On the Melting of Silver.
The processes of melting and properly mixing silver with its alloys in a crucible are among the first operations of the silversmith, and are, moreover, of great importance in the production of intimate and homogeneous alloys. In order to effect these, however simple they may appear, various precautions are necessary, and certain principles require carrying out to arrive at the best possible results, otherwise a great loss or waste of material may take place. To direct attention to those principles, which from very careful attention to the subject we have found to answer best, will first be our aim, and if we succeed in rendering some little service to our fellow-workers in the craft to which our toil and leisure have been devoted we shall feel highly gratified.
The weighing of the component metals, the selection of the crucible, the charging of it, and the attention it requires whilst in the furnace are considerations to which we cannot too strongly call attention. The regulations with regard to weighing should be strictly and accurately carried out. The best and safest plan is, after the various metals have been separately weighed, to re-weigh them, this time collectively, in order to ascertain whether the total weight corresponds with the previous calculation; if it does, the mixture has been properly prepared. We have known both time and trouble saved by the adoption of this precaution, after mistakes had occurred which could not have been detected until the weighing of the bar of metal had taken place after melting. There are various kinds of crucibles manufactured for the use of the precious metal workers. Crucibles were so-called from originally being impressed by the alchemists with the sign of the cross. They are calculated to bear very high temperatures, and consist of English, Hessian, Cornish, Black-lead, and Plumbago. The last two are by far the best; the plumbago, however, being the hardest, and capable of standing the highest temperature, is to be preferred before all others. It will also stand more frequent meltings than any of the rest. Such crucibles have been known to withstand the heat of the furnace for upwards of fifty times without giving way. The wear of them is very strong and resisting, as they only gradually become reduced in thickness, so that it is easy to distinguish their unfitness for use. Fluxes act on earthern crucibles, particularly English at a high temperature, whilst nitre and carbonate of soda soon destroy them.
Fluxes are necessary in most cases of metallic reductions: they protect the metal from the air, and dissolve impurities. They are of several kinds, as follows:—
Vegetable charcoal.
Carbonate of potash.
Carbonate of soda.
Common salt.
Sal-ammoniac.
Sal-enixum.
Saltpetre.
Borax.
Sandiver.
Yellow soap.
Black flux.
White flux.
Crude tartar.
Brown potash.
Sub-carbonate of potash.
All these fluxes have occasional duties to perform, and are therefore of great service to the metallurgist.
To prevent the cracking or flying of the crucible, when newly employed, it should, before being charged with the precious metal, be well annealed; that is, heated to redness upon a very slow fire—one that is gradually going down, and in which there is no blaze is to be preferred, because the flame has a tendency, on the introduction of a new crucible, to make it fly to pieces. When it has become red hot, if a cold bar of iron be introduced it will soon show whether there are any cracks, and if so the crucible should be rejected; on the contrary, if it withstands this test it may be placed aside until required for use, when it may be employed with perfect safety in the melting of silver and its alloys.
When copper and silver only form the alloys of the silversmith, they should both be added to the crucible at the commencement of the operation; and it is the best plan to put the copper at the bottom, because it is the most infusible metal. By doing so, it will receive the greatest degree of heat, which in jewellers' furnaces always comes upwards and the higher specific gravity of the silver has a tendency to force that metal downwards; consequently, when the two metals have become fused, upon well stirring—which should be done with an iron stirrer tapered at the point, and previously heated to redness—a perfectly homogeneous mass will be the result. When the more fusible metals of which we have spoken are to form the component parts of the mixture, different treatment with regard to them will be required. They should not be added at the commencement of the operation, but should be dealt with afterwards, in the following manner:—