Each class of work demands paper of a suitable character, but the principle of an easily detached surface-coating is the same for all. The main difficulty experienced is the liability of paper to stretch when damped, and various methods are devised to obviate this, either by employing paper which stretches very little when damp, or by making the paper partially waterproof before use. _Papier-mâché._--This name indicates a preparation of paper or paper pulp mixed with various mineral substances firmly cemented together by animal or vegetable adhesives. The _paper pulp_ used for high-class goods consists of pure wood cellulose, while for the commoner qualities mechanical wood pulp, waste papers, and any similar fibrous material are employed. The _mineral_ substances used are china clay, chalk, gypsum, barytes, ochre, sienna, and other mineral pigments. The _adhesive_ materials are glue, casein, gum, starch, paste, dextrine, Iceland moss, or wax. For experimental purposes, small quantities of papier-mâché may be prepared in the following manner:-- When old newspapers or brown papers are used as the fibrous basis of the papier-mâché, they are first torn up into small pieces, moistened with hot water, tied up in a small cloth bag or sack, which must only be half filled, and then immersed in a basin of warm water and thoroughly kneaded by hand, so that the paper is gradually reduced to the condition of pulp. If the kneading process is carried out thoroughly the paper is entirely reduced to pulp. The excess of water can be removed by pressure and the preparation of the final mixture completed by the incorporation of clay, pigment, and adhesive. In the preparation of papier-mâché for goods on a large scale a beating engine is used in order to break up the old paper or wood pulp into a fibrous condition. The following formulæ can be used for making papier-mâché:-- -----------+-----------+---------------+----------- (1) | (2) | (3) | (4) -----------+-----------+---------------+----------- Pulp 22 | Pulp 22 | Pulp 12 | Pulp 33 Clay 37 | Chalk 30 | Rosin size 22 | Starch 9 Casein 37 | Glue 4 | Flour 11 | Clay 9 Water 4 | Water 44 | China clay 11 | Water 49 | | Water 44 | --- | --- | --- | --- 100 | 100 | 100 | 100 -----------+-----------+---------------+----------- _Plaster Moulds._--Plaster of Paris or gypsum is the main article used for moulds and pattern. The preparation of gypsum for casting is made as follows:--The gypsum is gradually worked up into a creamy paste with water, the mixing being done quickly yet thoroughly.