COATING It will be found somewhat easy with a little practice to coat plates if the operation is prac tised first in daylight or gaslight, and for this purpose it is advisable to start with whole plates, assuming that one wishes subsequently to obtain quarter plates. A pneumatic holder (which see) should be obtained, and the sheets of glass thoroughly cleaned and stacked, with the surfaces to be coated away from the operator. The emulsion should be at a temperature of 95° F. (35° C.) in summer and 98° F. (36.6° C.) in winter ; and if the room is cold the glass itself should be warmed. The pneumatic holder is taken in the left hand, the bulb well squeezed, and the lip of the holder just wetted and then pressed on to the back of a sheet of glass in the centre and the pressure relaxed. The suction —really the pressure of the atmosphere on the surface of the glass—holds it firmly against the holder, wherein there is a partial vacuum. The glass should then be held horizontally, and the emulsion poured on to the middle, prefer ably from an earthenware teapot which has a spout that starts from near the base, as this avoids the air bubbles which rise to the top of the emulsion. Failing a teapot, the ordinary invalid's feeding cup would be a good sub stitute.
The pool of emulsion should be poured on to the centre of the plate, and, as soon as it covers about half the area, the plate should be tilted so as to cause the emulsion to run to the top right-hand corner, then to the top left-hand corner, then to the bottom left-hand corner, and finally to the bottom right-hand corner, and the excess drained off here. This must be done slowly, otherwise the emulsion will run over the edges ; and it is advisable to practise over a good-sized dish so as to catch any spillings. As soon as coated, the plate should be slid on to a sheet of plate glass accurately levelled, and allowed to set.
The coating of paper is not so easy, but it may be done by pouring the emulsion into a dish, tilting this, and drawing the paper over the top of the emulsion. At least a yard of
paper can be coated in this way, and with care but few bubbles will arise. But by far the simplest plan is to use one of the film develop ing dishes provided with a roller. Having the paper cut in long lengths, pass one end round the roller, and, keeping it tightly strained against the latter, pour in enough emulsion to cover a little less than half the diameter of the roller. Then the paper can be drawn through the emul sion and straight up, and enough will cling to it to give good results. Naturally, the emulsion must be kept hot.
Commercially, of course, special machinery is used both for plates and papers, and in the former case the cleaned glasses are fed on to the bed of the machine and coated with emulsion by various devices. Thence the glasses pass through an ice tunnel, which thoroughly sets the emulsion, and at the other end of the machine, which may be 3o ft to 4o ft. from the coating end, they are stacked in racks by hand and thence conveyed to the drying-room.
The commercial paper-coating machinery is usually arranged so that the paper, which is in long reels, passes round a roller through the emulsion. The coating is chilled either by a cold roller or by cold air ; the paper then passes on, is formed into loops or festoons, and traverses the drying-room, being again reeled at the other end.
In process work, coating is an important operation. For collotype, the gelatine coating is applied by levelling the glass plate and pour ing on a measured quantity of solution, guiding it to the edges by means of a catgut bow or a glass rod. For coating zinc or copper a whirler (which see) is invariably used, the coating being evenly spread by centrifugal force, while the sur plus is thrown off. In the case of very volatile mediums, such as bitumen, it is sometimes the practice to coat by pouring on the solution with a sweep of the bottle along the top edge of the plate, taking care to incline the plate so as to allow the solution to run down and the surplus to run off.