Water 96·0 parts. Gelatine 1·5 " Perchloride of iron (in syrupy condition) 6·0 " Tartaric acid 6·0 " Sulphate of iron 1·5 " The paper is coated with the solution. After printing, the image is developed with a solution containing Gallic acid 1 part. Alcohol 10 parts. Water 50 " A final washing of the print with water completes the operation. COATED PAPERS. This term should properly include all the varieties of special papers which are coated with extraneous matter for particular purposes, such as art, chromo, tinfoil, gilt, emery, carbon, photographic, marble, and sand papers. In practice however, the term is almost entirely limited to "art" papers used for illustration work and half-tone printing. An "art" paper, using the definition given above, consists of an ordinary sheet of paper, one or both sides of which have been coated by the application of a mixture of a mineral matter, such as china clay or satin white, and some adhesive, like casein or glue. The object of the coating is to impart to the paper a perfectly smooth surface, rendered necessary because of the conditions under which the printing of the illustrations is carried out. [Illustration: FIG. 44.--General arrangement of Plant for making "Art" Paper.] The machine used for coating the paper consists of a large hollow drum about 40 inches diameter and 48 inches wide. The paper is brought over upon the drum in a continuous sheet, and the coating mixture applied to the surface by means of a revolving brush or an endless felt which rotates in a copper trough containing a coating mixture which is usually maintained at a temperature of 120° Fahr. The amount of material put on to the surface of the paper is varied by altering the proportion of water in the trough. As the wet coated paper is drawn over the drum it comes into contact with a number of flat brushes which move from side to side and brush the coating well into the paper. [Illustration: FIG. 45.--Sectional Elevation of "Coating" Plant.] The last two or three brushes on the drum are made of very fine bristles, so that when the coated paper leaves the machine the surface is perfectly even and free from brush marks. The wet paper is then drawn up an inclined ladder by an ingenious device, which causes the paper to fall into festoons or loops, and these are carried bodily forward by means of travelling chains. The process, somewhat difficult to describe, is more easily understood by a study of the illustrations given. The paper is dried by a current of warm air which can be obtained by means of steam pipes placed below the festoons or with a special air blower. The dry paper is then led through guide rolls and wound up in the form of a reel.