Billon is an alloy of precious metal – most commonly silver, but also gold – with a majority base metal content, typically copper. It is used chiefly for making coins, medals, and token coins.
There is no fixed threshold for the silver content, but it is generally below 50% and can be as low as a few percent. Billon often has a dull grey appearance, darker than fine silver. It was widely used for low-denomination coinage in medieval and early modern Europe, when authorities sought to reduce the cost of coin production while retaining some silver content. It is often associated with monetary debasement, where the silver content of coins was progressively reduced over time.
The use of billon coins dates from ancient Greece. During the sixth and fifth centuries BC, some cities on Lesbos used coins made of 60% copper and 40% silver. Billon coins are perhaps best known from the Roman Empire, where progressive debasements of the Roman denarius and the Roman provincial tetradrachm in the third century AD led to declining silver and increasing bronze content. Eventually, by the third quarter of the third century AD, these coins were almost entirely bronze, with only a thin coating or wash of silver.
For collectors of European coinage before 1945, billon is most encountered in medieval pennies and deniers, and in the small-denomination issues of early modern German states, where silver content varied considerably from mint to mint and from year to year.