The composition of the film, the exposure of the plate through the back, and the careful treatment of the layer in the actual printing operations are all matters which considerably affect the “life” of the plate, and to which we shall again return. =Chloride of Zinc= is used for hardening special parts of the collotype plate, and is easily prepared by dissolving—to saturation—sheet zinc in hydrochloric acid, filtering, and reducing as circumstances may demand. =Tannin= and =Tannic Acid= in strong solution harden the film, and may be applied to certain parts to cause it to “take” the ink. For instance, to remove, or rather harden, spots which refuse the ink and consequently show upon a dark ground; and also to write names or titles to the plates. Written additions of this character must, of course, be reversed, as in lithography. The addition of tannin to the chromated gelatine before coating the plate is recommended by Waterhouse, he considering such addition confers greater lasting power to the films so treated. =Cyanide of Potassium and Ammonia.=—All alkalis or carbonates of the alkalis have a directly opposite effect upon the printed layer to that last referred to. By the judicious use of one or the other of these agents spots printing light or dark may be either removed or considerably modified. =Glycerine and Sugar= and similar easily soluble organic substances, added to the chromated gelatine, would assist the easier and evener damping of the plate, but render it more susceptible to damage, and the desired effect is better attained by suitable additions to the damping water. =Organic Acids.=—Citric acid and tartaric acid are recommended as additions to the chromated gelatine, partially as influencing the so-called “grain,” and also as additions to the damping solution to facilitate the production of cleaner prints. Experience with these substances shows that the print layer suffers from their use, and for clearing up a plate inclined to yield dirty impressions a weak solution of ammonia is far preferable. =Nitric Acid= has even been recommended for occasionally damping the plate in certain cases, after which the most delicate half-tones should be reproducible. An experienced collotypist ought never to be under the necessity of adopting such desperate remedies, which more often than not rapidly and entirely spoil the plate, and it is better to at once prepare a new one. =Ox Gall= remains to be mentioned as exercising a beneficial influence upon the print layer under certain circumstances. It considerably hardens the gelatine, and tends to reduce the annoying tendency of the film to adhere to the paper in the high lights. Ox gall was first used at the Royal Printing Works in the “glass” printing process there utilised for the reproduction of line subjects. Some operators of eminence consider that in collotype its use should be strictly confined to lineal representations, as by its continued use the half-tones are partially lost.