=Middle Hard Gelatine=, manufactured by Ferd. Friedr. Creutz, absorbs five times its own weight of water at 66°, without signs of disintegration after standing therein for thirty-six hours. If prepared with it and with chemically pure potassium bichromate, the plates after drying have a beautiful matt surface, from which long numbers can be printed without any falling off of the half tints, and, if the printing is done carefully, even the details in the shadows will be maintained to the last. This bichromated gelatine requires also only a very small amount of re-damping, reproduces faithfully the character of the negative, and the jelly can be used even after a period of four days. If, however, the work in hand requires a harder bichromated gelatine—as, for instance, reproductions of black and white—then the addition of a small quantity of ammonium bichromate will have the desired effect; in some cases equal quantities of the bichromates of potassium and ammonium. If ammonium bichromate is used instead of potassium in conjunction with middle hard gelatine, then the plates will acquire a shining, transparent coating, the manipulation of which is very difficult in printing, and which work too hard, being serviceable, at any rate, for reproductions without half-tint. If, however, the middle hard gelatine is washed before use, it will give, when mixed with the chromic salt, transparent films of insufficient durability. Middle hard gelatine should therefore be used unwashed for Collotype work, and mixed with chemically pure potassium bichromate. =The Production of Grain= in Collotype plates takes place in the last instant of drying the plates in the drying-cupboard; it depends entirely upon the larger or smaller quantity, and the consistency of the bichromated gelatine used. If the latter is of good quality, all artificial means for the production of grain are not only superfluous, but also disadvantageous. A larger addition of chrome salt generally gives rise to the crystallisation of the salt; chloride of sodium produces plates highly sensitive to moisture, but no grain; a larger addition of chrome alum gives plates which will lack vigour, and the manipulation of which, during printing, will be difficult, as they often will require an “after-etching,” so that even printing is made impossible. To test the suitability of gelatine for Collotype work, two small portions of the gelatine to be tested may be made up, adding chemically pure potassium bichromate to the one, and ammonia bichromate to the other, and a plate prepared with each solution. If both plates show a dull and untransparent coating, this indicates that it is a soft gelatine; if, on the contrary, both plates appear glossy and transparent, then the gelatine is hard. Should it, however, be middle hard, then the dried film prepared with potassium will be dull, but not rough; that with ammonia, however, shiny and transparent. CHAPTER XIII. COLLOTYPE IN NATURAL COLOURS.