ELECTRO-MET'ALLIIRGY. Within the last few years many advances have been made in the art of electro-metallurgy. Usually in electroplating, the silver presents a dull appearance when drawn out of the trough, requiring to be polished by a burnisher and a scratch-brush. If, however, bisulphide of carbon be added to the bath, or, better, if the article be dipped into a bath containing this liquid after immersion in the usual solution of silver, a thin but hard and bright layer of silver is deposited on the main layer, brilliant enough to do without polishing. There are certain difficulties in the process which render it only applicable in some circumstances.
Since 1869, the coating of other metals by an electro-deposit of nickel has been suc cessfully carried out on a commercial scale, and is becoming extensively practiced both in England and the United States. It is usually deposited from a solution of sulphate of nickel neutralized by ammonia. Nickel has the hardness of iron, and though it lacks the beautiful color of silver, it has nevertheless an agreeable grayish-white color, and does not tarnish like silver in impure air, besides being only about one fifth part of its price. Numerous articles of iron, steel, brass, and German silver for household use, but especially for ships and hotels, have been plated with nickel in recent years.
Very beautiful electrotype copies of art objects can now be made of iron, and even, it is said, of steel. As a consequence of this, the art of engraving on steel will soon cease to be practiced, as copper-plates, which are less costly to engrave, can now be coated with an electro-deposit of iron which can be renewed at will; thus practically yielding an indefinite number of prints. In the same way engraved zinc plates can be coated with copper, in cases where a lesser number of impressions are required.
Attempts have often been made to coat iron with an electro-deposit of an alloy such as brass, bronze, or German silver, and with all of these it has been attended with more or less success. The difficulty lies in getting the solutions of two metals to be of exactly equal conducting power. By employing a solution containing both the oxides and the cyanides of copper and zinc, along with some tartrate of ammonia, a very even and compact deposit of brass can be obtained.
There is a process now in use for purifying impure or blister copper by electrolysis_ The blister copper is cast into plates; these are placed in a solution of sulphate of cop per, and form the anodes or positive poles of a battery. Particles of pure copper, by the action of the electric current, separate themselves from the crude plate 'of pimple metal, and attach themselves to the opposite or negative pole. By this means plates of pure copper are obtained, provided that the blister-copper contained no metals which will deposit along with it. The impurities fall down as a residue.
It sometimes happens that iron ores are found mixed with the ores of copper or zinc in the state of sulphides. In such a case there is a difficulty in separating them by the ordinary ore-dressing processes, owing to their being of nearly the same specific gravity. If, however, the iron ore happens to be magnetic, or can be changed into the black oxide by roasting, then its particles can be conveniently extracted from the mixture by electro magnets.
The making of large copper or " bronze" statues by electro-deposit instead of by casting, is another department of the art which has made great strides. Messrs. Elking ton have produced many statues in this way from 6 to 14 ft. high. One of the earl of Eglinton weighs two tons. There is a trough or at the works of this cele brated firm, 15 ft. long, 9 ft. wide, and 9 ft. deep, that will contain 6,600 gallons of the copper solution.
The French are also making great progress in the art. M. ()wiry has taken electro casts of the bas-reliefs on Trajan's column at Rome, covering 700 sq. yards. He hay also coated many thousands of the iron lamp-posts of Paris with copper by electro deposit, as well as numerous fountains, etc., producing most of the good effects of bronze at a much smaller expense. This process of coating exposed iron with copper is not so much used as it might be. Few persons have any idea of the large extent of surface which even one halfpenny worth of copper will coat when deposited by the action of a galvanic battery from its solution either in the cyanide or chloride of potassium.
See GALVANISM.