Billon (alloy)

Billon is an alloy of a precious metal (most commonly silver, but also gold) with a majority base metal content (such as copper). It is used chiefly for making coins, medals, and token coins.
The word comes from the French "bille", which means "log".
History.
The use of billon coins dates from ancient Greece and continued through the Middle Ages. During the sixth and fifth centuries BCE, some cities on Lesbos used coins made of 60% copper and 40% silver. In both ancient times and the Middle Ages, leaner mixtures were adopted, with less than 2% silver content.
Billon coins are perhaps best known from the Roman Empire, where progressive debasements of the Roman "denarius" and the Roman provincial "tetradrachm" in the second century CE led to declining silver and increasing bronze content in these denominations of coins. Eventually, by the third quarter of the second century CE, these coins were almost entirely bronze, with only a thin coating or even a wash of silver.