As a general rule, if more than 5 grams of ore must
be taken, the crucible assay should be adopted.
[Illustration: FIG. 38.]
~Scorification Assay.~--The charge of ore is usually 3 grams, sometimes
5; the lead varies from 30 to 70 grams, and the quantity of soda, borax,
or powdered glass added varies from 0.3 to 3 or 4 grams. It is generally
recommended to have the lead granulated,[9] and to mix the ore with
about half of it in the scorifier; then to put on the rest of the lead;
and finally to sprinkle the borax or glass on the top. It answers just
as well, however, to use the lead in the shape of foil, and wrap the ore
up in it; and if the ore contains much sulphur, the borax may with
advantage be added (wrapped in a little tissue paper) some five or ten
minutes after the operation has started.
[Illustration: FIG. 39.]
The process of scorification is as follows:--A scorifier (fig. 38) of
convenient size having been selected (one 2-1/2 inches across is most
generally useful), it is dried at a gentle heat for about ten minutes.
The charge is then put into it, and it is introduced, with the help of a
scorifier tongs (fig. 39), into a muffle heated considerably above
redness. The muffle is then closed, and when the metal has melted down,
it is opened, but the temperature is kept up. A ring of slag will, after
a time, form around the metal, and when this appearance (known as the
eye) presents itself, the temperature may be lowered.
Pages:
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174