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BOOK IX.
smelter observe whether the ore is melting too quickly or too slowly, or
whether it is flaming in scattered bits, and not uniting in one mass; in the
first case the ore is smelting too slowly and not without great expense; in
the second case the metal mixes with the slag which flows out of the
furnace into the forehearth, wherefore there is the expense of melting it again;
in the third case, the metal is consumed by the violence of the fire. Each of
these evils has its remedy ; if the ore melts slowly or does not come together,
it is necessary to add some amount of fluxes which melt the ore ; or if they
melt too readily, to decrease the amount.
The second reason is that each time that the furnace is opened with a
tapping-bar, it flows out into the forehearth, and the smelter is able to test
the alloy of gold and lead or of silver with lead, which is called stannum16.
When the tap-hole is opened the second or third time, this test shows us
whether the alloy of gold or silver has become richer, or whether the lead is
too debilitated and wanting in strength to absorb any more gold or silver. If
it has become richer, some portion of lead added to it should renew its
strength ; if it has not become richer, it is poured out of the forehearth that
it may be replaced with fresh lead.
The third reason is that if the tap-hole of the furnace is always open
when the ore and other things are being smelted, the fluxes, which are easily
melted, run out of the furnace before the rich gold and silver ores, for these
are sometimes of a kind that oppose and resist melting by the fire for a longer
period. It follows in this case, that some part of the ore is either consumed or is mixed with the accretions, and as a result little lumps of ore
not yet melted are now and then found in the accretions. Therefore when
these ores are being smelted, the tap-hole of the furnace should be closed
for a time, as it is necessary to heat and mix the ore and the fluxes at the
same time; since the fluxes fuse more rapidly than the ore, when the
molten fluxes are held in the furnace, they thus melt the ore which does not
readily fuse or mix with the lead. The lead absorbs the gold or silver, just
as tin or lead when melted in the forehearth absorbs the other unmelted
metal which has been thrown into it. But if the molten matter is poured
upon that which is not molten, it runs off on all sides and consequently does
not melt it. It follows from all this that ores rich in gold or silver, when put
into a furnace with its tap-hole always open, cannot for that reason be smelted
so successfully as in one where the tap-hole is closed for a time, so that during
this time the ore may be melted by the molten fluxes. Afterward, when the
tap-hole has been opened, they flow into the forehearth and mix there with
the molten lead. This method of smelting the ores is used by us and by the
Bohemians.
The three remaining methods of smelting ores are similar to each other
in that the tap-holes of the furnaces always remain open, so that the molten
metals may continually run out. They differ greatly from each other,
^Stannum (Interpretatio,—werck, modern werk). This term has been rendered throughout as " silver-lead " or " silver-lead alloy." It was the argentiferous lead suitable for cupellation. Agricola, in using it in this sense, was no doubt following his interpretation of its use
by Pliny. Further remarks upon this subject will be found in note 33, p. 473.
