BOOK IX	191
	from them. After the metals have been removed from the latter ores the stone remains having been changed only slightly by the heat of the fire. We do not deny that slag forms from bismuth ore when it is smelted with silver ore. All slag is heavy when it first flows from the furnace because it contains some metal. After the slag is refined, as it usually is, it becomes lighter through loss of the metal. A second and third refining will make it still lighter. Tin slag is the lightest of all, especially after it has been re­fined for the third time. Slag is usually full of holes as if eaten away and the heavy ones are the most porous. It is usually black although that produced in refineries where silver is separated from copper is variegated and lead slag is light yellow. Some from the lead ore of Goslar is white, the color having come from the argentiferous pyrite in the ore. All have a good luster but only the thin lead slag that is tinted yellow like glass is transparent. All have the power of drying while that from iron is especially strong. When drunk with vinegar it counteracts aconite poison. Disks of stone, that is, slag, are taken from crucibles either with or with­out metal in the bottom part. The Germans call this stone by a name of their own6 just as the Greeks call it lithos. This stone commonly has a metal or metals mixed with it. The disks are broad at the top, tapering at the bottom and about an inch thick. Diphryges is made from these. There are four genera, all obtained from a stony cupriferous material that has been dried, burnt or smelted. The first genus comes from a mud taken from a certain cave in Cyprus. This mud is first dried and then burnt by surrounding it with brush. The name diphryges comes from the Greek meaning twice burnt, that is, once by the heat of the sun, once by the heat of fire. Dioscorides writes that it is produced only in Cyprus. Another genus resembles a certain sediment and is the excrement from copper smelting. After the molten copper flows from the furnace into the crucible this remains. It was found by Galen in Cyprus spread on the streets and between buildings used for storing metals. The smelter director said that after the cadmia had been taken out the material was valueless and similar to the ash left by burning wood. The third genus is obtained from cupriferous pyrite that artificers place on a hearth and then burn, just as they would lime, until it is completely changed to the color of red ocher. It is then removed from the hearth and preserved. It is also obtained from a cupriferous stone that is first dried in the air and then transferred to pits where it is burned. The diphryges forms around the sides and on the bot­tom. The fourth genus is obtained from stones called orbes. If these have resulted from the smelting of pyrite they only need to be dried twice or at the most three times but if they are natural stones they first must be dried in the air, then smelted in a furnace and finally burned five times in another furnace so that the material can be called properly έπταφρυγής. Even with all this treatment it still lacks the qualities of true diphryges.
	6 Stein.