Failures in Collotype.
IN THE PREPARATION OF THE PLATES.
AIR bubbles in the albumen substratum. These must either by filtration or by the aid of a piece of paper be carefully removed from the solution before it is applied to the plates. The latter must be well cleaned and dusted, and freed of emery powder if they have been re-ground.
To Prevent Dust or other matter rising from the edge of the plates by capillary attraction while standing, they should be allowed to rest on clean blotting paper, and preferably on one corner only.
Coating Plates.—Great difficulties are experienced by the beginner in coating the plates with the chromated gelatine. He seldom knows beforehand the exact quantity required, or the temperature of the solution. The latter ought to be ascertained, as it is an important factor in the thickness and flow of the coating; with a solution only just warm it is impossible to apply a very thin coating, while the hotter the gelatine the thinner the layer. Considerable dexterity and judgment are necessary to spread an even layer of the requisite thickness, which is a matter most essential to success, but, unfortunately, only to be mastered by actual experience.
Plates are not unfrequently spoiled by inclining them while conveying them to the drying box, as in this way the coating is frequently rendered uneven.
All solutions must be carefully filtered, and the plates freed entirely from dust. The drying box, particularly the lid, must be carefully dusted and cleaned, but not immediately before use.
Other necessary precautions have been pointed out while describing the preparation of the plates. By strictly adhering to the rules laid down it will be found possible, after a certain amount of practice, to avoid the pitfalls here enumerated, while with regard to faults in the plates, there only remain to be mentioned the consequences of drying at too high a temperature.
Thick Films are extremely liable to fly off the glass and damage its surface, if dried at too great a heat. The thicker the film the lower should be the temperature of the drying box.
Thin Films, dried at too high a temperature, even without exposure to light, present a brown appearance, their sensitiveness is impaired, and they take too much ink over their entire surface.