This
department occupies the largest part of the west side of the
building, on the second floor. Here the gold and silver used
by the Mint in the manufacture of coin and fine bars are
separated from each other, or whatever other metals may be
mixed with them, and purified. It goes to this room after
having been once melted and assayed. In separating and
purifying gold, it is always necessary to add to it a certain
quantity of pure silver. The whole is then immersed in nitric
acid, which dissolves the silver into a liquid which looks
like pure water. The acid does not dissolve the gold, but
leaves it pure. The silver solution is then drawn off, leaving
the gold at the bottom of the tub. It is then gathered up into
pans and washed.
The
silver in the condition in which it is received from the hands
of the depositor, and generally filled with foreign
impurities, is melted and then granulated, after which the
whole mass is dissolved with nitric acid. The acid dissolves
the base metals as well as the silver. the liquid metals are
then run into tubs prepared for it, and precipitated, or
rendered into a partially hard state, by being mixed with
common salt water. After being precipitated it is called
"chloride," and resembles very closely new slacked
lime. By putting spelter or zinc on the precipitated chloride,
it becomes metallic silver, and only needs washing and melting
to make the purest virgin metal. The base metals remain in a
liquid state, and being of little value are generally thrown
away. The process of refining silver is of two kinds; that of
melting it with saltpeter, etc., which was known some
thousands of years since, and the modern process of dissolving
it in nitric acid, like the method of extracting it from gold
in the above described operation.
After
the separating process has been completed, the gold or silver
is conveyed to the Drying Cellar, where it is put under
pressure of some eighty tons, and all the water pressed out.
It is then dried with heat, and afterwards conveyed in large
cakes to the furnaces.