Moulding.—The first operation in electrotyping is to run melted beeswax on to a perfectly level metal plate called a "case." This must be such that it will withstand the great pressure exerted on the moulding press. When the wax has had time to "set" the surplus is trimmed from the edges with a knife, the case then being put into a machine which shaves the surface of the wax down to a standard height, making it of uniform thickness. The wax is usually flared over with a gas flame to give it a smooth surface, the case having to be kept at a regular temperature until the moulder is ready to take a mould on the wax from the original. This is a most important operation. Before moulding, the case and the forme are brushed over with fine graphite, care being taken to have all the loose particles of black-lead removed before being placed in contact with each other. They are then put in a press and subjected to the necessary pressure. The presses are of great strength and are usually operated by hydraulic methods. When the mould is made the case is taken to a work man called a "builder" who trims the wax which has spread over the edges of the case with a knife, at the same time reducing to an even height the wax standing in high relief through being forced into the lower parts of the forme which do not have to print. The mould then goes to the "black shop" where it is care fully and thoroughly dusted and polished with graphite. This is done either by hand or machine, the purpose being to make the surface conductive, as otherwise the copper shell would not grow upon the mould. A thin piece of copper is now inserted into the wax at the top end of the case. In the copper there are two holes through which hooks are placed to enable the case to be hung on the negative rod in the depositing bath.
Previous to placing the mould in the bath "stopping out" takes place. This is done by running a hot iron over the surface of the wax to within about an inch of the actual moulding on three sides of the mould, thus removing the graphite, and rendering the surface non-conductive. If this is not done a waste of copper will be entailed, but care has to be taken that the copper con nection inserted into the wax comes within the area of conduc tivity, since otherwise it would not be possible to obtain a shell. The mould is thoroughly washed over with a strong force of water to remove all foreign matter from the surface. The modern tendency is to obtain moulds by the .lead process. A piece of specially prepared lead is pressed into the original forme by a press of great power. It is claimed that a greater degree of accuracy and a finer reproduction is obtained by this method because changes of temperature do not affect lead in the same way as wax.
Deposition.—The mould is now ready for the depositing bath, although it sometimes is previously prepared by covering it with a solution of copper sulphate into which fine iron filings are sprinkled and the whole stirred with a soft camel's-hair brush, thus precipitating a fine film of copper on the mould. Extreme care has to be taken to see that all connections are clean.
The depositing equipment consists of a bath or tank which is lead lined. This is filled to within about 6 in. of the top with water into which is placed copper sulphate and a percentage of sulphuric acid. When completed the solution should have a density of about Beaume. When working, the solution is agitated to keep the concentration uniform while a regular tem perature has to be maintained. The electrical current necessary for depositing is obtained from a dynamo situated in close proximity to the bath. The current is carried from the dynamo
through a resistance board to the bath by two copper rods which run along the length of the tank, one being connected to the positive pole and the other to the negative pole of the dynamo. Placed on the long copper rods are shorter copper rods running across the bath. On the short rod connected with the positive pole and suspended by means of hooks into the solution is a copper plate called an anode. The prepared mould is suspended in a similar manner on the other short rod connected with the negative pole with its face towards the anode.
The copper sulphate when dissolved in the water and sulphuric acid breaks up into two portions :—(1) a copper portion which is electrically positive, (2) a sulphate portion which is electrically negative. The positive copper portion is attracted to the negative pole, i.e., the wax mould and is there deposited while the negative sulphate portion travels to the anode (which is positive) and there combines with the copper to yield more copper sulphate so that the concentration of the bath is kept more or less con stant. In this way the shell is grown until a sufficient thickness is obtained.
Nickel is deposited in a similar manner, but the bath is com posed of a solution of nickel sulphate and ammonium chloride. Electrotypes are sometimes faced with a deposition of nickel to increase their durability and also to prevent certain chemical actions taking place which may occur when printing with coloured inks from a copper surface. The deposition of chromium is at the moment receiving much attention, it being one of the hardest metals and of great durability.
Backing.—When the deposition is complete the mould is taken from the bath and the shell removed by pouring hot water over it until the wax melts, the wax removed being melted and used again. The next operation is the backing up of the copper shell which when it is released from the wax is trimmed round the edges and prepared for receiving about a in. of molten metal. As the metal (which is composed of lead, tin and antimony) will not adhere to copper, the back of the shell has to be "fluxed." This is done by brushing it over with a soldering fluid and cover ing it with "tin foil," heat being applied until the two fuse together. The shell is then placed face downwards on to a pan or tray with flanges, and molten metal poured over the back and allowed to solidify.
When the backing is completed it is removed from the pan and the face cleaned free from wax. It is then trimmed and passed through a series of finishing operations such as roughing and plan ing the back, routing the "blanks," bevelling the plate and mount ing it on wood or metal, great care having to be exercised in every detail to ensure that the printing face is not damaged.
It is by the process of electrotyping that t'he duplication of the highest grade of printing plates is undertaken and that the reproduction of the finest half-tones and colour plates in accurate register and with fidelity to the original is made possible.
A Swiss inventor has made "shells" by spraying metal on to a mould, using a special tool, called a "pistol," through which metal in the form of wire is automatically fed. This is fused by gas and spread in a state of sub-divisions by means of com pressed air on to a matrix. Should this invention prove success ful, the whole method of the duplication of printing plates may become revolutionized.