A slight acidity must be maintained by occasional additions of a few drops of hydrochloric acid when required. Fizeau (Id. 133) dissolves 1 part dry chloride of gold in 160 parts of distilled water, and adds, little by little, carbonate of potassium solution until the solution begins to become cloudy, when it is ready for use. Another of his formulæ is a solution of 1 part of gold chloride and 4 of hyposulphite of sodium in 1000 parts of distilled water. Spencer has proposed the use of the bromide and iodide of gold for electro-gilding baths. His process is substantially as follows: He prepares a saturated solution of bromide of gold by adding gold leaf to bromine, until it will dissolve no more. This solution may used as it is, but to obtain a thick deposit, he recommends the addition of two parts of a saturated solution of acetate of ammonium to one part of the bromide solution. Where this solution is to be used to gild alloys that will receive a gold deposit by simple immersion in it, he advises the addition to the bath of an excess of carbonate and bicarbonate of sodium. In an analogous way Spencer prepares and employs a solution containing iodide of gold. The gold is converted into chloride, dissolved in the aqueous solution containing the other ingredients, and the resulting liquid forms the gilding bath. Hansen's patented process for electro-gilding, plating, or coppering upon glass or porcelain, which presents some differences to those described in Chapter XV., consists (Elsner, Chem.-Techn. Mittheil., 1876-1877) in preparing two solutions: one of sulphur in oil of lavender, which is concentrated until it has a thick syrupy consistence; and another, of chloride of gold or platinum, in sulphuric ether. He mixes these two solutions, and by gentle heating, evaporates, until the resulting mass has become pasty. This mixture is suitably laid on the glass or porcelain surface with a brush, to form the intended decoration, and the article is heated in a muffle, for the purpose of driving off the sulphur and other volatile and decomposable substances—the gold or platinum being left behind in the metallic state, firmly adherent to the surface of the object. Thus prepared, the metallized surface of the article may be plated with a coating of gold, silver, or copper of any desired thickness by galvanic means. For this purpose Hansen gives the composition of electro-baths of these respective metals which he prefers to use. It will be unnecessary to reproduce them, as any of the formula named in this work will serve the purpose. CHAPTER XXII. GILDING WATCH MOVEMENTS, AND OTHER SMALL ARTICLES FOR WATCHMAKERS. IN the gilding of watch movements, and other small articles for watchmakers, gold is seldom directly applied upon the copper. In the majority of cases, there is a preliminary operation, called graining, by which a very agreeable grained, and slightly dead appearance, is given to the articles. If one carefully examines the interior of a watch, the peculiar stippled matt lustre of the parts will be at once noticed. This peculiar bright-matt lustre, as Roseleur terms it, is totally different from what we have already mentioned. For instance, it does not resemble the dead lustre obtained by slow and thick electro-deposits of gold, silver, or copper, which is coarser and duller than that of watch movements. Neither does it resemble the dead lustre obtained with acids, which is the result of a multitude of small holes formed by the action of these agents; whereas the grained dead lustre is formed by the juxtaposition, upon a previously smooth surface, of an infinite number of granules of greater or less size, and always in relief. The graining may be produced by different methods, and upon gold, platinum, and silver; and since the latter metal is that preferred, we shall describe the process as applied to it. This species of gilding requires the following successive operations:— 1. Preparation of the Pieces.-Coming from the hands of the watchmaker, they still retain the marks of the file, which are obliterated by a rubbing upon a wet stone, and lastly upon an oilstone. 2. The oil or grease is next removed by boiling the pieces, for a few minutes, in an alkaline solution, made of one hundred parts of water and ten of caustic soda or potassa, and rinsing them in clean water, which should wet them entirely if all the oil has been removed. In going through this operation, the articles are threaded upon a brass wire. 3. A few gilders then cleanse them rapidly in the acid mixture for a bright lustre; others simply dry them carefully in white sawdust. 4. Holding the Pieces.-The pieces thus prepared are fastened, by means of brass pins with flat heads, upon the smooth side of a block of cork (Fig. 98). A few operators have lately substituted gutta-percha for cork, but this method does not appear to have given good results, since, so far as I know, it has not become popular. Fig. 98. 5. The parts thus held upon the cork are thoroughly rubbed over with a brush, entirely free from fatty matters, and charged with a paste of water and of the finest pumice-stone powder. The brush is made to move in circles, in order not to abrade one side more than the other. The whole is then thoroughly rinsed in clean water, and no particles of pumice dust should remain upon the pieces, or the cork. 6. Afterwards the pieces are plunged, cork and all, into a weak quicking solution, which very slightly whitens and is composed of— the copper, The pieces are simply passed through the solution, and then rinsed. This operation, which too many gilders neglect, gives strength to the graining which, without it, possesses no adherence, especially when the movements are made of German-silver, dignified by the name of nickel by watchmakers, or when the false cases contain tin in their composition. 7. Graining.-In this state, the parts are ready for the graining, that is to say, a light silvering obtained by a peculiar procedure. There are a great number of formulæ for preparing the graining powders; and it may be said that each gilder has his own formula, according to the fineness of the grain he desires to obtain. Herewith are given several formulæ which may be relied upon to give good results:— Silver, in impalpable powder* Bitartrate of potassium (cream of tartar), finely 100 66 *The silver powder is obtained by immersing cleansed copper plates in a very diluted solution of nitrate of silver made with distilled water. The more dilute the solution is, the finer will be the state of division of the silver, which will be deposited upon the copper as a species of moss, and the more easily may it be detached. The operation is generally conducted as follows: In a suitable vessel of glass or porcelain, 2 parts of crystallized nitrate of silver are dissolved in 1000 of distilled water, and five or six bands of cleansed copper about of an inch wide are plunged into it. These bands should be long enough to allow of a portion being above the liquid. The whole is kept for twenty-four hours in a dark place, and the liquid is now and then stirred with the copper bands. This motion is sufficient to loosen the deposited silver, and present fresh copper surfaces to the action of the liquid. When no more silver becomes attached to the copper, the operation is at an end, and there remains a blue solution of nitrate of copper. The silver powder is washed either by decantation, or upon a filter, until no trace of the copper solution remains. Lastly, another washing is had with distilled water, and the silver powder is dried. Great care is required in this operation to avoid contact with hard bodies, which may render the powder compact, and produce a sort of cohesion |