium, palladium, phosphorus (mixed with binder, sealed in resin cast), platinum, praseodymium (reactive, sealed in glass capsule), rhenium, rhodium, ruthenium, samarium (reactive, sealed in glass capsule), scandium, selenium, silver, strontium (reactive, sealed in glass capsule), sulfur, tantalum, tellurium, terbium, thallium (extremely poisonous; lead token clad on one side with thallium foil and sealed in resin), thulium, tin, titanium, uranium (not offered for sale), vanadium, ytterbium, yttrium, zinc, zirconium.