Tin. 
359 
the holes, is placed on the inside of the trough^ by which 
simple and effectual contrivance, the holes are prevented 
from being plugged up by the ore. In proportion as the 
tin- stone is reduced to the proper degree of fineness, it 
passes with the water through these holes into a labyrinth, 
of very simple construction ; here the oxyd of tin is sepa- 
rated from much of the lighter impurities, and by subse- 
quent washing on a wooden table, it is sufficiently dres- 
sed to be sent to the roasting furnace ; in this state it is 
called black tin, and is generally mixed in considerable 
proportion with mispickel, and iron and copper py- 
rites. 
It is now calcined at a low red-heat in a large reverbe- 
ratory furnace for several hours, in order to volatilize the 
arsenic and burn off the sulphur, (a part of this last after 
being acidified, combines with the oxyds of copper and 
iron.) The ore comes out of the roasting furnace, of a 
bright ochery red colour, owing to the decomposition 
and oxydation of the pyrites and mispickel, the oxyd of 
tin, if the operation has been well performed having under- 
gone not the least alteration. The ore is now washed a 
second time, by which nearly the whole of the impurities 
are separated. The water employed in this process, being 
considerably impregnated with sulphat of copper is re- 
served, and afterwards decomposed, by the addition of 
pieces of old iron. The next step is the reduction, pro- 
perly speaking; for this purpose a reverberatory fur- 
nace, about seven feet long and three and a half feet wide, 
is charged with seven hundred cwt. of roasted ore mixed 
with one- fifth of its bulk of culm (Welch small coal)* no 
lime or any other kind of flux being made use of ; the 
fire is kept up pretty brisk for about six hours, and the tin 
in proportion as it is reduced, sinks down to the bed of 
the furnace, being covered with a boiling hot bath of 
black scoriae. 
* Giving no smoke, T. C, 
