Billon is a silver alloy containing a low proportion of silver mixed with a base metal, typically copper. There is no fixed threshold, but the silver content is generally below 50%, and can be as low as a few percent. It often has a dull grey appearance, often darker than fine silver. Billon 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.
