PREPARATIONS FOR COATING.
In coating the plate, the first require ment is a levelling slab. The best form consists of a glass slab of patent plate A (Fig. 139), resting on a frame n, supported by three levelling screws c. This can be ad justed to a nicety. and may if desired be home-made. The details will be gathered from an examination of Fig. 139. This may with advantage be allowed to stand in a wooden dish n of sufficient size to allow of the screw c being easily manipu lated, according to the indications of the spirit level F.. All that is neces sary is to place the glass on the screws with a spirit level in the centre, and adjust one or other of the screws until quite level. To clean the glass, first wash it in a weak solution of caustic soda, rinse, and immerse in a weak solution, say 1 per cent., of nitric acid, again washing thoroughly. The final washing is best done in hot water, the frilling. Frilling is probably one of the most serious troubles the experimental plate maker will have to contend with.
It consists of an irregular expansion of the gelatine around the edges of the plate, causing it to pucker up and leave the glass. Once started, unless immediate pains are taken to harden the film and arrest. its progress, the latter is liable to completely leave its support and float off into the solu tion. The most likely time for such frill ing to occur is in the hypo. bath, particu larly if the weather is very hot or very cold, or the solutions of uneven tempera ture. In bad cases of frilling it will even start in the developer. To ensure a proper adhesion of the film to the glass is. there fore, most important..
over on to the arm—then finally to the bottom right-hand corner, pouring the ex cess into the coating pot. Figs. 142 to 195 will make this clear. If a certain quantity of emulsion be taken in a warm measure,