appears to take place in baths very rich in gold, and may be corrected by a judicious dilution of the solution.

On the other hand, when the solution is depositing less gold than is dissolved in the same time from the anode, less cyanide is liberated at the cathode. In this case a deficiency of gold in the bath may be suspected and remedied by a judicious addition of gold salt, or by the artifice of using for a time a very large gold anode and a comparatively small receiving surface. Where this unfavorable condition of the bath (i. e., a deficiency of gold) is pronounced, it is apt to manifest itself also by the anode becoming blackened and slimy, and evolving gas. This condition of things generally manifests itself when the bath is nearly exhausted of gold, as where a gold anode has been used which is very small in comparison with the surface of the article at the cathode. This slimy appearance of the anode should not be confounded with the yellowish, or greenish, or even dark-colored, incrustation on the anode, above referred to, as an indication of a deficiency of free cyanide in the bath, since in this case the addition of cyanide would be worse than useless. It should be remembered, therefore, that the surface of anode exposed should be regulated by that of the articles in the bath.

The experienced operator, familiar with the varying behavior of his cyanide gilding solutions, is enabled to correctly interpret the signs which they indicate, and to so manage them as to keep them in good working condition for years.

REMOVING GOLD FROM GILDED ARTICLES-" STRIPPING."

Artists and manufacturers are often required to remove the gold from gilded objects, either for repeating anew an imperfect gilding, or for extracting the gold from worn-out

articles to be consigned to the melting pot. The method varies considerably, according to the metal upon which gold has been deposited.

Iron and steel are" stripped" without any injury to themselves, by dipping them into a bath of 10 parts of cyanide of potassium and 100 parts of water, and connecting them with the positive pole of a battery. A wire or foil of platinum is attached to the negative pole. This is simply inverting the ordinary position of the poles; and the gold upon the iron or steel is partly dissolved in the solution of cyanide, and partly deposited upon the platinum anode, from which it is removed in a regular gilding bath. When there is only a mere film of gold upon iron or steel, it may be removed by the cyanide alone, without the aid of elec tricity, but this method is slow.

Silver, copper, and their alloys may also be stripped of gold by this process; but, as the cyanide at the same time. dissolves not only the gold but also the other metals, it is preferable to operate as follows:

For ungilding silver, it is heated to cherry-redness, and immediately thrown into a pickle of more or less diluted sulphuric acid. The gold scales off, and falls to the bottom. The operation is repeated until gold no longer appears upon the surface of the silver, which is then white and frosty.

This process is imperfect, and is not adapted to light and hollow articles, for which the first described process is preferable.

For copper and its alloys, such as bronze, Germansilver, brass, etc., two different methods may be followed, according to the nature and volume of the objects.

Small trinkets, thinly gilded, either by current or by immersion, are dipped into the following bath :

Sulphuric acid (oil of vitriol) .
Nitric acid (aqua fortis) .

Hydrochloric acid (muriatic acid)

10 volumes.

1 volume.

2 volumes.

The above mixture is simply aqua regia in a large quantity of sulphuric acid, which excess of sulphuric acid allows of the solution of gold, without sensibly attacking copper or its alloys.

Ordinarily, the sulphuric acid is placed by itself in a stoneware jar, and the mixture of hydrochloric and nitric acids (aqua regia), kept in a stoppered bottle, is gradually added to it as the operation of "stripping" proceeds. The same sulphuric acid may last a long time, provided it is kept well covered, and the action of the acid mixture is promoted by additions, from time to time, of nitric and hydrochloric acids.

The articles should be withdrawn often, in order to watch the operation, which is terminated when no traces of gold are seen, and when the copper has acquired a uniform blackish-gray coating.

We may also ascertain that all the gold has been dissolved, by plunging the objects into the mixed acids for cleansing (see ante), in which they are perfectly cleansed when the gold has disappeared.

Nitrate of potassium (saltpetre) and chloride of sodium (common salt) may be substituted for the nitric acid and hydrochloric acid respectively. The result is identical; but in this case the salts must be introduced into the acid in a finely powdered state, and stirred with a glass rod, in order that they shall dissolve, and form aqua regia by their decomposition.

A few operators ungild small trinkets by a more or less protracted immersion in the acid mixtures for bright or dead dipping; but they spoil their acids and lose the gold.

For large objects, such as clocks, mantel ornaments, chandeliers, etc. (and also for smaller articles), a good mode of procedure is the following (Fig. 108):

Fig. 108.

A glass or stoneware vessel, supporting two brass rods, is filled with concentrated sulphuric acid (66° Baumé). One of these rods is connected by a conducting wire with the last carbon of a battery of two or three Bunsen elements, in inverted order, and supports, by means of hooks, the objects to be ungilded, and which are entirely covered by the sulphuric acid. The other rod supports, also by means of a hook, a copper plate facing the object, and is connected with the last zinc of the battery. The current traverses the sulphuric acid, and carries the gold from the positive to the negative pole; and, as the copper plate is not prepared for retaining the gold, it falls to the bottom of the bath in the state of a black powder, which is afterwards easily recovered.

As long as the sulphuric acid is concentrated (even under the action of the galvanic current), it does not sen

sibly dissolve the copper.

But this is not the case when

it becomes diluted with water; and as it rapidly absorbs atmospheric moisture, the vessel in which it is contained should be kept hermetically closed when the "stripping" process is not in active operation. The pieces to be "stripped" should be dry when introduced into the acid.

For removing gold from old or imperfectly gilded articles, Napier, and also Watt, directs the articles to be placed in (pure) strong nitric acid, to which some common salt in crystals is then added. By this method, it is affirmed, gold may be removed from any metal, even from iron, without injuring it in the least. This method is no doubt an effective one, but will require close watchfulness on the operator's part to prevent the solution of the underlying metal. We give the several related methods of Roseleur, in which sulphuric acid is used, the preference.

When we intend to sacrifice the gilded articles of copper or silver, it is sufficient to let them remain in pure nitric acid, which dissolves all the metals except the gold, which either floats on the surface of the acid as a metallic film, or falls to the bottom as a blackish powder. If then the liquid be diluted with distilled water and filtered, all the gold will remain on the filter, and the solution will contain the other metals.

We shall give, at the close of this work, the simplest and most practical processes for recovering, for future use, the gold saved by these different methods of "stripping."