By the application of the processes about to be described, the finishing touches in their relation to articles or wares of silver manufacture are effected. These processes, as adopted by the trade, are various, almost every firm having a specially prepared mixture and mode of employing it. We shall refer only to those which, from their practical utility, are likely to be of service to those workmen who have to do with this particular metal. The branch of the art of which we are now treating comes only into operation when every other process of workmanship has been completed; and some of these processes must be executed in a perfect manner in order to arrive at the highest possible results in this one. The best and richest surface is produced when the metal to be operated upon is good in quality, and the workmanship of a fair order, so far as regards smoothness, and freedom from surplus solder-marks.
One of the oldest methods for producing a pure snowy whiteness upon articles of silver was as follows:—Take an iron or copper annealing pan (the latter is much to be preferred), place the work upon it in proper order, so that it may be heated all over alike. It should, previous to this, be immersed in a thick solution of borax, or otherwise brushed over with it. After the work has been properly arranged upon the pan for annealing, it must be sprinkled over with fine charcoal dust; the pan is then placed in the muffle upon a bright clear fire without blaze, and when the work has assumed a degree of heat approaching to cherry redness, it is withdrawn and allowed to cool. When this has taken place, it is removed and boiled out in a very weak solution of sulphuric acid, commonly called oil of vitriol. If the right colour was not then produced, the process was repeated as many times as circumstances permitted, though usually two or three times was found to be amply sufficient. The annealing process required great attention, for the work being in contact with borax, if slightly overheated, it was liable to become melted, therefore the operation was a delicate one to perform, and was only intrusted to such workmen as were experienced in the art. Small delicate articles were commonly treated with the mouth blow-pipe and gas-jet, and were placed upon a pumice-stone, or some other suitable substance capable of withstanding the power of burning. According to the inferiority of the silver alloy, is the difficulty of producing a good white surface on wares of such standards. Fine silver requires very little whitening, generally one process suffices to effect a good colour; but inferior standards require half a dozen or more to bring up the proper degree of whiteness; and those ranging below 9 oz. 12 dwts. to the lb. Troy cannot be whitened at all by the means we have described, but require the application of the modern chemical process, known as electro-plating.
The East Indian silversmiths never touch their manufactures with any kind of abrasive substance, from the most delicate, to the more strongly made article. But then it should be remembered, that in India the natives work from the pure material, a point which they rigidly adhere to; whereas we are compelled in this country to manipulate in all sorts of qualities; and some of these require no little trouble and difficulty to bring back to the surface the snow-white appearance of the pure metal. In the former case it is effected without any difficulty whatever—in fact, the metal scarcely undergoes any change throughout the whole of the manipulations to which it is subjected in the various processes of manufacture. The Indian mode of procedure is as follows:—Some juicy lemons are cut into slices; the silver articles are briskly rubbed with these for a short time, and subsequently covered with them, being placed in a suitable vessel for a few hours for the completion of the process. For very delicate articles of jewellery the natives cut a large lime nearly in two halves, into which they insert the work; the halves are then tightly closed up again, and placed aside for a few hours; when the article is removed, it is well rinsed in clean water, and consigned to a vessel of nearly boiling soap-suds, where it is well brushed, again rinsed in fresh hot water, and finally dried on a metal plate placed over hot water; the process is rendered complete by a little gentle rubbing with wash-leather if the work be of a plain nature. Green tamarind pods are also employed by them for the purpose of whitening silver, in the same manner as just described; they are great detergents both of gold and silver manufactures, and are largely employed by artisans in the East for the removal of oxides and fire-marks.
Another process for the whitening of silver goods is performed in the following manner:—Take the work, which must be cleanly prepared, and give it a coating of the following mixture:—Finely powdered vegetable charcoal four parts, saltpetre one part; the ingredients should be well mixed with a little water, and may be applied to the surface of the metal either by brushing over with a soft brush, or by dipping the work into it. The work is then placed upon the annealing pan and submitted to the heat of the muffle, until the wet powder has become perfectly dry and ceased to fly about; it is then withdrawn from the muffle, allowed to cool, and afterwards boiled out in a solution of potash prepared for the purpose, in the proportion of about one ounce of bi-sulphate of potash to twenty ounces of water. The boiling out is done in a copper pan ([Fig. 39]), and if the work be put through the above process two or three times a beautiful dead-white colour is the result. It is then washed in a hot solution of soda, soap, and water, or if preferred bright, scratched, or burnished, and the process is finally completed by drying it in fine boxwood sawdust, which should be made hot, but not allowed to char or burn in any way, as it would produce a stain upon the work very difficult to remove, and thus the finish would be considerably impaired.
Fig. 39. Boiling-out Pan.
In large manufactories the process of whitening silver goods is repeatedly required to be performed, and where such is the case, the above methods are found not only tedious and expensive, but occupy much unnecessary time and labour; to dispense with a portion of which, the custom of covering the work with a chemical preparation was accordingly departed from, and yet it was made to show its former brilliancy. To effect this object the liquid for boiling it in was differently prepared, which only required to be made of a proper strength to do all that the surface mixture had done before. The following is the method adopted in preparing the cleansing liquid. Boiling-out mixture:—To one pound of smoking salts, add two ounces of cream of tartar; well shake the ingredients together, so that they may be thoroughly incorporated. The smoking salts employed for this purpose are not the ordinary spirits of salts of commerce, but a preparation of the common oil of vitriol; therefore the one should not be taken for the other; the spirits of salts would turn the work black, whereas, if the proper ingredient were procured, a fine dead matte or blanched surface would be the result of its application. The mixture employed for boiling the work in consists of the proportion of one ounce of the above preparation to about thirty ounces of water. The silver articles simply require to be annealed, allowed to cool, and then boiled for a minute or so in this solution, when the desired result will be attained; if, however, the exact colour be not obtained the first time, the process should be repeated a second, and if necessary, a third time; the right colour will then be produced, if the articles are not made of a too inferior standard.