The pattern of which it is desired to form a mould must be coated with oil. Around the pattern placed on a table a wall of wood or pasteboard is fixed, so that a basin will be formed of suitable depth, preventing the gypsum from flowing away. Patterns of figures or of curved articles have to be made in two or more parts. For that purpose the pattern is usually cut into two pieces. Two moulds are now readily obtainable by first oiling the pattern and by pouring the gypsum in a thin state gradually over the surface, to avoid the forming of air bubbles. The rapid drying of the soaked gypsum is sometimes inconvenient, but the addition of a saturated solution of borax in water to the gypsum mixture can be resorted to as a check. Various means are employed for hardening and strengthening the plaster cast, such as the addition of coarse paper fibres, shreds of canvas, iron filings, or wire. _Colouring._--Usually a cheap water colour only is required; a light coating of a cheap varnish may be sufficient. In other cases a water colour serving as a filler for smoothing the surface may receive a finish of one or more coats of resinous solutions in alcohol or of copal varnish. Many goods are coated with asphaltum or Japan varnish and dried in cold or hot air. Some of the articles may be decorated with scrolls or arabesques in oil colours or enamels, or the lines may be covered with bronze powder, or with metal, gold, or aluminium leaf. _Varnishing._--The following varnish recipes are suitable:-- --------------+---------------+--------------+------------- (1) | (2) | (3) | (4) --------------+---------------+--------------+------------- Shellac 20 | Shellac 10 | Shellac 6 | Sandarac 15 Alcohol 70 | Rosin 10 | Sandarac 3 | Mastic 5 Lamp black 10 | Alcohol 60 | Mastic 18 | Turpentine 5 | Lamp black 20 | Alcohol 73 | Alcohol 75 --- | --- | --- | --- 100 | 100 | 100 | 100 --------------+---------------+--------------+------------- CHAPTER VIII CHEMICALS USED IN PAPER-MAKING The manufacture of paper is a highly technical industry, which requires a practical knowledge of mechanical engineering, as well as an intimate acquaintance with the many important chemical problems connected with the art. The following brief description of the various chemicals used in the manufacture of paper is divided into certain classes, based upon the order of the operations through which the raw material passes before its final conversion into paper:-- (1) The alkaline processes used for treating raw fibre: soda ash; caustic soda; lime; recovered ash. (2) The conversion of wood into sulphite pulp: sulphur; limestone. (3) The operation of bleaching: bleaching powder; antichlors; acids.