SILVER-IRON PRINTING PAPERS 627. Sepia Papers. " Sepia " paper (H. Shawcross, 1889), which is prepared by soaking thin translucent paper in a solution containing iron ammonium citrate, citric acid, and silver nitrate, is widely used for making copies from tracings. Such copies, in white lines on a brown ground, may be used as negatives for making prints on ferro-prussiate paper. In this way it is possible to obtain copies in blue lines on a white ground with the latter paper. In this case printing is done by exposing the sensitive surface of the " sepia " paper in contact with the side of the tracing bearing the design, while the blue print is made with the sensitive surface in contact with the image side of the negative.' The keeping properties of " sepia " paper are determined principally by the deterioration of the paper support, the mechanical resistance of which is very quickly reduced, especially at high temperatures.
During the exposure to light, the image appears gradually, and the printing can thus be judged by inspection. On removal from the printing frame, the paper is placed at once in a 2 per cent solution of hypo, in which it is allowed to remain for about five minutes ; it is then washed in several changes of water. Although the image does not appear very opaque, it absorbs nearly the whole of the radiations to which ferro prussiate paper is sensitive ; its effective opacity is thus very much greater than its apparent opacity.
A more opaque image may be obtained by replacing the above-mentioned fixing solution by a 15 per cent solution of crystallized sodium sulphite or a 7-5 per cent solution of anhydrous sodium sulphite (R. Namias, 19o1). Such treat ment, however, being more expensive, is not generally used.
Sepia paper negatives, made from tracings, are occasionally used for process work.
628. Local Sensitizing of Papers (Notepaper, Menus, etc.). Sensitizing solutions for use with a brush have been put on the market from time to time for the local sensitizing of drawing paper, Bristol-board, or notepaper ; these solu tions are practically identical with the sensitizing mixture given above. As an example, the
formula of such a mixture is reproduced here, according to the analysis of a commercial preparation (E. Valenta, 1899).
Dissolve separately— Ferric ammonium citrate A green) . . . 2 OZ. (Ioo grin.) Distilled water, to make . 20 oz, (1,000 c.c.) Silver nitrate . 1,220 gr. (14e, grin.) I )istilled water, to make i z-16 oz. (600-Soo c.c.) Add pure ammonia solution drop by drop to solution (B), shaking continuously, until the brown precipitate first formed is re-dissolved. If an excess of ammonia, which can be easily recognized by its smell, has been added in this way, the mixture should be rendered odourless by adding very dilute sulphuric or citric acid drop by drop.
The two solutions are mixed in the dark, and the sensitizer prepared in this way should be kept in the dark until required.
For printing from somewhat weak nega tives, a few drops of a 5 per cent solution of potassium bichromate may be added to the mixture.
The solution is applied with a brush (the brush should not be bound with metal of any kind) on the parts to be sensitized, using it either plain or after it has been thickened, just before use, by the addition of a little freshly prepared starch or arrowroot Fabrics should be sensitized preferably by immersion, after the liquid has been thickened with a little solution of white dextrine.
Sensitizing should be carried out in a very weak light or in artificial light. Drying is done preferably in complete darkness. The sensitized sheets may be kept for several days in dry weather, or very much longer in a desiccated atmosphere.
Printing done in bright daylight ; the greenish tint of the sensitized parts allows of their being easily located. When the image has attained a slightly greater depth than that finally required, it is fixed in a 3 to 5 per cent solution of hypo to which about I to 2 per cent of anhydrous sodium sulphite has been added. After remaining five minutes in this bath, it is thoroughly washed. The sepia tone obtained in this way may be modified before or after fixation by toning with gold, this process also improving the permanence of the image.