This book describes smelting, which Agricola describes as perfecting the metal by fire. The design of furnaces is first explained.
These are very similar for smelting different metals, constructed of brick or soft stone with a brick front and mechanically driven bellows at the rear. At the front is a pit called the fore-hearth to receive the metal. The furnace is charged with beneficiated ore and crushed charcoal and lit. In some gold and silver smelting a lot of slag is produced because of the relative poverty of the ore and the tap hole has to be opened at various times to remove different slag materials. When the furnace is ready, the forehearth is filled with molten lead into which the furnace is tapped. In other furnaces the smelting can be continuous, and lead is placed into the furnace if there is none in the ore. The slag is skimmed off the top of the metal as it is tapped. The lead containing the gold is separated by cupellation, the metal rich slags are re-smelted. Other smelting processes are similar, but lead is not added. Agricola also describes making crucible steel and distilling mercury and bismuth in this book.