Chloride of gold is the salt generally used in making gilding solutions and in preparing the other salts of gold. The chloride of gold is prepared by dissolving gold in a warm aquaregia which is a mixture composed of one part of nitric acid and two or three of muriatic acid. The gold should be cut up in small pieces and added slowly, care beirtaken not to inhale the gases that arise from the mixture. The gold dissolves very slowly but by gently heating the mixture its action is quickened. Four ounces of this liquid will dissolve about one ounce of gold, and form nearly one and a third ounces of chloride of gold.
When the solution has dissolved all the gold it can, evaporate it with gentle heat to a small bulk which will solidify when cooled. The resultant yellow salt is the chloride of gold which is soluble in water.
Almost all articles of gold contain traces of silver and this metal may now be observed in the solution in the form of chloride of silver, which is a white substance and insoluble in water, which may be removed, if desired, by pouring off the clear liquid, which holas the gold in solution, leaving behind the white chloride of silver, which should be preserved.
Any dark or brownish substance that will not dissolve is very likely metallic gold formed by the chloride being over heated and should be re-converted into chloride.
A solution of gold forms upon the addition of ammonia, a brown precipitate which, when dry, is one of the most powerful and dangerous ex plosives known, and \Wich detonates with the least friction or percussion. One little accident of this kind will put a sudden stop to any further experiments, therefore great care should be taken to prevent its formation, or if formed] to prevent its becoming dry.
The gilding solution is prepared as follows: Dissolve one pennyweight of chloride of gold in a gallon and a half of water; add nine ounces of caustic potash, one ounce of carbon-l ate of potash, and half an ounce of cyanide ofl potash. This solution should be used very hot, but not quite at the boiling point.
The previously cleaned articles of copper or brass are immersed for a short time in this solution, when they should be taken out and dried. Should a thicker coating be desired they should then be dipped in the cyanide of mercury solution (see page 40) and then after rinsing them, be immersed in the gilding solu¬tion again. By repeating this operation several times a very thick deposit may be obtained capable of resisting the action of the strongest acids for a long time.
This process of gold plating is more particularly adapted to the plating of articles not sub¬ject to much handling, as generally only a very thin coating is obtained by it. The work done by this method, however, looks fully as well as that done by the battery process, and to those having no knowledge ot the art of electro metallurgy is somewhat more simple.
The solution also improves with constant usage, acquiring a greenish tint from the presence of copper dissolved from the articles that have been plated in it. This however, does not interfere with its working unless there is a great deal of it held in solution , then it may be deposited with the gold, and impart a darker color to it.
As the solution gradually loses its gold by being deposited, it is necessary to add, from time to time, a small quantity of chloride of gold dissolved in a little water, 4in order to strengthen it, and after three or four such additions it may be necessary to add a small amount of the other salts, always preserving the proper proportions. By this means the solution may be kept in good working order for an in¬definite length of time.
Hiç yorum yok:
Yorum Gönder