Ancient coins

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 (Fig 1).  

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.)