Illustrated history of the United States mint / with a complete description of American coinage, from the earliest period to the present time

by Unknown

The Separating Room.

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 saltpetre, 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.