The original ought to be taken out of the mould before the latter becomes perfectly cold and rigid, or it will be very difficult to extract. Next pour in plaster of Paris, after having wetted the mould to prevent bubbles of air lurking in the small interstices; and if the mould be in 2 pieces, it is generally convenient to fill them with plaster separately before putting them together. Dry the plaster casts either wholly or partially. Paint the casts in water-colours, which must be fainter than the hues of the original, because the next process adds to their intensity. The delicate shades of colour in the original will be marked in the cast by the different quantity of the same colour which is taken up by the different textures of the cast. After drying the cast, steep it in hard paraffin: ordinary paraffin candles will serve the purpose. Cool, and hand-polish the cast with steatite.
(13) Photographic Plaster Casts
The following method of taking plaster casts by means of photography originated with Fink.
An ordinary piece of patent plate-glass, which should measure 2 or 3 in. each way bigger than the original, is coated in the dark-room with a mixture made up of the following solution: - In 15 oz. water is dissolved 1 oz. potash bichromate, the former being warmed gently, and then gradually 2 oz. gelatine are added. As soon as the latter has dissolved, and the solution has about reached the simmering point, it should be filtered through fine linen into a glass beaker, and then poured upon the glass plate above referred to. The gelatine solution is poured upon the centre of the plate, and then drawn towards the margins by means of a fine brush. It is applied again and again until the thickness amounts to about 1 1/2 line. As plates prepared in this way require 2 to 3 days to dry, it is well to prepare a good many at one time, and to place them when dry in a box well screened from the light. The sensitiveness of the plates has not been found to suffer, even after preservation for a period of 6 weeks.
When a suitable negative has been obtained of the object, and, furthermore, a diapositive from the negative, the prepared plate is placed, face downwards, against the collodion side of the cliche* in the printing-frame, and printed in diffused daylight for a period varying from 10 to 40 minutes. The plate is then taken out of the printing-frame (in the dark room), put into a dish, and poured over with lukewarm water until the relief is fully developed. The plate is then dried by means of filter-paper, and coated with glycerine (any superfluity of this substance being also removed with filter-paper), a fine and large badger brush being employed for the purpose.
The plate, which has hitherto been manipulated in the dark, may, after the development of the relief, be further manipulated in daylight, and the plaster cast proceeded with in an ordinary workroom. The manner of making the cast is as follows: - In a couple of evaporating dishes, some alabaster gypsum is put, and two mixtures are made with ordinary spring water, one having the consistence of oil, and the other that of thick cream. The gelatine mould is taken in one hand and a little of the thinner plaster liquid is poured upon it, the mould being at the same time tapped with the open hand from the bottom, in order that no air-bubbles be formed. After this the plate is placed horizontally upon a table, and the thicker paste is poured on, making a film 3/8 to 1/2 in. high. This latter, after it has stood for 15-18 hours, is carefully separated at the edge with a knife, and by employing a little force the cast is removed from the mould. This plaster cast may be employed for many purposes, and will serve for taking a casting from, with a metal fusible at a low temperature. .With amateurs and photographers, such a proceeding is, however, difficult, and if a metallic cast is required it is best to send the plaster one to a type-foundry or similar establishment.
Retouching may be done if necessary with a needle upon the plaster cast. There is not so much difficulty in taking metallic casts from the plaster moulds. It is only necessary to thoroughly bake the casts, and while still warm brush over with a little wax. (Eng. Mech.)
(14) Coats That Can Be Washed
The prize offered by the Prussian Minister of Commerce and Industry for a method of preparing plaster casts that permit of being washed was conferred upon Dr. W. Reissig, of Darmstadt. From Dr. Reissig's essay on the subject the following points are abstracted: -
In preparing these casts it is not only desirable to obtain a surface which should not wash away, but also to include a simple process for preventing dust entering the pores, and render them more easily cleansed. Laborious experiments showed that the only practical method of accomplishing this and retaining the sharpness of outline was to convert the lime sulphate into (1) baryta sulphate and caustic or carbonate of lime, or (2) into lime silicate by means of potash silicate. Objects treated in this way are not affected by hot water or hot soap solutions, but from the method of preparation, they remain porous, catch dust, etc, and when first put into water eagerly absorb all the impurities. To avoid this evil, subsequently coat the articles, now rendered waterproof, with an alcoholic soap solution, which penetrates more easily, deeper, and more freely into the pores than an aqueous solution. After the alcohol evaporates, a layer of soap remains, which fills the pores, and when washed it is converted into a sud which removes the dust without allowing it to penetrate.
This is the simplest, easiest, and cheapest method. It depends upon the fact that gypsum, or lime sulphate, is converted by baryta water into baryta sulphate (which is totally insoluble), and caustic lime, which latter is converted by contact with the air into lime carbonate.