Nine Books on the Nature of Things

Separation of metals in aqua fortis, aqua regis and other similar strong corrosives is carried out in this way. Take a metal mixed and joined with another, and reduce it to very thin plates or very small grains. Put it into a separating vessel and pour on enough ordinary aqua fortis. Let them stand and steep together until all the metal is dissolved into a clear water. If it is silver containing gold, all the silver will dissolve into the water, while the gold will be calcined and settle at the bottom like black sand. Thus the two metals, gold and silver, will be separated. If, however, you wish to separate the silver alone by aqua fortis without distillation, make it sink to the bottom like sand, and bring it from dissolution back into calcination, then cast a small copper plate into that solution. The silver in the water will immediately settle, covering the bottom of the glass like snow, and will begin gradually to consume the copper plate.