After treatment with alum, the tendency of gelatine to swell is greatly reduced, and its melting point is very considerably raised, some times to a temperature above 158° F.
In a warm and moist atmosphere spontaneous drying is generally slower than the growth of liquefying bacteria. Gelatine which has been suitably treated with alum will resist even the heat of direct sunshine, and drying can then be sufficiently rapid to prevent local liquefaction (L. J. Build, 1924).
4.03. The Chemistry of Fixation in Presence of Alum. The reactions of ordinary alum (double sulphate of aluminium and potassium 1 with hyposulphite of soda have been studied very completely by Seyewetz and Chicandard (1895) ; the reactions are almost the same with chrome alum (double sulphate of chromium and potassium).
On prolonged boiling, hyposulphite and alum are mutually and completely decomposed ; sulphur and alumina are precipitated, sulphur dioxide is liberated, and sulphate of soda is formed in solution.
In the cold the reaction is different ; hypo sulphite of aluminium tends to be formed. This substance is very unstable, and breaks down into aluminium sulphate and sulphuretted hydrogen. The latter, by reacting with excess of hyposulphite of soda, slowly yields bisulphite and hydrosuiphide of soda with precipitation of sulphur. The sulphate of aluminium being regenerated, the same reaction is repeated, but always slowly, because of the protective action of the bisulphite of soda, which is formed and which is only very slowly converted into thionates. 2 These reactions may be prevented or, at least, very much retarded, by the addition of sulphite or bisulphite of soda (Seyewetz and Chicandard), or of acetate or citrate of soda (P. Mercier, 1894).
Hardening by alums is favoured by neutral or alkaline condition, but when a mixture of ordinary alum and hyposulphite of soda is stabilized by sulphite, acid must be added, otherwise a white precipitate of aluminium sulphite will be deposited. This deposition of aluminium sulphite would occur, especially within the gelatine, on account of the alkali of the developer carried over with the negative, and this precipitate would be very difficult to remove.
For the different reasons given in the preced ing paragraph and above, an alum fixing bath must be acid to a sufficient extent to prevent precipitation of aluminium sulphate, even after a comparatively large quantity of developing solution has been added to it. It must, however,
not be so acid as to decompose the hvposulphite of soda.
Hardening by alum. is considerably reduced by the action of oxalic, tartaric, or citric acids, or of their salts, especially at high concentra tions.' Acetic, formic, and analogous acids and their salts are free from this drawback.
404. Fixing Capacity of Hyposulphite for Silver Halides. Hyposulphite of soda does not dissolve with equal facility the different silver salts occurring in an emulsion. Moreover, the power of the same weight of hyposulphite to dissolve the same silver salt increases as its concentration is raised. The following values correspond with saturation of the hyposulphite after long mechanical agitation, gelatine being absent (Richards and Burnells Faber, 1889)— Weight of halide dissolved per litre Concentration of .
Hyposulphite Silver chloride Silver bromide Silver iodide xo% 41 gem. 37 gym_ 3 gem.
20% 91 grm. 78 gem. to gem.
50% 213 g n - _ Under normal conditions of photographic practice the dissolving power of hyposulphite of soda is considerably less.
Almost all modem negative emulsions contain silver iodide, which is present also in some positive emulsions (notably in positive cinema tograph emulsions). This iodide tends to lower considerably the fixing power of hyposulphite. The inactivity of hyposulphite towards silver iodide explains the slow fixation of certain very sensitive emulsions, and especially anti-halation plates having a substratum of silver iodide.
It must also be remembered that every plate, film, or print introduced into the fixing bath, brings with it a certain amount of water and takes away with it an approximately equal amount of the fixing solution, when it is trans ferred to the washing tank. Thus the bath is gradually diluted, and at the same time the total amount of available hyposulphite decreases much more rapidly than it would do merely on account of the reaction of hvposulphite of soda with the silver salts.