If the article to be coated is dipped for a moment in a solution of one part of phosphorus to twelve parts of bisulphuret of carbon, on withdrawing it the bisul phuret of carbon, which is very volatile, will evaporate, leaving a film of phos phorus on the surface; the article is then immersed in a dilute solution of nitrate of silver, or sulphate of copper ; a pre cipitate of silver or copper is immediately formed, and thus becoming an electric conductor it may be introduced into the galvanic cell, and the process will proceed in the same manner as it does when plumbago is used in the first instance. By this simple and elegant method the most delicate articles, as feathers, flow ers, fruit, insects, &c., may be coated with metal. The surface should in all cases be free from moisture before it is introduced into the solution of phos phorus, which should be used with the greatest care, being highly inflammable. Phosphorus added to wax and stearine form an excellent coating for casts, as the surface becomes a conductor. Moulds of plaster of Paris being very porous, re quire to be saturated with wax, oil, var nish, or tallow, before receiving a coating of plumbago, otherwise when placed in the solution they will absorb the liquid, and the air which previously filled up the pores will he driven out, covering the surface of the mould with small bubbles. Flexible moulds for copying objects which are undercut, or overhung, may he made of a mixture of glue and treacle ; this mixture is easily removed from the pro jecting parts, immediately regaining its proper form.
The color of bronze is given to copper articles deposited by voltaic action by different methods. A very simple plan is to rub the article with plumbago immediately, or as soon as practica ble, after its removal from the battery ; afterwards heating it, and rubbing it with a hard brash. A lighter tint may be obtained by covering the surface with oxide of iron, and giving it a considers ble heat. Hydrosulphate of ammonia produces a fine color, and a dilute solu tion of chloride of platinum gives the object an agreeable tint.
On Dectro-Gilding, Plating, Platinia iog.—The metals redueeable by voltaic agency for purposes of utility are gold, sil ver, platinum, copper and zinc; these may be precipitated from their salts, or from the solutions of their salts in any material capable of dissolving them, and any de sired deposit may be made by adjusting the strength and temperature of the so lution to the intensity and power of the current of electricity employed. Gold may be deposited from its chloride, bro mide, cyanide, iodide, sulphite, and hy posulphite ; but for all purposes of gild ing, it is well to use a solution of the cyanide, which may be prepared by ad ding oxide of gold to the solution of cya nide of potassium. The most eligible proportions may be stated as follows : Two pounds of cyanide of potassium dis solved in one gallon of water, to which are added one ounce and a half of oxide of gold ; but if heat is employed, and the solution is raised to the boiling point, the quantity of water may be doubled. A single pint battery of Daniell's pos sesses sufficient intensity to gild any specimen, even large ores—the articles to be gilt must of course be attached to the zinc of the battery, and a plate of gold, of corresponding or greater dimensions, to the copper. The surfaces of all ob jects to be gilt, plated, or deposited on, must be thoroughly cleansed before being introduced into the solution ; oxide, grease, or other impurities, may be re moved by immersion in dilute sulphuric acid. The gold thus deposited may be colored to produce the red tint so gene rally admired, by being coated with a mixture of acetate of copper, sulphate of alumina, and bees'-wax, and exposed to heat till the whole is consumed ; and a rich orange color may be obtained by gently boiling the following ingredients together in water till they have a creamy consistency :—Five parts of nitrate of potassa, two parts of sulphate of alumina, one part of sulphate of zinc, and one part of sulphate of iron. The gilt object should then be dipped three or four times in the composition, and allowed to become nearly dry, and afterwards re moved to a stove, when, according to the length of exposure to beat the depth of color will be increased : it should finally be well washed, and cleaned with soap and water and a brush.
Silver can be precipitated from its cya nide, acetate, sulphate, sulphite, or hypo sulphite solutions. A solution of one pound and a half of cyanide of potassium in one gallon of water, to which two ounces of oxide of silver are added, answers ad mimbly for all purposes of plating. Fel some time after the introduction of elec. tro-gikling and plating, complaints were made of defective adhesion between the original and the deposited metal, and it was asserted by the manufacturers of similar articles on the old principle, that such plating and gilding would soon wear away, and exhibit the baser metal in all its original nakedness. There was, it must be admitted, some justice in this remark, though articles plated three years ago have been in daily use without showing any traces of the copper beneath. This defect was occasioned by the ab sence of an alloy, but the objection has lately been altogether removed by the use of mercury, for the purpose of alloy ing the two metallic surfaces ; for this purpose, nitrate of mercury is dissolved in water, and the copper article to be gilt or plated is plunged in the solution, and immediately withdrawn, then washed in water, and placed in the gold or silver solution. A. thin film of mercury is by this means distributed over the object, and amalgamating with both metals, completely alloys them. The articles after being gilt or silvered should be heated to Fahr., which dissipates the mercury. Another complaint against electroplating was, that the articles ra pidly tarnished on account of the purity of the. metal deposited. This may be obviated by brushing them over after re moval from the vat with a saturated so lution of biborate of soda, allowing them to dry so that a film of the salt may re main, repeating the _process a second or third time, till a slight but regular coat ing of borax covers every part • they should then be exposed to a red heat, and after being allowed to cool, immersed in dilute sulphuric acid, and dried in heated saw-dust. The metals' to which plating are most applicable are copper, brass, pewter, iron, steel, and gold ,• the process is also extensively employed for silvering articles formed of the alloy of nickel, known as German silver. Platinum may bo reduced from solu tions of its bromide, iodide, and bi-chlo ride, and the double chloride of platinum and sodium. The chloride is the salt commonly employed, but as considerable difficulty exists in depositing this metal in a ductile state, a very feeble current I of electricity should be employed, and I the plate of' metal introduced as an elec trode should be very small ; it is of great advantage to have the solution neutral, and some therefore recommend soda be ing added to it, thus forming the chlo ride of sodium and platinum.
Zinc may be deposited from its iodide, acetate, sulphate, and chloride ; also from the solution of oxide of zinc in potassa, or muriate of ammonia.
Copper may be thrown down from a conskkrable range of its salts ; those commonly used are the sulphate, nitrate, and cyanide.
Probably the most enormous applica tion of the electrotype art is made in the sculpture of the Cathedral of St. Isaac in Petersbnrgh. Seven doors of the ca thedral are of bronze and electrotype, the framework being of the former and the scultured posts of the latter. Three of these doors are 30 feet high and 44 feet wide, the four others 17 feet 8 inches wide. They contain 51 bas reliefs, 63 statuettes, and 84 alto relieve bests of religious subjects. The gilding of the. ca thedral was also done by this process. The quantity of metal employed in the dome was as follows :—Ducat gold, 247 lbs. ; copper, 521 tons ; brass, 8211 tons ; wrought iron, 5244 tons ; cast iron, 1068 tons ; total, 1,0661 tons. Casts in cop per have been taken from the daguerreo type plate, and impressions from these casts produced by electrotype by Dr. Paterson of Glasgow, Scotland. Smooth as a daguerreotype appears, the cast taken serves as a mould from which al most any number of impressions may be taken, which are as bold and as clear as the original type.