The first metal coins, which appeared in the Middle East around 700 BC, were made of gold and silver. These two metals were accessible at a time when the chemistry and technology of metal extraction from binary metal compounds were unknown. The chemical inactivity of gold meant that ancient gold coins, even after centuries of exposure to the atmosphere, or of burial and exposure to the corrosive chemicals in soil, are to be found in as good a condition as they were when they were in use.
Over the centuries, the base metals Fe, Cu, Ni, Zn, Al, Sn and Pb have been used as minor alloy constituents or as principal components in coins. Generally, a metal must be reasonably hard wearing to ensure the economic lifetime of a coin and must retain an acceptable appearance on exposure to the atmosphere. And it must not be too expensive. As a soft and expensive metal, gold is now usually alloyed with copper to give a hard-wearing alloy. British gold sovereigns, which are still minted, are made from 2 carats of alloy and 22 carats of gold, and comprise 91 per cent Au, 8.3 per cent Cu. (A carat is a measure of the purity of gold, pure gold being 24 carat.)
Platinum made only a brief appearance as a coinage metal. A few high-value coins in Russia were made in the mid-19th century, at a time when platinum had recently been found in the Ural mountains. For a while the value of platinum fell below that of gold, and this gave rise to the emergence of counterfeit sovereigns in the 1870s. These were made from platinum and had a thin layer of electrochemically deposited gold. Bronze (Cu-Sn alloy) farthings issued between 1897 and 1917 were darkened using Na2S2O3, resulting in a surface layer of copper sulphide. This was done to avoid confusion between a newly minted farthing and a gold half-sovereign, the latter being worth 480 times the former. Bronze pennies issued between 1944 and 1946 were similarly treated to deter hoarding of new pennies at the end of World War II.
Roman gold coin 43 AD
As chemical and technological knowledge improved, so did demands on the durability of coins increase. Modern conditions require that the metals and alloys used in coin production are extremely pure. Very small traces of non-metals, such as carbon, sulfur or phosphorus can cause brittleness, which cannot be tolerated in modern coin-striking equipment which operates at high speeds and pressures.
In the past 60 years electroplated iron and steel coins have appeared in many countries. These were a response to the increased costs of providing huge quantities of low-value coins for the prosperous consumers of the post-war years. From 1992 British 'bronze' coins have been made of copper-plated steel.
https://rbi.org.in/Scripts/ic_coins_5.aspx
