(a) Iron and steel articles are ungilt, without any injury to themselves, by dipping them into a bath of 10 parts potassium cyanide and 100 of water, and connecting them with the positive pole of a battery. A wire or foil of platinum is fixed to the negative pole. This is inverting the position of the poles: and in this case the gold applied upon the iron or steel is dissolved in the solution of cyanide, and partly deposited upon the platinum anode, from which it is removed in a regular gold bath. When there is only a film of gold upon iron or steel, it may be removed by the cyanide alone, without the aid of electricity, but this method is slow. Silver, copper, and their alloys may also be ungilt by this process; but the cyanide dissolves, at the same time, the gold, and part of the other metals; it is therefore preferable to operate as follows: -

(6) For ungilding silver, it is heated to a cherry-red heat, and immediately thrown into a pickle of more or less diluted sulphuric acid. The gold scales off, and falls to the bottom in the shape of spangles. The operation is repeated until gold no longer appears upon the surfaee of the silver, which is then white and frosty. This process is not adapted to light and hollow articles, for which the preceding process is better.

(c) For copper and its alloys, in small articles, such as false jewellery thinly gilt, either by battery or by dipping, use the following bath - Sulphuric acid, 10 parts; nitric acid, 1; hydrochloric acid, 2. The large quantity of sulphuric acid allows of the solution of gold, whilst it does not sensibly attack copper or its alloys. The sulphuric acid is put alone into a stoneware jar, and the mixture of hydrochloric and nitric acids, kept in a stoppered bottle, is gradually added to it as the operation proceeds. The same sulphuric acid may last a long time, if it is kept well covered, and its dissolving action promoted by successive additions of nitric and hydrochloric acids. The articles should be often withdrawn to watch the operation, which is terminated when no gold is seen, and when the copper has acquired a uniform blackish-grey coat; or by plunging the objects into the compound acids, they will be perfectly cleansed when the gold has all dissolved.

(d) Saltpetre and common salt may be substituted for nitric acid and hydrochloric acid; the salts must be finely powdered, and stirred with a glass rod.

(e) For large objects, such as clocks or chandeliers, concentrated sulphuric acid, 66° B„ is put into a glass or stoneware vessel supporting two brass rods. One of these rods is connected by a conducting wire with the last carbon of a battery of 2 or 3 Bunsen's inverted elements, and supports the objects to be ungilt, which are entirely covered by the sulphuric acid. The other rod supports a copper plate facing the object, and is connected with the last zinc of the battery. The electric fluid traverses the sulphuric acid, and carries the gold from the positive to the negative pole; as the copper plate is not prepared for retaining the gold, it falls to the bottom of the bath in a black powder, which is easily recovered. So long as the sulphuric acid is concentrated, and even under the action of the galvanic current, it does not sensibly corrode the copper; and as it rapidly absorbs the dampness of the atmosphere, the vessel in which it is contained should be kept perfectly closed, when the ungilding process is not in active operation; and the pieces for ungilding should be put in perfectly dry.

If it is intended to sacrifice the gilt articles of copper or silver, let them remain in pure nitric acid, which dissolves all the metals except gold, which either floats at the surface of the liquid as a metallic foil, or falls to the bottom as a blackish powder. If the liquor is diluted with distilled water, and filtered, all the gold will remain on the filter, and the solution will contain the other metals.

(f) Apply a solution of borax, in water, to the gilt surface, with a fine brush, and sprinkle over it some fine powdered sulphur. Make the piece red hot, and quench it in water. The gold may be easily wiped off with a scratch-brush, and recovered by testing it with lead.

(g) Gold is taken from the surface of silver by spreading over it a paste, made of powdered sal ammoniac, with aquafortis, and heating it till the matter smoker, and is nearly dry, when the gold may be separated by rubbing it with a scratch-brush.

Iridium Deposits

The problem of electroplating with iridium has been solved by employing a suitable solution of the metal and properly regulating the electric current. The solution is kept at uniform strength by using a plate of iridium as the anode. The metal takes a good polish. A buffing-wheel that will grind off nickel plating in a few moments only serves to polish iridium. Thin platinum foil, coated with iridium, retains its flexibility; and if the coating is not too thick, it will not readily scale off. Copper plates for engravings, faced with iridium, would possess marked advantages over steel engravings.

Iron And Steel Deposits

Iron may be deposited by the wet way," but is very easily oxidised. It is obtained by decomposing by the battery a perfectly neutral iron protochloride. This bath is rapidly altered by the air, and is transformed into sesquichloride, which is unsuited for the purpose. The double chloride of iron and ammonium, obtained by the protracted boiling of a solution of sal ammoniac upon iron filings, produces a very thin deposit of iron, very difficult to oxidise, which is employed for hardening the surface of engraved plates or of ordinary electrotypes. Double sulphates of iron and ammonia, or of iron and potash, and double chlorides of the same bases, have been successfully used for electro-deposits of iron.