The above method of mixing and baking is not the only one that can be used. The essential points are first to introduce the bond in the form of a solution into the body of the coating, and then to dry the coating upon the plate.
Instead of mixing the bond with the water and earths in the manner above described, the mixture may be made without the bond, and dried, with or without artificial heat, upon the base plate, and the coating may then be saturated with a solution of soda silicate containing 25-50 drops of soda silicate to 1 oz. water, and then baked as first above directed.
The formula above given may be widely departed from and good results still obtained, so long as the bond is of the proper nature and is used in the proper proportion. Either magnesia silicate or barium sulphate may be used alone in place of the mixture, though not so good. Barium sulphate is somewhat undesirable when used alone, because a coating made of it has a tendency to crack in drying.
All kinds of light-coloured earths may be used. Those which have the least attraction for water are the best. The clays are perhaps the least desirable of all, because of their tendency to crack when drying. It is desirable where they are used to mix them with an equal bulk of plaster of Paris or with some other substance which will prevent cracks, as the bond will not perform that function when used in the proper proportion.
The most desirable earths - naming them in their order of merit - are soap-stone, tripoli, talc, quartz, and chalk. Barium sulphate, though excellent when mixed with magnesia silicate, is not so good when used singly. Mixing a very light with a very heavy earth is a good course to follow. The specific gravity of a mixture of barium sulphate and magnesia silicate in the proportions named is the most desirable.
Where less than two drops of bond are used, the plate is very poor, because insufficiently bonded. Where more than eight drops are used, the coating is made too hard to be entirely satisfactory, and is more or less liable to break away from the base plate when being engraved.
A test as to the amount of water to be used is that the mixture when made should be thin enough to pour out of a containing vessel, but no thinner than necessary for that purpose. It should be of about the consistency of batter, so that a small portion will cling to the side of the vessel from which it is poured. Clay is an exception to the general rule as to the amount of water necessary. Where it is used, only one-half the quantity of water necessary for other substances should be used.
(a) Provide a smooth level plate of some suitable hard material and of a size somewhat exceeding that of the engraving which it is desired to produce. A piece of plate glass will answer admirably. The face of this plate is covered with a thin coating of tallow, lard, oil, or beeswax, spread evenly and enabling a sheet of tinfoil to adhere to the composition with such tenacity as to prevent it from being displaced when manipulated.
The tinfoil, having been attached to the composition on the plate, is ready for the draughtsman or artist, who, with a style, pencil, or other instrument, proceeds to draw the sketch of which an engraving is required, taking care to exert sufficient pressure to indent the tinfoil to the depth of the base plate, the intervening layer serving to give depth and tone to the lines, which may be made fine or coarse by using suitably pointed tools. In this manner letters may be written and sketches or drawings executed very rapidly and with great facility.
The next step is to place the prepared plate in a level position and to pour over it a quantity of plaster of Paris, of which the matrix over which the stereotype or electrotype afterward to be made is formed. As soon as the plaster is sufficiently hardened or set, the matrix is removed from the bed plate. This may be easily accomplished by slightly heating the under side of the plate, thus melting or softening the intervening layer, when the tinfoil, with its plaster backing, may be readily removed. - The face of the matrix (which is formed by the tinfoil) is now washed with benzine, turpentine, or other material, for the purpose of removing any portion of the intervening substance which may still adhere, and the matrix is now ready for stereotyping or electro* typing.
By drawing in the tinfoil with suitable tools, lines may be obtained as clear and sharp as the finest line engraving, the intervening layer serving not only to hold the foil upon the bed plate, but to give depth, tone, and richness. Its thickness may therefore be varied to suit circumstances.
To obtain correct likenesses of individuals, scenes from nature, etc, such likenesses are to be photographed upon the tinfoil, to serve as a guide for the draughtsman,' whose task is thus reduced to a mere mechanical one.
(b) First, provide a bed or base plate of steel or other suitable material that will resist heat, the top of which should be polished smooth and placed in a perfectly level position. Next, provide the plate with a coating of plastic material which will resist heat - such as slaked lime, kaolin, or trippli, which, when mixed with a suitable quantity of water, will form a plastic composition which may be spread evenly upon the top of the plate to form a coating of uniform thickness, and which will not melt or "run"by the application of heat.
The next step is to carefully cover the coating with a layer of foil which will not melt at the temperature where type-metal melts. Thin copper foil is well adapted for this purpose, and this should be provided with a thin film or coating of tin or solder. As thus prepared, the base plate is ready to receive the engraving by indenting the foil, down through the layer of plastic material, to the base by means of a stylus or other suitable instrument, according to the figure, sketch, or lettering which it is desired to represent in the engraving. The plastic layer operates to give depth and tone to the lines, which may be made fine or coarse by using different kinds of tools; and it will readily be seen that as the engraver has soft and yielding material to work in, the engraving, or rather indentation, may be executed with great rapidity. The plate having been prepared, the thin film or coating of foil is carefully moistened with muriatic acid, to prepare it to receive and firmly adhere to the type-metal backing. The plate is placed in a frame, after which a quantity of molten type-metal is poured over the foil to form a plate or backing of the desired thickness.
The type-metal, as it flows freely over the foil film, unites therewith without disturbing in the least the underlying foil or coating, and after hardening, the plate or relief block thus formed is removed from the coated base plate and is ready for use, after washing its face to remove any trace of the coating material, and suitably finishing the back.