~--Ores of this class may be easily
recognized, either by the appearance of the minerals they contain or by
the odour of sulphurous oxide (SO_{2}) which they evolve when roasted on
a spatula. The sulphides most commonly present, in addition to the
sulphurized minerals of silver, are pyrites, galena, blende, and
mispickel. When they are present in only a moderate amount, their effect
is simply to increase the weight of the button of lead; and this is
easily counteracted by reducing the amount of flour, or by omitting it.
When in larger amounts, they not only yield large buttons, but also
render the metal sulphury, sometimes even giving a button of regulus
instead of lead. This last evil may be remedied (1) by putting in a rod
of iron as soon as the charge has fused, or (2) it may be counteracted
by a proper addition of nitre, or (3) when the sulphides present are
only those of iron or copper the sulphur may be removed by calcining,
and the ore converted into one of the class containing metallic oxides.
The calcination is effected as follows:--Weigh up 20 grams of the
powdered ore and place it in a wide-mouthed crucible sufficiently large
to perform the subsequent melting down in. The roasting must be done at
a gentle heat at first, so as to avoid clotting: the mouth of the
crucible should project considerably above the coke, and should slope
forward towards the worker.
Pages:
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180