BOOK IX.

386

however, for the tap-hole of the first of this kind is deeper in the furnace andnarrower than that of the third, and besides it is invisible and concealed.It easily discharges into the forehearth, which is one and a half feet higherthan the floor of the building, in order that below it to the left a dipping-potcan be made. When the forehearth is nearly full of the slags, which well upfrom the invisible tap-hole of the furnace, they are skimmed off from the topwith a hooked bar ; then the alloy of gold or silver with lead and the meltedpyrites, being uncovered, flow into the dipping-pot, and the latter are made intocakes ; these cakes are broken and thrown back into the furnace so that alltheir metal may be smelted out. The alloy is poured into little iron moulds.

The smelter, besides lead and cognate things, uses fluxes which combinewith the ore, of which I gave a sufficient account in Book VII. The metalswhich are melted from ores that fuse readily in the fire, are profitable becausethey are smelted in a short time, while those which are difficult to fuse arenot as profitable, because they take a long time. When fluxes remain in thefurnace and do not melt, they are not suitable ; for this reason, accretions andslags are the most convenient for smelting, because they melt quickly. It isnecessary to have an industrious and experienced smelter, who in the firstplace takes care not to put into the furnace more ores mixed with fluxes thanit can accommodate.

The powder out of which this furnace hearth and the adjoining fore-hearth and the dipping-pot are usually made, consists mostly of equal pro-portions of charcoal dust and of earth, or of equal parts of the same and ofashes. When the hearth of the furnace is prepared, a rod that will reach to theforehearth is put into it, higher up if the ore to be smelted readily fuses, andlower down if it fuses with difficulty. When the dipping-pot and forehearthare finished, the rod is drawn out of the furnace so that the tap-hole is open,and through it the molten material flows continuously into the forehearth,which should be very near the furnace in order that it may keep very hot andthe alloy thus be made purer. If the ore to be smelted does not melt easily, thehearth of the furnace must not be made too sloping, lest the molten fluxesshould run down into the forehearth before the ore is smelted, and the metalthus remain in the accretions on the sides of the furnace. The 'smelter mustnot ram the hearth so much that it becomes too hard, nor make the mistakeof ramming the lower part of the mouth to make it hard, for it could notbreathe 17 , nor could the molten matter flow freely out of the furnace.The ore which does not readily melt is thrown as much as possible to theback of the furnace, and toward that part where the fire burns veryfiercely, so that it may be smelted longer. In this way the smelter may directit whither he wills. Only when it glows at the part near the bellows’ nozzledoes it signify that all the ore is smelted which has been thrown to the side ofthe furnace in which the nozzles are placed. If the ore is easily melted, oneor two wicker baskets full are thrown into the front part of the furnace so thatthe fire, being driven back by it, may also smelt the ore and the sows that

17 Expirare ,—to exhale or blow out.