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passed through the sulphuric acid bath just described are next immersed in a bath of this kind. They should be allowed to remain in this until the red coating of suboxide of copper has entirely disappeared, leaving, after rinsing, a uniform metallic lustre.

The dipping in old aqua fortis is not absolutely necessary, although we recommend it for two reasons, the last being the more important: First, it is economical in saving the cost of fresh acids; second, as its action is slow, it prevents the too rapid corrosion of those portions of the copper already cleansed, during the time that the solution of the oxide, which still adheres to certain parts of the surface, is being effected. If some operators frequently complain of the burned or pitted appearance of their pieces, it is due to the neglect of this operation, and direct dipping in strong aqua fortis.

We shall indicate further on the processes for utilizing and extracting all that can be saved from old aqua fortis and other dipping baths.

Dipping in Strong Aqua Fortis..

After a preliminary rinsing in fresh water, the absolute necessity of which has already been pointed out, the articles, well shaken and drained, are plunged into a bath composed of

Nitric acid of 36° Baumé (straw-colored

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This mixture attacks the metal with the greatest energy, and the pieces should therefore not remain in it more than a few seconds. The volume of acid should be from twenty to thirty times as great as that of the articles to be cleansed, for the chemical action being energetic a small

volume of the liquor would become heated, and the acid. would be rapidly weakened or "spent."

After this bath, and rapid rinsing before the acid has time to give off reddish nitrous vapor, the pieces present a fine red color, sometimes gold-yellow or greenish-yellow, according to the alloy employed, and one might be led to suppose that they had been entirely deprived of foreign matter. It is not so, however; for, if the pieces in this state are plunged into a gilding or silvering bath, they become entirely black and without any metallic lustre.

If, on the other hand, we put aside without rinsing, the pieces removed from the dipping bath, there is produced on their surfaces a greenish froth, with the evolution of abundant nitrous vapors, which indicate the complete decomposition of the acid which remains upon them. When the vapors have disappeared, the pieces, even after washing, remain black and tarnished, on account of the formation of a basic copper salt, which is not soluble in

water.

This last mode of operating, called blacking by aqua fortis, is preferred by a few gilders, varnishers, and color fixers, who find it economical to allow the production of nitrous vapors while the pieces are draining on top of the vessel which contains the acids. At all events, any subsequent operation is to be prefaced by rinsing in fresh water.

When small objects, such as pins, caps, eyelets, pearls, etc., are to be dipped, they are often put into a stoneware pot with a small quanity of aqua fortis, and then rapidly shaken and stirred. In this case, the acid is entirely used up with the production of abundant vapors, and the objects remain blackened and ready for further cleansing.

The soot in the above bath acts as a reducing agent. The carbon and hydrogen which it contains deoxidize a small proportion of the nitric acid, transforming it into

nitrous acid, which, uniting with the chlorine of the sodium chloride present, forms a small quantity of aqua regia, which is necessary in the operation above described. The operator is required to be careful in the choice of his aqua fortis. Three kinds of nitric acid of 36° are to

be found in the market:

One is perfectly colorless.

The other is colored straw-yellow.

The last is of a more or less dark-red color.

The colorless acid does not cleanse well, especially when freshly used.

The red acid, which often contains chlorine, bromine, or iodine, acts too powerfully and pits the copper.

The straw-yellow acid, on the contrary, is to be preferred to the others. Nitric acid of 40° is too energetic and costly; certain operators however, who have to cleanse large quantities of copper-ware daily, prefer it on account of the rapidity of its action.

Aqua fortis is spent when its action on the articles of copper becomes too slow, and when the objects removed from the bath are covered with a kind of bluish-white film. Such acid is then preserved for the preceding operation, that is to say, dipping in old aqua fortis; or for dipping in the whitening bath, which will be described at the proper time.†

* Certain gilders have taken advantage of this property of bromic and iodic acids to pit copper surfaces, to produce what they call “mat anglais," which has a fine effect on smooth surfaces.

Very good aqua fortis may cleanse imperfectly when the temperature is too low or too high. This accounts for the difficulty in cleansing in frosty weather, or during the great heat of summer.

Aqua Fortis for Bright Dipping.

Before quitting this subject, we may indicate an excellent method of restoring the brilliant lustre of metallic surfaces that have been dulled by bad dipping, or slightly pitted or otherwise injured by the acids used for "stripping" or removing the gold or silver from plated articles. For this purpose it is sufficient to allow them to remain for a few minutes in a bath composed of

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The pieces, when removed from it, are entirely black, and must be thoroughly rinsed in clear water to remove the black slime which covers them. They are then cleansed and dipped anew.

This bath is in constant use by varnishers on metals, and though scarcely known to electrogilders, will certainly be found useful by them. It will be found to answer admirably for removing the moulding sand adhering to castings of copper and its alloys. With pieces of considerable size, however, it will be necessary to prolong the immersion for 20 or 30 minutes and sometimes much longer, as this mixture acts very slowly on copper and its alloys.

Acid Mixture for "Bright" Dipping.

These acids, which were rightly considered as indispensables by the first gilders by simple immersion, are of two kinds according to the object in view.

If the pieces must have a bright lustre, they are stirred for one or two seconds in a liquid (prepared the day previous, and cold) composed of

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On removing the articles from the bath they should be thoroughly swilled in an abundance of water.

In the preparation of this dipping liquid, the acids should be added in the following order: First, the nitric acid should be poured into the containing vessel, and then the sulphuric acid, as otherwise the latter, by reason of its superior density, would remain at the bottom of the vessel without mingling with the former.

For these

While the acids are being mixed, and especially when the common salt is added, considerable heat is developed, accompanied by the disengagement of an abundance of acid fumes, which are dangerous to inhale. reasons it is prudent to conduct this operation in the open air, or under a good chimney-hood provided with a movable glass sash. As this mixture must be used cold, it will therefore be necessary to prepare it some time in advance.

In an emergency the mixture may be used the same day that it has been prepared, by placing the containing vessel in ice or cold water until the acid mixture is cooled off.

Copper articles, after this dipping, are lighter colored and much brighter than after the passage through aqua fortis. They may then be considered as completely cleansed.

The above acid mixture is too energetic for small articles, such as pins, hooks, pearls, etc., which are generally cleansed in stoneware colanders, the holes of which are more or less obstructed, so that the acid liquid shall not drain off two rapidly, but if permitted to remain too long in contact with the copper, it becomes heated, fumes,

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