POWDER PROCESS, also known as the " dusting on " process.—A method of producing photographic images in any color upon paper, glass, metal, or other supports. The principle of the process is this : An organic tacky substance is sensitized with potassium dichromate, and exposed under a reversed positive to the action of light. All those parts acted upon become hard, the stickiness disappearing according to the strength of the light action, while those parts protected by the darker parts of the positive retain their adhesiveness If a colored powder be dusted over it will be understood that it will adhere to the sticky parts only, forming a visible image, the same being a reproduction of the positive printed from. The process is very useful for the production of lantern slides and transparencies, or for the reproduction of negatives. Any of the following formula may be employed for the manufacture of the organic substance :— The gum is first dissolved and the remainder of the ingredients added. It may be neces sary to warm the solution in a hot water bath to dissolve it. It is then filtered through flannel or clean muslin, and preserved for use in well-stoppered bottles. With this solution clear glass plates are coated and dried by a gentle heat over a small spirit lamp. The plate while still warm is exposed under a reversed positive* for from 2 to 5 minutes in sunlight, and from io to zo minutes in diffused light. The image is then but slightly visible. On removing from the print ing frame the plate is laid in the air (protected from light) for a few minutes to absorb a little moisture from it. The next process is the " dusting on." If the image is required to be black, fine Siberian graphite is spread over it with a soft flat brush. This will adhere to the parts unaffected by light, giving an image of the positive. Any colored fine powder may be used, giving images in various colors. When fully developed the excess of powder is dusted off and the film coated with collodion. After this it is well washed to remove the unaltered gum and dichromate salt. The film may, if desired, be detached from the plate and used for enamels, ivory, wood, textile fabrics, opals, etc.
By the above process it will be seen that to obtain a positive we require a positive. Mr. G.
W. Wood has recently patented a method of producing positives from negatives by this method. The principle of the process consists in using a black or dark-colored material, such as ebonite, No. r. Tannin, pure to to 15 grains Water (distilled) r ounce No. 2.
Gum arabic go grains Sugar candy zo grains Water (distilled) 54 ounces • No. 3.
Coffee (Mocha) ounce White sugar go grains Distilled water (boiling) 51 ounces No. 4—Sot. A.
Gum arabic 20 grains Sugar candy 5 grains Water 6 drachms Sot.. 13.
Gallic acid 3 grains Water 2 drachms Sol. B is prepared by heat, and mixed with sol. A. (See also Collodion Process, Dry.) Preservative for Sensitized Paper.—It is well known that albumen paper, sensitized with silver, will not keep for any length of time. Various preservatives have been recommended to improve this. The commonest consists of an addition of citric acid to the silver bath. The sen sitizing bath then reads:— Silver nitrate 6o grains Citric acid grains Water r ounces Paper thus prepared will keep for months. Debenham advocates the addition of io drops of pure perchloric acid to each fluid ounce of the silver bath instead of the citric acid. Commer cial sensitized paper is usually silvered with an ordinary bath, and the paper afterwards floated on a preservative solution of vulcanite, ferrotype plates, or glass coated with a black or dark-colored varnish as the support or ground, and dusting on a suitable lustrous substance, such as white, gold or other light-colored bronze powders or aluminium, magnesium, zinc, tin, silver, or other suitable metal or alloy reduced to powder.
It is obvious that if the ground be dark and the bronze powder be light we get the opposite effect to that obtained with a light ground or support and a dark-colored powder. By this means we are therefore able to get a positive (viewed by reflected light only) from a negative.
PRECIPITATION.—Solid matter thrown down from a state of solution by the action of heat, light or chemical reagent, is termed a precipitate. If one solution be added to another to form a precipitate the former is termed the precipitant.