The electrotypes obtained from metallic moulds prepared with the turpentine solution have a bright surface, which is not liable to change easily ; but if the mould has been prepared with oil or composed of wax or plaster, the metal will either be dark, or will very easily tarnish.
In putting moulds into the copper solution, the operator is often annoyed by small globules of air adhering to the surface, which either prevent the deposit taking place upon these parts, or, when they are very minute, permit the deposit to grow over them, causing numerous small hollows. To obviate this, give the mould, when newly put into the solution, two or three shakes, or give the wire attached to it, while the mould is in the solution, a smart tap ; but the most certain means is to moisten the surface with alcohol just previous to putting it into the copper solution.
When busts or figures, whether of wax or plaster of Paris, are to be coated with copper, with no other conducting surface than black-lead, it is attended with considerable difficulty to the inexperi enced electrotypist. The deposit grows over all the prominent parts, leaving hollow places, such as armpits, neck, and the like, without any deposit ; and when once missed, it requires considerable management to get these parts coated, as the coated parts give a sufficient passage for the current of electricity. It is recommended by some electrotypists to take out the bust, and coat the parts deposited on with wax, to prevent any further deposit on them ; but this practice is not good, especially with plaster of Paris, for an electrotype ought never to be taken out till finished. Sometimes the resistance of the hollow parts is occasioned by the solution becoming exhausted of metal, from its position in regard to the positive pole. In this case a change of position effects a remedy. It may be remarked, that when a bust or any large surface having hollow parts upon it, is to be electrotyped, as many copper connections as possible ought to be made between these parts and the zinc of the battery. Let the connections with the hollow parts be made with the finest wire which can be had, and let the zinc plate in the cell have a large surface compared to the surface of the figure, and the battery he of considerable intensity ; if attention is paid to these conditions, the most intricate figures and busts may be oovered over in a few hours. Care has to be obseived in taking off the connections from the deposit, or the operator may tear off a portion of the deposit ; if the wires used are fine, they should be cut off close to the deposit surface.
Copper deposits are obtained either by simple dipping or galvanic methods. Copper deposits by dipping are seldom practised except upon iron, and are generally wanting in lasting qualities, since, from the thinness of the deposit, the iron is not protected against atmospheric influences. If the iron is steeped in a solution of sulphate of copper, 8 oz. ; sulphuric acid, 3i oz. ; water, 1 to 2 gal., for a short time, it becomes covered with a coating of pure copper, having a certain adhesion ; but shourd it remain there for a few minutes, the deposit of copper is thicker and muddy, and does not stand any rubbing. For coating large objects a solution, 1 part sulphate of copper, 2i parts soda lime, 7i parts sodio-potassic tartrate, and 25 parts by weight of water has been found effective, but they require several hours' immersion. In this case, compress it by means of rollers or a draw plate, in order to impart a certain cohesion to the particles of copper. Small articles, such as hooks, pins, or nails, are coppered by jerking them about for a certain time in sand, bran, or saw-dust, impregnated with the above solution, diluted with three or four times its volume of water.
Battery electro deposits of copper are obtained by decomposing a double salt of copper with another base, such as the double cyanide of potassium and copper. This process is equally well adapted to all metals, and the deposits are fine, lasting, and their thickness is entirely regu lated by the will of the operator. Dissolve about 16 oz. of sulphate of copper in 2 gal. of water, and add a solution of carbonate of soda until no more precipitate is formed ; collect the green preci pitate, carbonate of copper, thus obtained upon a cloth filter ; stir the washed carbonate of copper in water, to which cyanide of potassium is added until the carbonate is entirely dissolved, and the solution is colourless ; a small excess of cyanide will increase the conducting power of the liquor. This bath may be employed hot or cold and requires an intense electric current for its decomposition ; a copper plate or foil forms the anode, which must be removed when the bath does not work. This bath is much used, but the following formula is preferable. Water 2 gal. ; acetate of copper, crystallized ; carbonate of soda, crystals ; bisulphate of soda ; cyanide of potassium, pure, 7 oz. of each ; the acetate of copper is put in first and moistened with sufficient water to make a homogeneous paste, then add the carbonate of soda and some water ; after stirring, a light green precipitate is formed.