The rules which must be followed in order to ensure the effective fixation of photographic plates, films, and papers follow logically from these facts.
401. The Mechanism of Fixation. The mech. anism of fixation has been studied specially by Sheppard and Mees (1906) and by Warwick (1917). These workers used very different ex perimental methods, but arrived at identical conclusions, which are in conformity with the general laws of physical chemistry.
A fixing bath dissolves per unit time a con stant fraction of the mass of silver bromide existing in the coating at the commencement of the interval of time considered.
The magnitude of this fraction depends on the temperature and on the effective concentration of the bath ; it is independent of the silver content of the emulsion, of the quality of the gelatine, of its degree of swelling, and even of previous hardening of the film ; 3 it is greater for silver chloride than for the bromide, and for the bromide than for the iodide ; for the same silver salt it is greater if the emulsion is one consisting of fine grains.
The disappearance of the milky layer of silver bromide does not indicate that the solution of this substance is complete, but only that the opalescent layer is so much reduced as to be invisible ; at this instant there may be still more than 5 per cent of the original silver halide undissolved. 1 In the same way that the best time of develop ment may be determined by multiplying by a suitable factor the time taken for the first details of the image to appear (§ 384), so, by multiplying the time of disappearance of the silver bromide by a factor, the time necessary to give satis factory fixation may be calculated. The best margin of safety is obtained, when single-bath fixation is employed, by keeping the negative in the fixing solution after the apparent dis appearance of the silver bromide for a time equal to that taken for the milky appearance to disappear. 2 The curves in Fig. 172, taken from the experi ments of Warwick, show, for a surface of i sq. dm, of negative emulsion of average coating weight, the quantities of silver bromide (ex pressed in weights of metallic silver) to be dis solved and already dissolved at the various stages of fixing in a 20 per cent solution of hvposulphite of soda at 65° F. The curves
shown as heavy lines refer to an undeveloped plate ; the finer lines refer to a developed plate. A study of the curves shows the gradual progress of fixation.
402. Additions to Fixing Baths. At the instant of plunging negatives or prints into the fixing bath, the gelatine (and in the case of papers the base also), in spite of intermediate rinsing, is impregnated with the developing solution, especially in the deeper layers of the film. There is thus danger of development continuing in irregularly - distributed zones. Further, the accumulation of these substances in the fixing bath as more and more negatives are treated tends to produce troubles such as dichroic fog (§ 433) or general coloration of the gelatine by the oxidation products of the developer. Lastly, under these conditions, the fixing bath would become slightly alkaline, and it is in such alka line media that maximum swelling of gelatine occurs, so that it becomes very tender and tends to reticulate 434). These difficulties may be avoided by acidifying the fixing bath to a slight extent by the addition of bisulphite of soda (J. M. Eder, 1889), or of acetic acid in presence of sulphite of soda,' (A. Lainer, 1889) or of boric acid (H. Reeb, 1906; J. I. Crabtree, When the temperature of the bath or of the wash water, or of the air in which drying is to be done, rises above 68° F., it is advisable to harden the gelatine. From every point of view it is best in such cases to combine this operation with that of fixation by adding an alum to the fixing It remains, therefore, to decide which of the two available alums, ordinary alum (white) or chrome alum (violet), is preferable. Taking into account only hardening efficiency, choice should be made of chrome alum, and this is, in fact, always used in very hot climates ; its price is, however, considerably greater than that of ordinary alum, and also certain printing papers when treated with chrome alum retain— even after washing--a very slight green tint.