This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
The Effect Of Backing. The immediate effect of the use of backing can be easily observed by again taking the piece of clean glass and holding it at an angle, to obtain the reflection of a candle flame or other bright light.
167. If a dab of backing (as described later) is spread over the under side of the glass, the secondary image will promptly disappear, leaving only the bright upper surface image (See Illustration No. 26). The image on the under surface has been absorbed by the backing, and if the mixture is properly constituted so that it has the same refractive index as glass, every ray of light that strikes the surface coated with it will be absorbed instead of striking back and fogging the image above.
Non-Halation Plates. Numerous methods have been advocated, from time to time, to prevent undue halation. At the present time most of the leading dry plate manufacturers supply their plates prepared with two emulsions, the one next the glass being very slow, while the upper emulsion is of regular speed. When the image is exposed on such a plate the rays from the strongest lights will, of course, quickly penetrate the upper film, but when the slow emulsion is reached they will proceed with less speed, and very seldom will the rays of light penetrate through to the glass. As the object of this plate is to do away with halation, it is termed a NON-HALATION plate.
169. The Lumiere Non-Halation Plate has instead of the slow emulsion next to the glass, a coating composed of a brown pigment, which, of course, holds back all rays of light, not allowing them to reach the glass. Upon this brown coating is the regular emulsion. The plate is developed and fixed in the usual manner. The brown dye is removed in the fixing bath.
170. Non-halation plates are a great boon to the busy worker who has neither time nor inclination for backing his own plates. For those who desire to back their own plates, there are several methods at hand for expeditiously accomplishing the operation in a very satisfactory manner.
Methods Of Backing. A printing frame is probably the best piece of apparatus that can be used to assist in backing, but when a frame has been once used for this purpose it should be kept apart, and not employed for any other purpose.
Backing Mixtures. Caramel dissolved in alcohol or water forms the ideal backing mixture. It is for this reason that the following method of backing plates is recommended, as the mixture not only takes some little time to dry, but also is very sticky.
173. The following formula is a representative one for caramel backing, and can be recommended:
Gum solution (ordinary mucilage)........
Burnt sienna, ground in water...........................
Mix and add
174. The plate should be placed in the frame, glass side out, and a piece of smooth chemically pure, perfectly dry, fluffless blotting-paper placed on the film side, to protect it from injury when the back is put in place. The backing mixture is applied with a stubby, soft, hog-hair brush, and smeared lightly all over the glass surface. Pieces of tissue paper cut slightly smaller than the size of the plate should be prepared beforehand. As soon as the backing is applied, a piece of the tissue should be pressed into contact with the glass, and gently rubbed down. The plate should now be removed from the printing frame with the tissue paper adhering. It will be found that the edges of the plate are perfectly clean and free from backing. Further smoothing down of the paper will result in no particle of backing remaining in view to stick to anything. The plates can then be put into the plate holders, without danger of the backing coming off when dry, unless its removal is desired. All operations of backing must be carried on in the dark room, and as far from the ruby lamp as possible.
175. Another good method of backing plates is as follows: The backing is composed of burnt sienna mixed with water, and sufficient dextrine added to make the solution of a thick, creamy consistence. Upon a sheet of glass some of the color is distributed, then a roller is passed over the mixture and applied once or twice to the back of the dry plate, which will result in a perfectly even coating. If your desire is to dry the mixture very quickly, methylated spirit