Billon is an alloy of equal parts silver and copper. Historically, this was used to mint coins - though it is possible to do the same in this game, they will not be used as currency.
Two billon bars can be made at a smelter from 1 chunk of silver ore and 1 of copper ore, or 1 silver bar and 1 copper bar. One unit of tetrahedrite counts as both copper and silver ore at the same time, and will yield two bars of billon all by itself; this may be considered a bug. This recipe will only work when no silver is available to the smelter, otherwise silver nuggets will be used, the tetrahedrite acting as copper.
Billon can be used to make furniture and 'other objects' at a Metalsmith's forge.
temperature values unknown, used copper