Employ a mixture of ground potters' clay and plaster of Paris, nearly equal parts, moistened with water to the consistency of mortar; but ground soap-stone, chalk, or other material may be used instead.
The material is spread upon the metal plate and scraped down to any desired thickness, accordingly as lines are required in high or low relief, the thickness of the coating determining the relief elevation of lines in the finished plate.
Usually dry the plaster coating before any portion is cut away, but this is not essential.
Design may be pencilled, traced or transferred upon the surface of the plaster, or an artist sufficiently skilled may work without any copy. The coating is then cut away for the lines, entirely through to the metallic plate.
The plate at this stage of the operation resembles a mould for a stereotype plate as used in the clay or plaster process of stereotyping, except that the lines and letters are cut entirely through the plaster.
The mould or matrix is now made ready, and a metal stereotype plate is cast upon it in any manner usual in the stereotyper's art. This plate is finished up in the ordinary manner, and if more relief is desired for the lines, the low portions of the plate may be cut or routed out.
Door plates and other ornamental relief line plates may be made in the same manner.
Relief line plates can thus be made in a very short time. It is especially adapted for the speedy reproduction of plates for maps, diagrams, plans, etc.
The base plate of the engraving plate for stereotyping purposes is preferably a polished blued steel plate of suitable superficies. It withstands the heat incident to stereotyping, and its tint, when a light-coloured coating is used, presents a marked and agreeable contrast to the coating, and enables the engraver to readily judge the effect of his work. For electrotyping purposes, a base plate of glass is preferred. Glass is also an excellent material in combination with the special coating, and its transparency enables the engraver to examine his work by holding the plate to the light.
For the coating of the plate a finely powdered inorganic substance which will withstand the heat of molten stereotyping metal - is used. The more thoroughly and evenly the material is comminuted, and the more marked the contrast in colour between the coating and the base plate, the better is the engraving plate adapted for the purpose in view.
The leading features are that the particles of the coating next to the base plate adhere thereto more strongly than the particles above them adhere either to them or to each other, and that the coating is very friable and the particles very loosely as well as very evenly bonded together, so that they readily separate without caking and without breaking away between the lines when the plate is engraved; bond-ing the particles of the coating to each other and to the base plate with soluble glass or an equivalent soluble mineral alkaline bond; bonding the particles of the coating to each other and to the base plate by treating the particles with a solution of soluble glass, and then baking the coated base plate until the coating is dried; mixing together the ingredients to be bonded, then adding a water glass in solution, and then subjecting a base plate coated with the mixture to a heat beneath the boiling point and not under 100° F. until the coating is solidified, after which the heat may be increased without injury to the coating; the special combination of substances given in the formula is used to form the coating upon the base plate.
In engraving a design in the matrix, it is highly desirable that the engraver shall be able to see distinctly the lines made through the coating, and thereby' judge correctly the work being done. To this end the coating is made white or light-coloured, while the surface of the base plate appears dark, and the end is more effectually attained by employing a blued steel plate for the base plate.
The more perfectly it is mixed, the better the plate. A good way to make the mixture is first to mix the earth with water, and work and rub the ingre clients in a mortar until the mixture becomes smooth, and then add the solution of soluble glass and mix it with the other ingredients as thoroughly as practicable. The mixture should then be spread evenly over the base plate, shaking it slightly to settle the coating evenly. The coating should, after being placed on the plate, be dried by heat, preferably between 180° and 190° F., until it solidifies, after which the heat may be increased as high as 300° F. without injury to the coating. The object in keeping the heat beneath the boiling point while the coating remains in a semi-liquid state is to prevent its boiling. The coating should be thoroughly dried. The coating at its top is usually incrusted, and after the plate has cooled and before it is engraved, the crust should be scraped oft, and the coating made of a uniform depth over the surface of the base plate.
When the coating is mixed very slowly, it may be baked at once; but otherwise it should be allowed to stand after mixing for at least 5 minutes before baking, and is rather improved by being allowed to stand longer. The object is to allow the air contained in the mixture to escape as far as possible, and to prevent the coating from curling up and cracking while being dried.