235. Another process is that suggested in 1891 by A. and L. Lumiere, and applied in 1892 by Magersted (Isolar plates). Before being coated with emulsion the support is coated with collodion or hardened gelatine coloured red with dyes which do not diffuse into the emulsion and no injurious effect on it, and which will also, as far as possible, be decolorizecl by the acid fixing bath. For this purpose rosolic acid and its salts, aurine, coralline, and congo red, have been used. Red dyes are used for ordinary and orthochromatic emulsions ; dark brown or black dyes for panchromatic ones.
H. Oakley proposed, in 1895, to coat the glass with a solution of gelatine and, after drying, to treat the gelatine with permanganate to form the brown hydrated manganese dioxide. This would be eliminated, when fixing, by the sodium bisulphite usually present in the fixing solution. Anti-halation plates have since been prepared by coating with an emulsion of colloidal hydrated manganese dioxide in gelatine, which gives a transparent yellow-brown Such a substratum does not interfere at all with the control of development by examination by transmitted light, when the operations are carried out in red light. At the most it inter feres with the examination of the image by viewing the back of the plate, but we do not regard this as a real drawback.
236. Many English plates, especially those of the panchromatic kind, are protected by a coating of lamp-black in dextrine or caramel, applied to the back of the plate after the emulsion is coated (backed plates). Such a coat ing should not be too smooth, which would result in confusion between the two faces of the plate when the slides are filled in darkness, nor should it be for detached particles depositing on the emulsion would cause pinholes, worse in their effect than halation. It should not become tacky, as it would then adhere to the dark slide, and it must be quite soluble in water, or at any rate must soften in water, so that it may be easily removed with a sponge in the early stages of We also mention the use on plates and films of a dorsal anti-halation coating consisting of a very thin layer of varnish (shellac or cellulose derivatives soluble in water) deeply stained by dyes which are not readily soluble in water, but which are decolorized in the developer which also dissolves the varnish.
The user may himself coat any plates which he cannot obtain ready backed. The following method of working (A. Helain, 1901) gives excellent results, the addition of the ammonium chloride to the dextrine ensuring perfect adhe sion of very thin layers of the backing— To prepare the backing Lamp-black (refined) . oz.
(to to 12 grm.) Yellow dextrine . 4 oz.
(too grm.) Ainmonium chloride I oz.
(6 grrn.) Water . . . 4 oz.
(oo to too ....) The lamp-black is moistened with a little denatured alcohol ; the dextrine is then added, and then the water in which the ammonium chloride has previously been dissolved. The mixture is stirred with a spatula until even, and is not used until it has stood for 24 hours ; it is then again stirred. The quantities given are sufficient for from 21 to 27 sq. ft. (200 to 250 sq. decimetres). Coating is best carried out with a stiff flat paint brush (a very thin, wide brush, rectangular in shape), of such a size that one stroke is sufficient to cover a plate. Practice in daylight on plain glass is useful. It is well to coat the plates in pairs. For this purpose the plates are held, emulsion sides in contact, either in the hand or in a screwed polishing clamp. The coating should be very thin, but no portion of the glass should be left bare. 2 The coated plates are left to dry in a cupboard in the dark-room, or in a light-tight cupboard (approximate time, 3 hours).
237. A French brand of plates has, for a long time, been made with a black anti-halation backing film. This film, which contained gelatine, glycerine, ox-gall, and a black pigment, adhered perfectly to the glass until the plate was placed in any solution, when it was readily stripped off in one piece. (L. Lesueur, 1907.) A similar result may be obtained by the user, at least as far as plates insensitive to red are concerned, by coating or brushing on to the backs of the plates collodion (P. and P. Henry, 1890) or celluloid varnish coloured with chry soidine and safranine or with other dyes soluble in collodion solvents, or even with a little bitumen varnish.
238. Reference may finally be made to anti halation backing sheets, made by coating ordinary or mounting paper with a mixture of glycerine, gelatine, and fine black pigment. These arc applied to the backs of the plates by a squeegee or a rubber roller, and may be removed, for subsequent use, by stripping after the exposure has been made. These backing sheets may also be prepared by coating waste photographic paper with glycerine. Prior to this treatment the paper is darkened by exposure to a uniform source of light, and by development if necessary, and is then washed thoroughly.