In the assays for the mint in this coun try, two assays are always made of the same mass of metal, and no sensible dif ference between the weights of the but tons is allowed to pass in scales that turn with the part of a grain, troy. they differ, the assay is repeated.
The process is considered as well per formed, when the button adheres but slightly to the cupel ; when its shape is very considerably globular above and be low, and not flattened at the margin ; when it is quite clear and brilliant, and not folded or spotted with any remaining li tharge; and especially when the surface is disposed in minute scales, the effect of a hasty crystallization, which gives it a play of light very different from that of a perfectly even surface of a white metal. The scales are of a pentagonal form, slightly depressed at the centre. When any alloy remains in the silver, the sur face appears, under the microscope, smooth, as if varnished, and scarcely at all scaly in texture.
In the common assays of plate, either gold or silver, copper is the alloy usually met with ; if the fine metal be nearly pure, the cupel round the bottom is only stained yellow by the litharge ; if copper is contained, it leaves a brown stain. The other metals, except bismuth, scarcely penetrate the substance of the cupel, but remain on the edges of its cavity, in the form of coloured scoriae ; of which iron is black, tin grey, and zinc a dull yellow.
The management ofthafire is a point of great consequence in cupellation. When silver is kept in fusion in a very high heat a portion of it is volatilized, as Mr. Tillet found that a button of pure silver, kept in a very high heat, lost a twentieth part of its weight ; which loss would cause a great error in assaying. On the other hand, when the fire is too slack, the Mb arge is not absorbed by the cupel, but lies on the surface as a red scoria. The heat is known to be too great, when the cupel can scarcely be distinguished from the muffle, and the ascending fume can scarcely be seen for the dazzling heat. Towards the end of the operation the fire should be gradually increased, for in pro portion as the lead is abstracted from the alloy, it becomes less easy of fusion ; and at last a heat fully equal to the melting of pure silver is required.
As the cupellation requires a free ac cess of air, as well as an high degree of heat, the stopper of the muffle is always removed as soon as the metal is put into the hot cupel, to allow a current of air to pass through the muffle : but to prevent this from cooling the muffle too fast, several round pieces of charcoal are heaped up, in front of the muffle, on an iron plate placed there to hold them, which burn with sufficient force to heat the air as it passes to the cupels. The furnace should be made so that the heat of the fuel within may be readily increas ed or diminished, but at the same time so that it can be kept up with steadi ness.
The time taken up in making one assay of silver, is generally from 15 to 25 mi nutes. The proportioning of the lead to the supposed alloy in the silver to be as sayed is of great importance ; if too little is employed, some of the alloy will remain in the mass ; but if too much is used, some of the silver will be wasted ; for Mr. Til let has found, that when the proper quan tity of lead is used, it carries down a por tion of the silver into the cupel, which he has ascertained by accurate experiments to amount toT7Y of the lead in the cu pel ; whereas the natural admixture of silver in lead is only But when an excess of lead is employed for cupella lion, this loss of silver is somewhat great er, though it does not increase in the ra tio of the excess of lead ; for ten parts of lead to a given alloy will not carry down twice as much silver as five parts, though the difference of loss will be very sensi ble. When the litharge carried into the cupel is reduced to reguline lead, on be ing cup elled a second time, it will yield a button of silver, fully equal to the loss of this metal in the first assay. In all these reductions the silver appears equally dis tributed through the lead, for Mr. Tillet found that separate globules of the lead, spurted out by accident upon an empty cupel in the muffle, each left a minute atom of silver lying upon the spot where the globules had scorified.