3731. To Electro-Gild Iron, Tin, and Lead

3731.    To Electro-Gild Iron, Tin, and Lead. Iron, tin, and lead are very difficult to gild direct; they therefore generally have a thin coating of copper deposited upon them by the cyanide of copper solution (see Nos. 3754 (Cyanide Solution of Copper or Zinc) and 3755), and immediately put into the gilding solution.

3732. Conditions Required in Electro-Gilding

3732.  Conditions Required in Electro-Gilding. The gilding solution generally contains from one-half to an ounce of gold in the gallon, but for covering small articles, such as medals, for tinging daguerreotypes, gilding rings, thimbles, etc., a weaker solution will do. The solution should be sufficient in quantity to. gild the articles at once, so that it should not have to bo done bit by bit; for when there is a part in the solution and a part out, there will generally be a line mark at the point touching the surface of the solution. The rapidity with which metals are acted upon at the surface line of the solution is remarkable. If the positive electrode is not wholly immersed in the solution, it will, in a short time, bo cut through at the surface of the water, a:3 if cut by a knife. This is also the case in silver, copper, and other solutions.

3733. To Maintain the Strength of the Gold Solution

3733. To Maintain the Strength of the Gold Solution. As the gold solution evaporates by being hot, distilled water must from time to time be added. The water should always be added when the operation of gilding is over, not when it is about to be commenced, or the solution will not give so satisfactory a result. When the gilding operation is continued successively for several days, the water should be added at night. To obtain a deposit of a good color, much depends upon the state of the solution and battery; it is therefore necessary that strict attention be paid to these, and the more so as the gold so-lution is very liable to change if the size of the article receiving the deposit is not the same as that of the positive electrode plate. The result of a series of observations and experiments, continued daily throughout a period of nine months, showed that in five instances only the deposit was exactly equal to the quantity dissolved from the positive plate. In many cases the difference did not exceed 3 per cent., though occasionally it rose to 50 per cent. The average difference, however, was 25 per cent. In some cases double the quantity dissolved was deposited, in others the reverse occurred - both resulting from alterations made in the respective processes ; for in these experiments, the state of the solution and the relative sizes of the negative and positive electrodes were varied, as far as practicable. The most simple method of keeping a constant register of the state of the solution is to weigh the gold electrode before putting it into the solution; and, when taking it out, to compare the loss with the amount deposited. A little allowance, however, must be made for small portions of metal dissolved in the solution, from the articles that are gilt, which, when gilding is performed daily, is considerable in a year. A constant control can thus be exercised over the solution, to which there will have to be added from time to time a little cyanide of potassium, a simple test of requirement being that the gold positive electrode should always come out clean, for if it has a film or crust it is a certain indication that the solution is deficient of cyanide of potassium. Care must be taken to distinguish this crust, which is occasionally dark-green or black, from a black appearance, which the gold electrode will take when very small in comparison to the article being gilt, and which is caused by the tendency to evolve gas. In this case an addition of cyanide of potassium would increase the evil. The black appearance from the tendency to the escape of gas has a slimy appearance. This generally takes place when the solution is nearly exhausted of gold, of which fact this appearance, taken conjointly with the relative sizes of the electrodes, is a sure guide.