When the cupel has received the above proportion of metal, the addition of the alloy ceases, and the silver is allowed to purify. The litharge which passes off towards the close of this process will be richer in silver than in the former one; consequently it is found best in practical metallurgical operations to treat in a special manner the last part of silver cupelling on the large scale, for it needs very careful management indeed to secure all the silver, especially to do so in a fine state. Towards the completion of the process the fire should be increased considerably, in order to keep the silver thoroughly melted, and also to oxidize and completely remove every trace of lead that is possible. As it begins to purify itself from the remaining lead a characteristic brightness will be perceived. When this takes place the fire must be lowered, the wind or blast stopped, and the metal left to cool gradually. This latter proceeding is of some importance, as a too sudden cooling of the surface causes the interior of the metal to expand and shoot, by which means little globules of silver may be lost; therefore it should be allowed to cool very slowly.
The iron ring encircling the cupel with its contents may now be drawn from beneath the arch of the furnace, and the cake of silver taken from its bed in the bone-ash which formed the vessel, and cleaned of any impurity; when it may be re-melted in a plumbago crucible, and cast into ingot moulds. These moulds should be made of iron, and should always, when used for this purpose, be warmed and greased a little, previous to the introduction of the melted material, to prevent the metal from spitting and adhering to it. If skilfully treated during the process of cupellation, the desilvered lead seldom contains more than ·002 to ·003 per cent. of silver to the test assay of 200 grs., or between six and ten pennyweights to the ton, beyond which point it is unprofitable to carry on the operation.
The litharge which is formed and passes off during the process gradually grows richer in silver towards the end of the cupellation. It probably contains after concentration about thirty to forty ounces of silver to the ton of litharge. This is again subjected to the several operations of the same kind for the recovery of the silver.
It is somewhat remarkable that the present method of recovering and purifying this metal bears a strong resemblance to that employed in ancient times, and which is spoken of in the Holy Scriptures by the prophet Ezekiel (xxii. 18 and 20): “Son of man, the house of Israel is to me become dross: all they are brass, and tin, and iron, and lead, in the midst of the furnace; they are even the dross of silver.” And also, “As they gather silver, and brass, and iron, and lead, and tin, into the midst of the furnace, to blow the fire upon it, to melt it; so will I gather you in mine anger and in my fury, and I will leave you there, and melt you.” The celebrated metallurgist Dr. Lamborn says, “Only those who have seen, beneath the glowing arch at the smelting works, flames surging wave after wave across the surface of the liquid metal, carrying all the substances, here called dross, from the pure silver; and only those who have heard the roar of the fiery blast, that ceases neither day nor night, until its task of purification is accomplished,—can appreciate the terrible force of the figure made use of by the prophet.” According to the above scriptural passage it is evident that the ancients were in possession of the first rudiments of assaying, and understood to some extent the purification of metals; but scriptural testimony does not point out with what amount of skill and success these operations were performed. Judging from the appliances which have been handed down from generation to generation, we are inclined to think they must have been practised somewhat rudely; for it has been left to the present school of scientific and practical metallurgists to found and develop the art in the direction of that commercial success to which it has at the present day attained.
This plan of cupellation which we have just described is still adopted in many continental works in the assaying of silver-lead ores. In England the system has been almost entirely superseded by one invented by the late Mr. Pattinson of Newcastle, and which is confidently stated to be far more convenient in practice.
The Alloys of Silver.
Fine silver enters freely into combination with nearly all the useful metals, but its most important alloys are those prepared from copper, the latter substance being more suitable for the production of silversmith’s work than any other; whilst it produces a more pleasing effect, if not over-alloyed, in regard to finish. Silver articles, especially of the filigree kinds, if the designs are good, possess a very tasteful appearance. In treating of the alloys of silver, it is our intention, first, to give a cursory glance at the chemical and physical properties of the metals which form these alloys. Such a description, although brief, will, we believe, prove of essential service, not only to working silversmiths and metalsmiths, but also to goldsmiths and jewellers, who are constantly manipulating with these inferior metals in precisely the same way as the silversmith. Besides, such information cannot, we apprehend, fail to be useful, whether to the student, the theorist, or the practical worker.