SILVER SOLUTIONS.] UNDULATORY FORCES. ELECTRO-METALLURGY. 



225 



nitric acid, and then precipitating it by caustic potash ; 

 he then dissolves the oxide, together with sixteen ounces 

 of cyanide of potassium, in two gallons of water, and 

 uses the resulting liquid for depositing solid articles in 

 silver. 



139. Mr. Edmund Tuck took out a patent, June 4th, 

 1842, for "improvements in depositing silver upon 

 German silver." For plating the commoner qualities 

 of this alloy, he uses a solution composed of sulphate of 

 silver dissolved in a solution of carbonate of ammonia ; 

 and for the best quality, he uses cyanide of silver dis- 

 solved in a solution of carbonate of ammonia. The 

 solutions are formed by dissolving 70 parts of carbonate 

 of ammonia in distilled water, then adding 156 parts of 

 sulphate of silver, or 134 parts of cyanide of silver, and 

 boiling the liquid until the salt is dissolved : for coating 

 commonJGerman silver, he adds half an ounce of sulphate 

 of silver to 107 grains of bicarbonate of ammonia. 



140. For depositing purposes, a solution composed of 

 water 20 parts, cyanide of potassium four parts, and 

 acetate of silver one part, conducts very freely, and 

 yields a fine white deposit of silver. A solution com- 

 posed of water 25 parts, prussic acid 65 parts, "black" 

 cyanide of potassium 12 parts, and cyanide of silver 10 

 parts, is also a very good one. 



141. Many electro-platers use a cyanide solution con- 

 taining about half an ounce of silver to the gallon, and 

 add a very large proportion of free cyanide to make it 

 conduct freely : such a solution has the advantage of 

 being comparatively inexpensive in its first formation, 

 quick in working, and yields metal of an average cha- 

 racter; but it is rather difficult to manage in hot weather, 

 and dissolves the anode very rapidly, on account of the 

 large proportion of free cyanide. In practice, the amount 

 of silver to the gallon varies from half an ounce to about 

 four ounces, but ordinary solutions contain about one or 

 two ounces ; the amount of free cyanide of potassium 

 also varies from about half the weight of the silver dis- 

 solved in the liquid to five or ten times this quantity : a 

 very good proportion is about three-fourths of the weight 

 of the dissolved silver; but there is no rule generally 

 recognised in the trade upon this point; some manu- 

 facturers use a very large and others a very small pro- 

 portion. 



142. A good plating liquid should contain one equiva- 

 lent (65 parts) of pure cyanide of potassium, and one 

 equivalent (134 parts) of cyanide of silver, besides free 

 cyanide, and sufficient water to form a thin liquid. It 

 is necessary to have free cyanide, because, in working the 

 solution, insoluble cyanide of silver is formed, and 

 requires free cyanide of potassium to combine with it and 

 form the soluble double cyanide ; at the same time, cyano- 

 gen and cyanide of potassium are set free at the cathode or 

 receiving surface by the deposition of the silver ; and as 

 it requires some time for those substances to mix with 

 the liquid and reach the dissolving plato, free cyanide 

 must be provided. The necessity of having sufficient 

 water to form a thin liquid, arises from the double 

 cyanide formed at the dissolving plate, being specifically 

 heavier than the liquid, having a tendency to sink to 

 the bottom ; whilst the cyanogen 'and cyanide of potas- 

 sium set free at the surface of the articles, being 

 specifically lighter, tend to rise to the surface. At the 

 same time, each of them mixes more or less with the sur- 

 rounding liquid by capillary attraction or adhesion ; and 

 the more dilute the liquid is, the more mobile are ita 

 particles, and the more rapidly does this mixture take 

 place. This explains why strong silver solutions require 

 more frequent stirring than weak ones to keep them 

 uniform. In some manufactories, where they have 

 steam-power at command, the articles are kept in con- 

 stant motion by machinery swinging them gently to and 

 fro; but in ordinary electro-plating establishments, the 

 silver solutions are stirred every evening. 



143. If a solution contain but little water and a 

 large supply of free cyanide, and, from any cause, the 

 battery current become suddenly weak towards the 

 evening, the silver deposited upon the articles will be 

 redissolved, in consequence of tho liquid about the 



VOL. I. 



dissolving sheets having, by the day's work, become satu- 

 rated with silver, and that about the articles become full 

 of free cyanide; the two electrodes (i.e., the dissolving 

 plates and the articles) form a kind of voltaic battery 

 (one metal in two liquids), which develops a current of 

 electricity in an opposite direction to the original one, 

 and thus redissolves the deposited silver. 



144. Bright Silver Solution. Much practical interest 

 was for a long time attached to the anticipated discovery 

 of a solution by which silver might be deposited in 

 a bright condition, and the labour of burnishing be 

 thereby avoided or lessened. This discovery was at last 

 effected as follows : Some operators at the electro-plate 

 works of Messrs. Elkington and Mason, Birmingham, 

 were engaged in experiments on moulds containing 

 bisulphide of carbon : whilst these moulds were beiug 

 coated with silver in the depositing vats, very peculiar 

 appearances upon the various articles receiving a deposit 

 in .the vat were noticed, some having very bright patches 

 upon them like burnished metal. From the known 

 presence of bisulphide of carbon, experiments were tried 

 of adding that substance to a quantity of silvering 

 liquid, which ultimately resulted in success, and a 

 patent was taken out by Messrs. Lyons and Millward, 

 March 23rd, 1847 ; in which they give the following 

 instructions for forming a " bright solution :" " Add to 

 the usual solution of silver in cyanide of potassium, 

 bisulphide of carbon, terchloride or other chloride of 

 carbon, sesquichloride of sulphur, or hyposulphite of 

 either potash or soda. The bisulphide of carbon may bo 

 used alone or dissolved in sulphuric ether ; or it may bo 

 used in conjunction with any of the other substances 

 mentioned above ; but the patentees prefer using it as 

 follows : six ounces of bisulphide of carbon are put into 

 a stoppered bottle, and one gallon of the usual plating 

 liquid added to it ; the mixture is then shaken and set 

 aside for twenty-four hours ; two ounces of the resulting 

 solution are then added to every twenty gallons of tho 

 ordinary plating solution in the vat, and the whole 

 stirred together : this proportion must bo added every 

 day, on account of the loss by evaporation ; but wheu 

 the mixture has been made several days, less than this 

 proportion may bo used at a time ; when hydrocarbons 

 are used instead of the bisulphide, a much larger quan- 

 tity must be added. This proportion gives a bright 

 deposit ; but, by adding a larger proportion, a dead surface 

 may bo obtained, very different to the ordinary dead 

 surface. This substance is generally employed through- 

 out the trade, although few are licensed to use it. Other 

 compounds are also used, but to a very limited extent : 

 among these are sulphur and collodion. A solution of 

 iodine and gutta-percha in chloroform is said to be more 

 permanent in its effect than the bisulphide of carbon. 

 The liquid is generally added to the vat in the evening, 

 after the work has been taken out. A method of bright 

 gilding has also been recently brought into use in the 

 trade. 



145. Method of Making Cyanide of Potassium. As 

 nearly all the solutions which are used for electro-silver- 

 ing and gilding contain cyanide of potassium, and as 

 this substance is used extensively in electro-deposition 

 generally, it will be necessary for the practical plater to 

 understand how it is made, and to possess information 

 respecting its impurities, and tho method of testing its 

 quality. It is nearly always made by the following 

 process : Take ferrc-cyanide of potassium (yellow prus- 

 siato of potash), pound it fine, and gently heat it in an 

 iron pan, w-th constant stirring until quite dry ; treat a 

 quantity of tho best quality of carbonate of potash in a 

 similar manner. When they are perfectly dry, add 

 about three parts of tho carbonate to eight parts of the 

 ferro-cyanido, and thoroughly mix them ; heat the mix- 

 ture rapidly in an iron ladle or crucible, until it melts 

 into a clear liquid, when gas will bo evolved from its 

 surface. It should be maintained at a moderate or dull 

 red heat about fifteen or twenty minutes, or until the 

 end of a cold iron rod dipped into it shows a white 

 sample. The fusion should not be continued until tho 

 evolution of gas ceases, or tho product will be of a grav 



2o 



