Specification enrolled 3rd December, 1842, of a Patent granted 4th June, 1842, to Edmund Tuck, of the Haymarket, in the county of Middlesex, silversmith, for "certain improvements in the covering or plating with silver various metals and metallic alloys."
This invention consists in the use and application of either of the two carbonates of ammonia, (viz., the sesqui-carbonate and the bi-carbonate,) as one of the ingredients in the mixtures or compounds employed for covering or plating with silver various metals and metallic alloys by the action of electricity. And it is found that by the use of the said carbonates, the silver so deposited and constituting such covering or plating, has not a crystalline character, and is firmly adherent to the surface of the metal. Bi-carbonate of ammonia may be purchased of the manufacturing chemist, or may be prepared in various ways; among others is the following: take any quantity of the solid carbonate of ammonia of commerce (called sesqui-carbonate of ammonia), reduce it to a coarse powder, and expose it to the air till it ceases to exhale or give off the pungent odour characteristic of that salt. This change will take place, under ordinary circumstances, in twenty-four hours, more or less, and the salt will have lost from forty to about fifty-two per cent. of its original weight, and will be found to have become bi-carbonate of ammonia; the composition of which is one equivalent (or by weight about 17 parts) of ammonia, two equivalents (or by weight about 44 parts) of carbonic acid, and one equivalent (or by weight about 9 parts) of water.
The material of the articles usually plated with silver is either copper or its alloys, and of these alloys, especially that which is commonly called German silver. This latter consists essentially of copper, nickel, and zinc; but the different manufacturers of this alloy do not use precisely the same proportions of the said ingredients, nor are these ingredients (especially the nickel) brought respectively to equal degrees of purity. Hence the process of plating German silver requires certain adjustments in the proportions of the substances used, and in certain variations in the manipulation, in order to secure in all cases the best result. But these adjustments and variations it is not possible to point out precisely, depending as they do on the varying quality of the alloy.
It has been already stated that bi-carbonate of ammonia is one of the ingredients in the mixture or compound which is employed by the patentee for plating. The other ingredient which the patentee employs is a salt of silver; and he finds it advantageous to vary the salt of silver according to the nature of the metal or alloy intended to be plated. For the common alloys of the so-called German silver, he uses the compound or mixture of a solution of bi-carbonate of ammonia with the sulphate of silver; for plating on copper or on good German silver, he uses the compound or mixture of bi-carbonate of ammonia with the cyanide of silver; and he observes, that though the latter compound may be used to plate the common alloys of German silver, and the former compound to plate copper or good German silver, the use of these respective compounds, for such purposes, is not so advantageous, according to his experience, as those firstly mentioned. Both of these salts may be purchased of most manufacturing chemists, or may be made in the following way : to prepare the sulphate of silver, he takes a cold saturated solution of the salt called nitrate of silver, and adds gradually to it strong sulphuric acid, as long as any white precipitate comes down ; he then washes the precipitate with small portions of cold distilled water, till the excess of acid is got rid of; he then dries the powder at a gentle heat: when it is fit for use. The sulphate of silver thus prepared is composed of one equivalent (by weight about 40 parts) of sulphuric acid, and one equivalent (by weight about 116 parts) of oxide of silvers The cyanide of silver may be made by putting any convenient quantity of the yellow salt called prussiate of potash, or ferro-prussiate of potash, or ferro-cyanide of potassium, into an earthen crucible, and exposing it to a low red heat for a quarter of an hour; the black mass thus produced is to be digested in warm distilled water, which when passed clear through a filter is a solution for the most part of cyanide of potassium; to this is to be added, gradually, a cold saturated solution of nitrate of silver, as long as a white or greyish white precipitate comes down: the precipitate, when washed in cold water and dried, is cyanide of silver, and consists of one equivalent (by weight about 108 parts) of silver, and one equivalent (by weight about 26 parts) of cyanogen. The way of preparing the plating mixture or compound, which he has found to afford the best results, is to take one equivalent (that is 70 parts by weight) of bi-carbonate of ammonia, and one equivalent (that is 156 parts by weight) of sulphate of silver, or one equivalent (that is 134 parts by weight) of cyanide of silver. The bi-carbonate of ammonia is to be dissolved in distilled water, and the sulphate or cyanide of silver (whichever is made use of,) is then to be added, and the liquor is to be boiled till the salt of silver is entirely dissolved. The strength of the solution, that is the. preparation of water, must be regulated by the strength of the galvanic battery made use of. The strongest solution used by the patentee for coating bad German silver, was composed of half an ounce of sulphate of silver, and 107 grains of bi-carbonate of ammonia, dissolved in one pint of water.
The kind of electricity whereby the plating mixtures above mentioned are made to deposit their silver on the metal or alloy to be plated, is that which is produced by means of a galvanic or voltaic battery; and of the various forms of such battery, which have been invented and published, the patentee prefers a modification of that commonly known by the name of Daniell's Constant Battery, of which a description is given in the 439th page of a book, intituled, "An Introduction to the Study of Chemical Philosophy, &c.," by J. F. Daniell, F.R.S. The modification of this battery which is employed, is a rectangular trough of seasoned wood, the cavity of which is divided longitudinally by a thin partition of porous wood into two cells, one of which is twice as wide as the other. The wide cell is again subdivided, by a longitudinal partition, into two equal cavities, which have free communication, because the partition does not quite touch the two ends of the trough. Of these two cavities, the outer is to be filled with crystals of sulphate of copper, and the inner one with a solution of sulphate of copper and water; a plate of copper is also inserted in this cavity: the other cell is filled with water, and a plate of zinc is also put into it. Near the above mentioned trough or battery is placed another trough, having no partition, and containing the plating mixture or compound. The article to be plated, and a piece of zinc, are put into the plating mixture on opposite sides of the trough, the article to be plated being connected by means of a strip of copper with the zinc plate of the battery, and the piece of zinc being connected by a strip of copper with the copperplate of the battery. The general proportion of the copper to the zinc surface of the battery, is as four of copper to one of zinc, but the proportion of copper must be increased when operating on those metals or alloys that have but a feeble aflinity for silver. The article to be plated must first be made quite clear. The method which the patentee uses for this purpose, is to put it for two or three hours in a cold solution of common carbonate of potash and water, and then to wash it in cold water, and afterwards dip it in a pickle, which is a mixture of aqua-fortis and water, the proper strength of which depends on the nature of the metal or alloy of which the article is made. It is next to be washed and dried, and well rubbed with rotten-stone on a rag or piece of leather. In operating on German silver, the quality of the alloy may be judged of by its appearance when taken out of the pickle, the best kinds having a perfectly white surface, the inferior kinds having a surface more or less dark coloured. Immediately before immersing the article in the plating liquid, it is dipped in a solution of common salt, in which a little gum has been dissolved. The strength of the battery must be such as to give a slow and even deposit of silver on the surface of the metal or alloy to be plated. The usual time for obtaining a satisfactory deposit is three or four days; a perfect coating, indeed, is obtained in an hour, but the further length of time required depends on the intended thickness of the plating.
The patentee states that he does not claim the exclusive use of any of the several processes herein before described, except so far as any of them may be used in connexion with, or in aid of his said invention, which he declares to consist in the use of either of the two carbonates of ammonia, namely, the sesqui-carbonate, and the bicarbonate, as one of the ingredients in the mixtures or compounds employed for covering, or plating with silver, various metals and metallic alloys by the action of electricity.
Source: The Record of Patent Inventions - Alexander Prince - 1842