PLATES, FILMS, AND NEGATIVE PAPERS 227. Supports of Sensitive Coatings. The terms plate, film, and photographic paper are applied to photographic products resulting from coating sensitive emulsion on to glass, flexible trans parent material, and paper, respectively.
In Daguerreotype, which was the first practical photographic process, a thin sheet of silver plated copper was used as the support, and the sensitive coating was made by treating the surface of the silver with iodine vapour. Later, in the case of ferrotype, a process still used by itinerant photographers, sheets of iron covered with black varnish and then coated with a collodion emulsion were developed to give an image appearing more or less white on the opaque black ground, thus forming a direct positive.
Many attempts have been made to use alu minium, or of steel plated with nickel, as a sup port for gelatino-bromide emulsions. either as a flexible band for amateur cinematography (the image being observed or projected by reflec tion), or as a rigid plate instead of glass in accurate photo-cartographic work, where meas urements are made directly on the negative and where it would be advantageous to make use of sensitive materials as rigid as glass plates but less fragile.
228. Glass. The glass used in the manufacture of photographic plates is specially manufac tured, and is, in fact, the monopoly of certain Belgian firms. It must be fairly flat, of uniform thickness, almost colourless, and as free as possible from bubbles or black spots. These qualities are rarely united in one glass, even in the highest qualities for picture framing, which are of much better quality than window glass. The sheets of glass, received in crates, are sorted according to thickness and quality, and stored for distribution as required. Except in the case of very large plates the size required is not the size coated. Medium-size plates are prepared by coating a sheet which is twice or four times the size required, afterwards cutting at right angles, with automatic machinery. For small
plates, a sheet corresponding with eight or more is used. The glass is cleaned on both sides with alkaline solutions on machines having either rotating or reciprocating brushes. The plates, carried on endless belts, are rinsed in a large amount of water and then coated on the concave face, on which the emulsion will ultimately be coated, with a substratum consisting of a small quantity of gelatine in strong alum solution. This enables the emulsion to adhere to the glass 2 ; the plates then pass through a tunnel through which a current of warm air passes, and arrive at the end dry and ready to be transferred, on wooden racks, to the coating room.
The plates, loaded end-to-end on to the end less band of the coating machine, pass first under a coating trough, which distributes to them a uniform layer of liquid emulsion, and then through a cold tunnel, where the emulsion sets. The plates next pass to another endless band which, through moving more quickly than the first, separates them. On reaching the unloading station they are placed on racks for transport to the driers (drying rooms or con tinuous conveyor driers).
After drying is complete the plates are exam ined, cut and wrapped, etc., for delivery to customers.
229. Flexible Supports. Celluloid is still the best flexible support for films. For its manu facture in thin transparent sheets (H. Goodwin, x887; H. M. Reichenbach, x889), collodion, of a consistency approximating to that of honey, is prepared in mixing tanks by dissolving nitro cellulose and camphor (natural or artificial), in a mixture of methyl alcohol with the addition of another less volatile solvent such as amyl acetate. The highly-viscous collodion is passed through a filter press and on to the coating machines. The evaporation of the more volatile solvents leaves a very compact solution in the less volatile one, which results in a homogeneous, non porous, and transparent film which is actually a solid solution of nitro-cellulose in camphor.