CHAPTER VIII.

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:--

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.

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.

The bar of metal upon cooling should be weighed, and the difference--as most meltings show a little--noted. This is _loss_, but it will be very little, if the above instructions have been strictly adhered to from the beginning of the operation. With the charcoal flux we have referred to, very nice and clean bars of metal can be produced. This flux is always floating upon the surface of the mixture, and, with a little dexterity in the pouring, it can be prevented from coming out of the crucible with the metal; its proper place is at the end of the pouring. When tin is employed, either in alloys or solders, its treatment is similar to that described for zinc; such alloys should not be kept too long in the furnace after they have become fused, as they rapidly become oxidized, especially if brought into contact with the air.

The waste in silver, and in fact of all alloys, is entirely dependent on the duration of the time of fusion. If it is prolonged after the addition of the fusible metals, the loss is greater in every case, than when once melted. The metals should be subjected to the beat of the furnace for the shortest possible period. The alloys of silver with zinc would lose more than the alloys of silver with tin, because zinc rapidly volatilises when heated above the temperature of its fusion, and this is especially the case when it enters into combination with silver and copper in the fused state; its vapours can be seen to rise and burn in the air, producing light and white flaky fumes, and, chemically speaking, forming the _protoxide_ of zinc. With care and manipulative skill during the process of fusion, the proportion of waste can be reduced to a minimum; and when this is exactly ascertained an allowance can be made in the preparation of the mixture for the crucible. From the above remarks it will be apparent that when both tin and zinc form component parts of a mixture, either to be used as an alloy or as solder, the tin should be added to the other metals, and well stirred, so as to obtain an intimate mixture, before the addition is made of the zinc.

Scrap silver should be carefully sorted before undergoing the process of re-melting, and if possible all foreign substances removed. It may, if preferred to work it in that way, be melted into a separate bar, or otherwise used as an addition to a new mixture. When, however, it is separately melted, a flux, such as carbonate of soda, may be employed, on account of its cheapness, in small proportions to the charcoal flux already alluded to. In brittle and troublesome alloys we have found charcoal and a small quantity of borax extremely effective. Saltpetre is a very useful flux in dissolving impurities, but in some alloys its presence is injurious. Sandiver will remove iron or steel from the mixture. Corrosive sublimate destroys lead and tin. We have found the sub-carbonate of potash one of the best fluxes for silver, when matters have not been quite so straight as they should be in the working of the metal; it is used in melting the difficult alloy of 18-carat gold, and is considered a secret not generally known to the trade. Sal-ammoniac is an excellent flux for producing clean and bright ingots and tough alloys. We invariably use it with all our alloys, mixed in small quantities with charcoal, and prefer it to all others.

Lemel, that is the filings and turnings produced during the process of manufacture, should have quite a separate method of treatment. It is best prepared for the crucible by passing it through a fine sieve, afterwards thoroughly burning it in an iron ladle, and then intimately mixing it with a flux of the following nature and proportions:--

Silver dust 24 parts Carbonate of soda 4 " Common salt 2 " Sal-enixum 1 " ---------- 31 parts. ==========

The sal-enixum prevents the rising of the mixture in the crucible--which should be of the skittle shape (Fig. 21)--and keeps it from overflowing; it also possesses a refining capacity the same as saltpetre, and is much cheaper. The burning of the lemel has a great tendency to destroy all organic matter that would be likely to cause the mixture to overflow during the period of fusion; but if such a thing should be at all likely to take place, the addition of a little dried common salt would remedy the evil, a small quantity of which ought always to be provided for the purpose. The common carbonate of soda is also a cheap and useful flux to the silversmith. Five-sixths of the above flux should be well mixed with the stated proportion of lemel, then placed in the pot, and the one-sixth remaining placed upon the top of the mixture, when it may at once be transferred to the furnace. Great heat is required in this operation, and it also requires careful supervision to prevent, if possible, waste of material. When the mixture has become perfectly liquid, the heat of the furnace should not be allowed to decrease, but continued for half an hour longer, and if the use of it be not further required, the fire may then be allowed gradually to die out. The mixture will require repeated stirrings during the period of fusion, in order to dissolve such portions as might otherwise not come immediately under the action of the flux. When the operation of fusion has been completed, the crucible is withdrawn and allowed to cool, the solidification of the metal is then perfect, and it may be recovered by breaking the pot at the base, when it will fall out in a lump corresponding with the shape of the crucible. The lump of metal should then be carefully weighed, the loss ascertained--which always varies in proportion to the amount of organic matter contained therein; it may then be sold to the refiner, or exchanged for new metal.

In this process it will be observed that the crucible is broken every time a fusion takes place, consequently some little expense is incurred in providing crucibles for the purpose. To obviate which the following plan may be economically and successfully employed; and especially when the metal is sold to the refiner by assay, the method about to be described will be found most advantageous, for it should be borne in mind that the lump of metal from the previous fusion has to be again run down in another crucible and poured into an ingot mould before the refiner will consent to take his assay from it. In this latter process the whole work is performed in one fusion, and the expense of a new crucible thereby saved. The flux employed in the reduction of the metal is also considerably reduced. The plan is performed after the following manner:--

Take a plumbago crucible of the shape shown in Fig. 22, and capable of holding the required mixture; put the lemel into it, and then place on the top one ounce of finely powdered carbonate of soda; this is all the flux the mixture requires, and it is then quite ready for the furnace. When the lemel has become properly fused, for facilitating which it is repeatedly stirred with a thin iron rod, it is withdrawn and poured into an ingot mould prepared for it as previously described. The flux and other organic matter, which always accumulates upon mixtures of this kind, is held back by the timely application of a thin piece of flat wood to the mouth of the crucible. After the withdrawal of the bar of metal from the ingot mould, it is cooled and weighed, and then it is quite ready for the operations of the refiner.