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Pickling of Metals

Metals are pickled for the purpose of removing the oxide and producing a lustrous surface. An excellent pickle for brass consists of ten parts of water and one of sulphuric acid. Dip into this pickle, wash, dry, and immediately dip into a second pickle consisting of two parts of nitric acid and one of sulphuric acid and rinse thoroughly. This dissolves the zinc from the brass, and gives the metal a brilliant surface. All pickling operations with either hot or cold pickle should be carried on in the open air or in the draft of a well drawing chimney, as the vapors arising from the acids are very injurious. In order to retain the luster, a good transparent varnish should be applied.

Pickle for German Silver. To twelve parts of water add one part of nitric acid; immerse the article in this, quickly remove, and place in a mixture of equal parts of sulphuric and nitric acid, rinse thoroughly in water and dry in sawdust. In all cases of pickling it is essential that all traces of acid be removed by frequent washings in clean water.

Pickle for Gold Alloys. Gold alloys, especially those containing copper, assume an unsightly dark brown exterior, owing to the copper oxide generated by the repeated glow heating necessary during work. In order to remove this the object must be pickled, and either highly diluted sulphuric or nitric acid is used for the purpose, according to the color the article is designed to have.

If working with an alloy consisting only of gold and copper, either sulphuric or nitric acid may be used indifferently, since gold is not attacked by either of these acids, while copper oxide is easily decomposed thereby, and after having been pickled, the article will assume the color of pure gold, because its surface is covered with a layer of the pure metal. If the alloy is composed of pure gold and silver however, only nitric acid can be employed, and the article is left immersed in it only for a short time; this acid dissolves a very small portion of the silver, and the article also assumes the color of pure gold.

When working with an alloy which, besides the gold, contains both copper and silver, the process of pickling may be varied in accordance with the color desired to be given to the article. If the pickling is performed in sulphuric acid, the copper alone is dissolved, the article assuming a color corresponding to a gold-silver alloy, which now constitutes the surface of the article.

In nitric acid is used it will dissolve the silver as well as the copper and in this case a pure gold color is produced.

Pickling is done by first feebly glow heating the article and cooling it; this operation is for the purpose of destroying any fat from the hands or other contamination adhering to the article. If it was soldered with some easily-flowing solder, this glow-heating must be omitted, but it may be cleansed from impurities by immersing it at first into very strong caustic lye, and rinsing it with water; it is then laid into the acid. The acids are employed in a dilute state, taking forty parts water to one part concentrated sulphuric or nitric acid. If more articles than one, they had best be laid beside each other in a porcelain or stoneware dish, the diluted acid is poured over them, and some article is lifted out from time to time to watch the course of proceedings, whether it has assumed a yellow color. When to satisfaction, they are rinsed with clean water and dried. While pickling for the purpose only of causing the color peculiar to gold to appear, the process of coloring has for its object to lend the appearance of very fine gold to an article of an indifferent alloy. Various mixtures may be employed for the purpose, and we give two receipts below which are very appropriate: Mix two parts saltpeter, I part table salt and six parts alum with 6 1/2 parts water, and place in a porcelain dish for heating. As soon as you notice that the mixture begins to rise, add I part of muriatic acid, raise the whole to boiling and stir with a glass rod.

The article to be colored, and previously treated with sulphuric acid, as specified, is suspended to a hook, either of sufficiently thick platinum wire or glass; it is then introduced into the rather slow boiling bath, and moved around in it. It is to be taken out in about three minutes, and rinsed in clean water, inspecting its color at the same time. If not to satisfaction, it is returned to the bath, and this withdrawing it or introducing is repeated until the desired color is obtained. By the latter immersions the article is left only one minute at a time in the fluid.

When sufficiently colored the article, after rinsing, will be of a high yellow and mat color; it is washed repeatedly in water to remove the last traces of the bath, and then dried in hot boxwood sawdust.

In place of drying in sawdust the article may also be dipped in boiling water, leaving it in for a few seconds; the adhering water will evaporate almost instantaneously.

The second coloring method consists in pouring water over a mixture of 115 parts table salt and 230 nitric acid, so that the salt is dissolved; it is then to be heated until a dry salt residue is again present. This residue is mixed with 172 parts fuming muriatic acid and heated to boiling for which purpose a porcelain vessel is to be used.

As soon as the pungent odor of chlorine gas begins to evolve, the article to be colored is immersed, and left for about eight minutes in the fluid for the first time; in other respects, a similar treatment as specified above, is also used for this method; if the article colored was polished previously, a subsequent polishing is unnecessary.

On account of the vapors evolved by the coloring baths, which are very dangerous to health, the operations should be performed either under a well-drawing flue, or what is still better, in open air.--Goldsch Miedekunst.