SENSITISING THE PLATES.
A good formula for sensitising is as follows : Creutz middle hard gelatine ... 1 oz.
Potassium bichromate ... 50 grs.
Ammonium bichromate ... 30 grs.
Chrome alum ... „. 1 gr.
Water ... 10 ozs.
water is divided into two portions, the gelatine being in one and the rest of the ingredients in the other. When the gelatine has absorbed the water in which it is placed, it is melted at a gentle heat, and the second solution added gradually, stirring well all the time. The mixture must now be filtered (before it has time to cool) through swansdown calico or wash-leather, preferably by means of a filter pump (see Fig. 13S, p. 67), although an arrangement of the kind shown by Fig. 8S4 is also very suitable. The Warm plates are then taken out of the oven and levelled on a tripod (Fig. 885) or on levelling sorews (Fig. SSG). About fice. minims of solution per square inch will be required, a sufficient quantity being placed in a warm measure and poured in a pool on the plate, tipping the latter to cause the solution to flow all over, and helping it, if necessary, with a clean glass
rod. When evenly coated, the plate is replaced in the oven and allowed to remain with the lid closed for about two hours, the temperature being kept constant throughout. 'When dry, the film should be of uniform colour, free from specks or streaks of any kind, and of matt surface. If it is shiny the oven was too cold, and the results will be unsatisfac tory. Wavy marks or shiny streaks are caused by a draught or by vibration. On no account should the oven door be opened while the plates are drying. They will keep for some little time if well pro tected from light.