Lead. 
193 
of smelting lead ore there is no appearance of the sulphur 
it contains, it is consumed by the flame of the furnace, as 
soon as it is separated from the ore ; an attentive observer 
may, indeed, by looking into the furnace, distinguish a 
diversity in the colour of the flame, at different periods of 
the process ; during the first three or four hours after the 
ore is put into the furnace, the flame has a bluish tint, pro- 
ceeding no doubt from the sulphur which, in being sub- 
limed from the ore, is inflamed : after all the sulphur is 
separated from the ore, the flame has a whitish cast, and 
then, and not before, the fire may be raised for finishing 
the operation ; for if the fire be made strong before the 
sulphur be dispersed, the quantity of lead is less, probably^ 
for two reasons ; the sulphur unites itself in part to the 
lead which is formed, and by this union becomes insepa- 
rable from it ; for the sulphur cannot without much diffi- 
culty be separated from an artificial mixture of lead and 
^sulphur, when the two ingredients have been fused to- 
gether.— 2. The sulphur, whilst it continues united to 
the lead in the natural ore, renders the ore volatile, so that 
in a strong heat a great portion of it is driven off Hence^ 
very sulphureous ores should be roasted for a long time 
with a gentle heat, and in this proper management of the 
fire, principally consists the superiority of one smelter 
above another. 
An old lead smelter informed me that he had often re- 
duced a ton of ore to sixteen hundred weight by roasting 
it, but that he did not obtain more metal from it by a sub- 
sequent fusion, than if he had fluxed it without a previous 
roasting. This may be true of some sorts of ore, but it 
is not true of very sulphureous ores. Indeed the fire may 
be so regulated in a cupola furnace, as to make it an- 
swer the purpose of a roasting and a smelting furnace at 
the same time, I have seen much lead lost by smelting 
a ton of sulphureous ore in eight hours, which might 
Vol. III. " Bb 
