BOOK IX.                                          407
	third smelter, as I have said, adds to the tertiary cakes when he re-smelts them, while from the " final " cakes, thrice roasted and re-smelted, is made black copper46. The rudis copper from which pure copper is made, if it contains little silver or if it does not easily melt, is first smelted in the third furnace of which the tap-hole is always open ; and from this are made cakes, which after being seven times roasted are re-smelted, and from these copper is melted out ; the cakes of copper are carried to a furnace of another kind, in which they are melted for the third time, in order that in the copper " bottoms " there may be more silver, while in the " tops " there may be less, which process is explained in Book XI.