Zinc is one of the more fusible metals, and is sometimes employed by the silversmith in his alloys, for the purpose of imparting a greater degree of whiteness to them, as well as rendering inferior silver more easily bleached or whitened; thus assisting to bring back the natural colour of fine silver to manufactured articles, which have partially lost it by the addition of alloy of some other colour. Zinc, when employed in silver alloys, should be cautiously used, and care should be taken not to add too much to a given quantity of material. The solder used with silver-zinc alloys should be far more fusible than that employed with the other alloys. If too much zinc be added in the preparation of these alloys, in the course of the work, particularly in the process of soldering, they have a tendency to sweat, and sometimes to eat the metals into holes around the parts to be united; such alloys, therefore, render this process very difficult to perform, besides entailing more labour in the production of a clean and smooth finish.
Fig. 18. Plumbago Crucible for melting.
In melting an alloy of silver, copper, and zinc, the silver and copper should first be melted in a plumbago crucible of the form shown in [Fig. 18], and well stirred together in order that they may become properly mixed. The zinc is sold in flat cakes under the name of spelter, and, when required, is usually cut up with a chisel into pieces of various weights suitable for the object in view. When the copper and silver have become well incorporated, the mixture should be protected from the air by a suitable flux, charcoal being the best for this purpose. The most suitable time to add it to the crucible in the furnace is when the metals are just beginning to fuse. This flux covers the whole of the surface of the molten mass, and so prevents the action of the air from destroying some of the baser metal. The charcoal should be perfectly pure and in a finely divided state, for if adulterated with any gritty matter (and sometimes such is the case) a very indifferent working material is produced, the evil results of which show themselves in every stage of manufacture. These instructions with regard to melting the more infusible metals having been carried out, the zinc is taken with a long pair of tongs ([Fig. 19]), and held within the furnace, over the mouth of the crucible, until the temperature has almost reached the melting point, when it should be carefully dropped into the fused mass below, quickly stirred, so that it may become intimately mixed with the other metals, and at once withdrawn from the furnace and poured into a suitable ingot mould ([Fig. 20]). The ingot mould should be clean and smooth inside, slightly greased, and dusted over with fine vegetable charcoal; this latter substance prevents the metal from adhering to the sides of the mould. It is, perhaps, almost unnecessary to state that the ingot mould requires heating to a certain temperature before the melted composition is poured in, otherwise serious spouting takes place, resulting in a great loss of metal. On the other hand, the operator should be cautious not to over-heat it, as the same evil consequences may result.
Fig. 19. Tongs for Melting.