smelters by which of these methods they can obtain the most metal from
any one ore. Moreover, while sometimes there are many methods of
smelting the same ore, by which an equal weight of metal is melted out, yet
one is done at a greater cost and labour than the others. Ore is either melted
with a furnace or without one ; if smelted with a furnace the tap-hole is either
temporarily closed or always open, and if smelted without a furnace, it is done
either in pots or in trenches. But in order to make this matter clearer, I will
describe each in detail, beginning with the buildings and the furnaces.
