The electro-deposition of metal is much employed as a means of reproducing printing surfaces, especially being adapted for duplicating engravings and delicate work rather than ordinary type. The conduct of the process demands the provision of 3 separate rooms, for taking the moulds, working the battery, and finishing the plates respectively, efficient ventilation being a primary necessary. The battery room should not have a lower temperature than 50°-60° F. (10°-15 1/2° C), and demands the greatest attention in ventilation.
The fittings comprise a bench 2 1/2-3 ft. wide, placed in a good light; a small steam or gas stove, for melting the wax; and a few imposing surfaces. Also a cupboard for storing the acids and solutions in proper stoppered jars or bottles. The floor of the battery room is best of brick; a wooden floor may be covered with sheet lead or a bed of sawdust 3-4 in. thick, with some old plaster or mortar distributed through it. Sabots and a large apron are desirable for the workman.
The pot used for melting the wax and keeping it liquid generally consists of a round sheet-iron pedestal 3 ft. high and 18 in. wide, fitted inside with a zinc or sheet-iron pan about 18 in. deep. If a steam jacket is used, it should surround the whole of the pot; if gas-heating is adopted, the burners are restricted to the bottom of the pan.
The iron moulding-boxes generally measure 10 in. by 13 in. inside, the top rising 2-3 in. for the connecting-hooks, and the sides about 1/8 in. to accommodate the proper depth of wax; 2 holes are made in the head for connection by hooks to the copper rods. If the moulding frame is made with an electric connection gripper, the pan is of brass, and fitting inside is a conducting-pan, attached by clamps to a long bar 2 in. Wide, curved at the end to admit of hanging on the rods. This latter plan obviates the stopping-out process, as only the portion of the pan containing the wax is subjected to the chemical action.
The press employed may be either of the "hydraulic" or of the "toggle" pattern, according to choice.
The so-called process of "blacklead-ing" the mould, that is coating it with graphite, is often performed by hand; but a machine for the purpose is advisable, as being certainly more economical, not only in saving labour but also graphite. When done by hand, a large percentage Hies off in dust; but in the machine, the mould is so effectually closed in that no waste occurs, the surplus being received in a slanting box underneath the table.
The machine simply consists of a large box supported on legs, having in the middle a table formed of a series of rafters. To this is given a slow motion to and fro, the driving and reversing gear being placed at the back of the box. Fixed to 2 arms extending from the main shaft is a long brush, the same width as the table, having a vibrating motion of about 400 per minute. The whole is covered in by a box having a door at the end. The mould is placed on the travelling carriage or table, and a quantity of graphite is put over the surface. The machine being set in motion, the carriage carries the mould slowly backwards and forwards under the vibrating brush, and the graphite by this means is thoroughly beaten into the mould. The machine can be worked by hand.
Sometimes, when "blackleading" is done by hand, a glass-top case is used, with one end open to admit the mould and the hand of the operator. This method saves a certain - quantity of graphite, but it is questionable whether, considering the loss of time caused by the necessarily confined space, and awkward position of the operator's hand, it is really better than the usually adopted plan of merely placing the mould on its back, sprinkling the graphite on the face, and brushing it well in.
Blackleading brushes are made of goats' hair, the best quality only being used, or the moulds are likely to get damaged. Economy in brushes is effected by using the blackleading machine, as the action of the brush is vertical. In some machines, canvas replaces wood. for the bottom of the box (under the table) to catch the graphite; -it is fixed somewhat loosely, and sloping towards the front. The vibration caused by the rapid movement of the brush slightly shakes the canvas, and the graphite is thus automatically collected at the front or lowest part.
The battery jar is a thick, cylindrical stone jar, capable of holding about 8 gal.
The depositing trough must be of a size adapted to the amount of work to be performed. It matters little what its length is, but it should be at least 3 ft. deep, and wide enough to take 2 4to moulds suspended on the same rod. The trough must be of sufficient depth to afford space under the bottom of the moulds for the sediment of the solution to settle without touching the wax, as the copper of the solution becomes denser at the bottom, and if the moulds were to be allowed to dip too low, the deposit would be uneven, being much thicker at the base, while the quality of the metal would be inferior. The trough should be made of wood, about l 1/2-2 in. thick, firmly bolted together on the outside. Various materials are used for the lining, thick plate-glass being sometimes adopted. In this case the joints should be made with marine glue, Canada balsam, or gutta-percha; the whole can be cemented to the wood by liquid asphaltum or pitch. It may, however, be urged against this method of lining, that the joints are apt to "perish" by the action of the acids; and should a mould by accident fall into the solution, there is a probability of the glass becoming cracked.
With the joints well burned together, is the best material; solder is destroyed in time.
Several methods will suggest themselves for securing the rods which support the copper plate and the moulds. Thus a copper rod about 3/4 in. thick may be placed on each side of the trough, extending from end to end, the wires or copper bands secured to these rods, and the electricity passed to the copper plates and moulds; the connections being made at the end near the battery, one rod to the zinc or positive, and the other to the silver or negative pole. Or, on the top of the side of the trough, near the wall, may be secured 2 long copper bands about 1/2 in. thick and 1 1//2 in. wide, to which may be soldered, at proper intervals, upright sockets, in which one end of the rods rests, the other end being supported by nicks cut into the wood on the other side of the trough; one band being connected to the zinc and the other to the silver of the battery. But always take care to secure the rods in such a manner as to prevent their being accidentally shifted while the battery is in operation.