The two essential things regarding every piece of metal offered in payment of any dues were, first, the weight or quantity, next, the fineness or purity of the same. The process of weighing even the baser metals used in coining must be conducted by the careful use of accurate scales, with precise notes of the results. In precious metals, gold, silver, and their high grade alloys, a very small variation in the fineness makes a great difference in the value. Nothing is more essential than the accurate determination of the weight of the sample and of the metal obtained from it. It requires keen sight and most delicate adjustment in the hand which manipulates the Lilliputian scales of an Assayer’s table. The smallest weight used in the Mint is found in the Assay Room; it is the thirteen-hundredth part of a grain, and can scarcely be seen with the naked eye, unless on a white ground. The Assay Department is strictly a technical and scientific branch of the service. It has been practically under one regime, for the last fifty years. There have been but three Chief Assayers in that time, the only removals being by death, the only appointments by promotion. Its workmen are all picked men, selected from other parts of the Mint for special fitness and good character.
The Assaying Rooms.
These are on the second floor, in the southwest corner of the building. In one of these are fires, stills, and other appliances used in the delicate and complicated process of assay, by which the specific standard of the fineness and purity of the various metals are established and declared.
Assaying Gold.
The gold is melted down and stirred, by which a complete mixture is effected, so that an assay piece may be taken from any part of the bar after it is cast. The piece taken for this purpose is rolled out for the convenience of cutting. It is then taken to an assay balance (sensible to the ten-thousandth of a half gramme or less), and from it is weighed a half gramme, which is the normal assay weight for gold, being about 7.7 grains troy. This weight is stamped 1000; and all the lesser weights (afterwards brought into requisition) are decimal divisions of this weight, down to one ten-thousandth part.
Silver is next weighed out for the quartation (alloying), and as the assay piece, if standard, should contain 900-thousandths of gold, there must be three times this weight, or 2700-thousandths of silver; and this is the quantity used. The lead used for the cupellation is kept prepared in thin sheets, cut in square pieces, which should each weigh about ten times as much as the gold under assay. The lead is now rolled into the form of a hollow cone; and into this are introduced the assay gold and the quartation silver, when the lead is closed around them and pressed into a ball. The furnace having been properly heated, and the cupels placed in it and brought to the same temperature, the leaden ball, with its contents, is put into a cupel (a small cup made of burned bones, capable of absorbing base metals), the furnace closed, and the operation allowed to proceed, until all agitation is ceased to be observed in the melted metal, and its surface has become bright. This is an indication that the whole of the base metals have been converted into oxides, and absorbed by the cupel.
The cupellation being thus finished, the metal is allowed to cool slowly, and the disc or button which it forms is taken from the cupel. The button is then flattened by a hammer; is annealed by bringing it to a red heat; is laminated by passing it between the rollers; is again annealed; and is rolled loosely into a spiral or coil called a cornet. It is now ready for the process of quartation. This was formerly effected in a glass matrass, and that mode is still used occasionally, when there are few assays. But a great improvement, first introduced into this country by the Assayer in 1867, was the—“platinum apparatus,” invented in England. It consists of a platinum vessel in which to boil the nitric acid, which is to dissolve out the silver, and a small tray containing a set of platinum thimbles with fine slits in the bottom. In these the silver is taken out, by successive supplies of nitric acid, without any decanting as in the case of glass vessels. The cornets are also annealed in the thimbles; in fact there is no shifting from the coiling to the final weighing, which determines the fineness of the original sample by proportionate weights in thousandths. In this process extra care has to be taken in adding the proportions of silver, as the “shaking” of any one cornet, might damage the others.
Assaying Silver.
The process of assaying silver differs from that of gold. To obtain the assay sample, a little of the metals is dipped from the pot and poured quickly into water, producing a granulation, from portions of which that needed for assay is taken. In the case of silver alloyed with copper there is separation, to a greater or less degree, between the two metals in the act of solidification. Thus an ingot or bar, cooled in a mould, or any single piece cut from either, though really 900-thousandths fine on the average, will show such variations, according to the place of cutting, as might exceed the limits allowed by law. But the sudden chill produced by throwing the liquid metal into water, yields a granulation of entirely homogeneous mixture that the same fineness results, whether by assaying a single granule, or part of one, or a number.