A text-book of assaying : $b for the use of those connected with mines.Beringer, C. (Cornelius)
Science
A text-book of assaying : $b for the use of those connected with mines.
Beringer, C. (Cornelius)
Assaying
The flask is then filled with water. A parting cup (size B) is
then placed over its mouth, like a thimble on the tip of a finger. This
cup is of unglazed porous earthenware of such texture that it absorbs
the last few drops of water left on drying; and with a surface to which
the gold does not adhere even on ignition. The gold should fall out
cleanly and completely on merely inverting the cup over the pan of the
balance. The flask and cup are then inverted so that the flask stands
mouth down in the cup; a little of the water from the flask flows into
the cup, but only a little. The gold falls steadily through the water
into the cup. When time has been allowed for even the finest of the gold
to have settled into the cup, the flask is removed. This is easiest done
under water. The cup, with the flask still resting in it, is dipped
under water in a basin; as soon as the neck of the flask is immersed the
crucible can safely be drawn away from under it and then lifted out of
the water. The flask should not be taken away first, for the rush of
water from it may easily sweep the gold out of the cup. The water in the
cup is then drained off and the cup is dried at not too high a
temperature; for if the last drop or two of water should boil there is
danger of spattering the gold out of the crucible. When it is dry, the
cup is heated on a pipe-clay triangle over a Bunsen burner, or on a slab
of asbestos in a muffle, to a dull-red heat. This brings the gold to
"colour"; that is, the loose tender dark coloured gold becomes bright
yellow and coherent; and is in a state fit to be transferred to the
balance and weighed. All unnecessary transferences must be avoided. As
soon as the cup is cool it may be inverted over the pan of the balance,
when the gold will fall out cleanly or, at the worst, a gentle tap with
the finger will be sufficient to detach it.
_Parting in test-tubes_, or in the smaller conical flasks, is used in
the assay of gold ores of ordinary richness. The work is exactly like
that just described in all its main features. Generally speaking much
less acid will be used; for example, in test-tubes and for small
buttons, 3 or 4 c.c. of each acid is quite enough. Again, the action
need not be so prolonged; 10 or 15 minutes in each acid is sufficient.
So, too, the heating may be less; it is very convenient to support the
test-tubes in a water-bath, or merely to rest them in a beaker of
boiling water; and there is no serious objection to doing this. A
smaller parting cup should be used; the A size is suitable. The button,
on the other hand, should be beaten thinner than is needed for the
larger partings. If the silver should be in excess and the gold becomes
much broken up, ample time should be given for subsidence from the
test-tube or flask into the parting cup.
Public-domain text, read in full here on John Shaqi.
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