From this sample the weight of 1115 thousandths is taken; this is dissolved in a glass bottle with nitric acid. The standard solution of salt is introduced and chloride of silver is the result, which contains of the metallic silver 1000 parts; this is repeated until the addition of the salt water shows but a faint trace of chloride below the upper surface of the liquid. For instance: if three measures of the decimal solution have been used with effect, the result will show that the 1115 parts of the piece contained 1003 of pure silver; and thus the proportion of pure silver in the whole alloyed metal is ascertained. Extensive knowledge and experience are required in such matters as making the bone-ash cupels, fine proof gold and silver, testing acids, and other special examinations and operations. The Assayer must, himself, be familiar with all the operations of minting, as critical questions are naturally carried to him. The rendering of decisions upon counterfeit or suspicious coins has long been a specialty in this department. Once a year the President appoints a scientific commission to examine the coins of the preceding year. There has never yet been a Philadelphia coin found outside of the tolerance of fineness.
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.
The Melting Rooms
are on the first floor, in the west side of the building. Here all the metal used in coining is alloyed, melted and poured into narrow moulds. These castings are called ingots; they are about twelve inches long, a half-inch thick, and vary from one to two a half-inches in breadth, according to the coin for which they are used, one end being wedge-shaped to allow its being passed through the rollers. The value of gold ingots is from $600 to $1,400; those of silver, about $60. The fine gold and silver bars used in the arts and for commercial purposes, are also cast in this department.
CASTING INGOTS.
