A used bath can be frequently revived by the addition of fresh solution of somewhat greater strength, but after a time it is necessary to discard it and to recover the 526. Use of Salted Papers. Prints on salted papers which are intended only to act as guides for drawing are simply fixed ; after the drawing is finished the image is destroyed by immersion in one of the " surface " reducing solutions previously mentioned (§§ 458-460). The paper is then washed and dried.
In all other cases the image usually has to be toned. While toning can be done after fixing, it is quicker and gives more regular results if done before fixation.
Gold toning is best done in an alkaline bath, and notably by " chalk toning " (§ 534) ; black tones can be obtained by toning with platinum (§ 538) after previous partial toning with gold.
Economy of the toning bath can be effected by washing the prints in several changes of water before toning ; a preliminary treatment in a chloride solution, recommended below for emul sioned papers, considerably slows the toning of prints in a gold toning solution which contains no solvent of silver chloride.
Fixing should be in an alkaline solution and should be followed by washing in plenty of water.
527. Albumenized Papers. From 18v until 1890 almost all photographic printing was done on albumenized paper (Blanquart-Evrard, 1847). Smooth paper was floated on the surface of a solution of albumen (white of egg) containing sodium chloride and citric acid. The paper was supplied ready for sensitizing, in the manner already described for salted paper 525), or ready for use. The introduction of emulsion coated print-out papers caused the gradual disappearance of albumenized paper, as com monly used.
Meanwhile papers sized with vegetable albu men, with casein and with other colloids, have appeared under the name of matt-albumen paper, and were used chiefly by professional portrait photographers. These papers require negatives which are much less vigorous than those for printing on to the older type of albumen paper, and softer still than those for salted paper. They arc usually prepared by coating a sensitive emulsion on to the paper, and the method of use is, therefore, similar to that recommended for emulsion-coated print-out papers. But the sensitive film is much less coherent, and all rubbing should be avoided, particularly when the prints are wet.
528. Sensitizing Silk. The working methods for sensitizing silk fabrics do not differ from those employed in the preparation of salted paper, except in the choice of a sizing material. This latter must be removed in the course of the various operations in such a way that the normal appearance of the fabric shall be preserved. A mucilage of lichen (moss) is usually prepared for this purpose in the following manner— Infuse 45 gr. (5 grin.) of Iceland moss (phar macist's moss) in about 20 oz. (L000 c.c.) of boiling water ; decant and filter the solution (which should be of a thin, syrupy consistency) while hot. In i8 oz. (900 c.c.) of the filtered solution dissolve 350 gr. (40 grim) of sodium chloride (kitchen salt) and acid 2 oz. (i00 c.c.) of glacial acetic acid to keep down all fermentation of the infusion. Keep the mucilage in a corked bottle.
Pour the solution into a perfectly clean dish and float the pieces of silk (which should have been previously marked on the back with pencil) on the solution, taking care to remove air bubbles and to prevent the liquid reaching the back of the material. This is done by taking the material by two opposite corners and allow ing the tip of the loop to touch the liquid first ; the corners are then lowered gradually until the whole is touching the liquid. After about two minutes the material is removed from the bath by taking hold of two adjacent corners ; it is dried by pinning to a stretched cord.
The pieces of material, which should be thoroughly dry and free from any smell of acetic acid, are then floated for about two minutes on the silver bath already described for salted papers (§ 525), drained, and dried.
As the image on silk is more or less buried in the fibres, the contrasts suffer, so that a vigorous negative is called for, particularly with material with a pronounced grain.
After printing, with the precautions specified in § 5o6, toning follows, and then fixing in the manner described for salted paper. After wash ing and drying, the tissue should be smoothed with a hot iron. Such tissue can be cleaned when need arises by the methods which would be used for the plain material itself.