The base for this form of matrix is a metal plate whose surface is slightly roughened with sand-paper. It is next rubbed over with white of egg, and flooded with the chalk wash made as follows: Flong paste (described under Papier Maché Matrices, [page 80]), six ounces; whiting, twenty-four ounces; water, three pints. The whiting is softened by soaking for an hour or more. The whole must be intimately mixed. It should cover the plate to the depth of one-thirtieth to one-twentieth of an inch. The plate is dried in a perfectly horizontal position.
When dry the design or writing, etc., is made with a smooth steel point, the lines being carried clear through the white layer to the metal. The mould is now baked at a temperature well above boiling water; as high as 392° F. (200° C.) may be reached without harm.
If the coating seems too thin, an extra coat can be given between the lines especially over the larger areas. This must be done before the baking. A pipette may be used for putting on this coat. This deepening has the bad effect of increasing the chance of the coating stripping from the metal.
The matrix thus prepared is used in the press just as is the ordinary plaster matrix. It is suited for reproduction of autographs, scrip, diagrams, etc.
CHAPTER IX.
THE MAKING OF VARIOUS SMALL ARTICLES OF INDIA RUBBER.
India rubber can be so readily shaped in moulds and the latter are so readily made of plaster of paris that any one who is interested in such things will find endless amusement in working out different designs. Before suggesting any specific articles the following are the general points to be kept in mind.
The material may be uncured mixed sheet of any thickness. As we have seen this material when heated and pressed runs together. It can be forced into any shape by comparatively slight pressure. So exactly does it reproduce the smallest line or mark, that care must be taken to have the moulds very smooth and free from defect. Powdered soapstone is used to prevent adherence to the mould, but great care must be taken not to mix it among the pieces of the india rubber, where several are used in one article, as it will prevent their coalescing or running together.
Another point is to contrive to introduce the proper quantity of rubber. The aim must be to have a slight excess, but to avoid waste this should be as little as possible. Unless some rubber is squeezed out there is no certainty that the mould has been filled. Any projecting “fins” from the overflow are cut off with a knife or scissors after the article is removed from the mould.
Plaster of paris or dental plaster mixed with dextrine or gum arabic water or the zinc oxychloride cement, already described, is to be recommended for the moulds. They should be made, if deep, in frames or “flasks” of tin, as plaster if unsupported is liable to split open when the rubber is forced home.