The Silversmith's Handbook / Containing full instructions for the alloying and working of silver

by George E. Gee

The mode we have ourselves adopted for the colouring or whitening of silver goods is somewhat different, and still more simple than even the above. We will proceed to give the details of the process. A mixture of very dilute sulphuric acid is first provided, in the proportion of one ounce to forty ounces of water, and well mixed together; the work, after being heated to a good red heat, is boiled in this, which soon removes the oxide from the surface, and shows the fine white colour of the pure silver. For fine silver work, such as Indian filigree, one process will generally suffice, for English standard quality two, and for low qualities three, but these latter must not by any means be too low; if so, no colouring can take place by the method just described. Objects of delicate workmanship are usually annealed by the gas; being placed on a pumice-stone of light material, the flame of the gas is blown with the mouth blow-pipe, in such a manner that the object gradually becomes heated all over alike; and the work should be well heated, as this facilitates the process of oxidation, and subsequently that of whitening. The oxidation takes place at the expense of the copper in the silver alloy, and this is only effected by raising the articles to a very high temperature, which produces the oxidation of the copper coming in contact with the air, and which necessarily exists upon the surface of the alloyed goods. Whitening silver goods then is nothing more than the removal of the base alloy from the surface, leaving the pure metal behind with its full rich colour. Therefore to be clear, the process of annealing in contact with cold air oxidizes the copper upon the surface, and the pickling mixture so dissolves and removes it, that it gradually undergoes a process of refining, and is ultimately made to represent the finest material in all its purity.

Sometimes silver work is to be seen having a brown colour upon it; this is produced when the acid employed for cleansing has been too strong; it can only be remedied by another annealing and boiling-out in a much more diluted mixture. There are various other methods employed in the trade for the purpose of whitening silver work of the best quality; and although annealing is always a part of the process, other ingredients, such as salt and tartar, permanganate of potash, cyanide of potassium, alum, &c., have been severally used for the cleansing or whitening mixture. They may be useful in their application to plated work (articles that have received a coating of pure metal by means of the electro-metallurgical process), for cleansing purposes only, but for all practical purposes the process to which we have called special attention is to be much preferred.

Common articles of silver cannot be whitened by annealing and boiling-out in a diluted acid; a thin film of pure silver must be deposited upon their surface by the process of electro-deposition, or by the action of some chemical preparation in which fine silver forms the principal ingredient. Such preparation, however, as the latter can be used only to plain surfaces, therefore they are not applicable to all kinds of work. They are composed of the following chemical ingredients:—1st, chloride of silver 1 part, cream of tartar 1 part; 2nd, chloride of silver 1 part, common salt 1½ parts; 3rd, chloride of silver 1 part, prepared chalk 1 part, pearl-ash 1 part; 4th, chloride of silver 1 part, alum 1 part, common salt 2 parts. The chloride of silver is easily prepared by precipitating it from the nitrate with a solution of common salt or hydrochloric acid. The various mixtures should be worked up with water into a thin paste, and applied to the work by rubbing with a soft cork or piece of wash-leather, or by thoroughly stirring it about in the mixture until it has acquired the requisite degree of whiteness. For the purpose of silvering watch and clock faces, &c. these mixtures may be used with advantage and entire success.

Other solutions are sometimes employed for similar purposes and are very useful; being simple in their preparation and easy of management. We have selected the following as being the most practical:—Take one ounce of the nitrate of silver and dissolve it in one quart of pure distilled water, or if this cannot be procured, water which has been boiled, by which it loses some of its impurity. When the nitrate of silver has become thoroughly dissolved, throw into the mixture a little powdered hyposulphite of soda, this will precipitate the silver, and when it has taken place, a further addition of hyposulphite of soda should be made, which will eventually re-dissolve the precipitate, and the solution is then ready for use. To produce a good mixture, the salt of soda should be added slightly in excess. The solution is used by simply dipping a sponge in it and rubbing it over the surface of the articles to be coated, and this is continued until they have assumed the desired colour.

For improving the colour of silver and electro-plated wares, the following mixture has been strongly recommended:—Nitrate of silver 4 pennyweights, cyanide of potassium 5 ounces, and water 1 quart; the ingredients should be well mixed together, and applied by means of a soft brush or sponge to the surface of the work. In using this cyanide solution, the operator should be careful to guard against a too frequent contact with it, as it is decidedly injurious to the hands, especially if there be any abrasion of the skin; it being one of the deadliest poisons known. Sufficient details of the process of silver whitening and cleansing having now been given to assist the workman who manipulates in this particular metal, and to enable him to select a form of recipe in every way adapted to the kind of work in hand, we shall now proceed to the modern process of electro-plating, and give a practical description of it in its applicability to the trade of the silversmith.

This art is decidedly of modern origin, as far as concerns its employment for commercial purposes. The invention is supposed to be due to the electrical and chemical researches of Mr. Spencer, of this country, and Professor Jacobi, of Russia, both of whom claim to have found out the art of depositing one metal upon another, somewhere about the same period. Of course it was left to others to apply the invention to the industrial arts, and it was not until after the discovery of the Constant Battery, by Professor Daniell, about half a century ago, that the art began gradually to extend in the direction of commercial pursuits. The Messrs. Elkington, of Birmingham, were the first to employ it in their manufactures, with a success which their enterprise thoroughly merited. This took place about the year 1840, and since that time the art of electro-plating and gilding has wonderfully developed, in its application to the various manufactures of the country. Its progress would be a subject highly interesting, were we to trace the general details of it, but the part of it we are considering being the practical mode of its employment in manufactures, we shall at once direct our attention to it, by giving a complete description of the process; so that the ordinary silversmith may be enabled to employ it in his business with safety and advantage.

The first thing to be considered in electro-plating is what Battery to employ, which will be the most simple, inexpensive, and effective one. When the battery is only occasionally required for use we prefer the Smee before any other. It is a small portable apparatus, and consists of a high, but narrow, glass or stoneware jar, in the form of a cylinder, capable of holding about two quarts; inside this jar is fitted a thin plate of platinized silver fitted to a frame with two zinc plates, one on each side of it, the zinc plates being held to the frame by means of a binding screw. Strong copper wires are firmly secured to these screws, which serve as the positive and negative poles of the battery. Those parts of the plates which are not exposed to the action of the acid solution may be advantageously coated with sealing-wax varnish or melted paraffin wax, to protect them from the destructive influences of the battery acid, and to prevent it from creeping upwards, which destroys the connections. The jar is filled with water acidulated with sulphuric acid, in the proportion of 1 of acid to 20 of water; the frame containing the plates is then lowered into the solution, and the battery is ready for use. In the above form of battery for occasional use we have one simple in construction, easy of management, of fair constancy, and when once prepared very inexpensive, merely requiring a little free acid at times to keep up the strength of the current. Two cells of this form of battery, each holding two quarts of mixture, will be found sufficient for all ordinary purposes. The zincs should be well amalgamated, and not touch the bottom of the cells. The connections should be regularly examined, and kept perfectly free from corrosion, which would stop the passage of the current. For plating small delicate articles of jewellery one cell of the above form will be found powerful enough for the purpose.

The battery that we prefer and have of late years employed, for regular continuous working, is the Bunsen, consisting as before of a cylindrical glass or stoneware jar of the same size and dimensions, fitted with a well amalgamated cylinder of zinc and a copper wire secured to it; a porous cell is placed in the centre, and a bar or rod of carbon is put into this cell with a copper wire also secured to it. The porous cell is filled with a mixture of equal parts of nitric and sulphuric acids, or sulphuric acid alone; we prefer the latter, as it does not give off such fumes as does the other acid; a little of the more powerful acid, however, is sometimes required to be added in order to increase the action, as with this acid alone it sometimes becomes slow. The outer cell is filled with a mixture of 1 part of sulphuric acid to 20 parts of water, and the connections being in proper order the battery is then ready for use. In action this form of battery is regular and continuous, it lasts a long time upon one charge, and is therefore inexpensive in use; if the two cells are coupled for power or intensity, an unusual quantity of work may be got through in a given time. This cell is admirably suited to the work of the manufacturing silversmith, and to those who prefer doing their own plating.

The amalgamation of the zinc is effected as follows:—The cylinders are best treated by putting some mercury into a coarse flannel bag, dipped repeatedly into muriatic acid and applied to the surface of the zinc, both inside and out; and when they present the bright characteristic appearance of mercury they are sufficiently operated upon, and may be rinsed and set aside to drain. The zinc plates may be advantageously amalgamated by placing some mercury in a shallow dish with a little muriatic or sulphuric acid, a hare’s foot or a piece of cloth tied to the end of a stick is then dipped into the mercury and acid, and rubbed over the plates until they are sufficiently protected with mercury, when they should be rinsed in clean water and set aside to drain. If possible the process of amalgamation should always be conducted in the open air, as the fumes which are given off, if breathed, are highly injurious. The best possible way to amalgamate rods of zinc is by pouring mercury into the melted metal just before casting it into rods, in the proportion of 1½ oz. of mercury to the pound of zinc. This makes the rods exceedingly brittle, and they should therefore be handled with care. The mercury should not be added to the zinc when the latter is at too high a temperature, and the best manner of testing this is by the application of a piece of paper to the molten metal, when if it takes fire, the temperature is still too high; it should be allowed to cool until the paper refuses to ignite, then and not till then is the proper time for the addition of the mercury.

The copper conducting wires and binding screws must be cleaned when they become much corroded; if not they add resistance to the current, and it will become considerably diminished, or cease altogether. The cleaning may be effected by simply annealing and then plunging them while still hot into dilute sulphuric acid pickle, or dipping them into nitric acid for about an instant.