The acceleration of fixa tion by the addition of ammonium salts is, how ever, counterbalanced by a disadvantage (Lumiere and Seyewetz, 1908). The com plex hyposulphites of silver 0.11,1 C1.1111.11l alla 1,11111G, these circumstances are much less stable than the complex hyposulphites of sodium and silver ; the practical limit of use is only about half, and the risks of discoloration of the image are increased. This acceleration, although advantageous in cases of extreme urgency, has no practical application in ordinary practice, particularly when negatives are to be kept.
406. Fixation in Two Successive Baths. In all industrial operations involving the extraction of a substance by a solvent, the mass to be treated is at first extracted with solvent already almost saturated in previous operations, fresh solvent being only used to extract from material which is almost exhausted. Often, indeed, the process is made continuous by causing the solvent to circulate in the opposite direction to that of the material to be treated. This method of system atic extraction, which allows a solvent to be used most efficiently, may be applied with advantage to photographic fixation ; it permits of making the most of a solution of hyposulphite whilst giving perfect fixation and permanence of the images. Furthermore, it has the advan tage that the residual silver salts are left at higher concentration, and therefore may be recovered more economically, such recovery being always carried out more easily when the substances are in concentrated solution.
In 1894 A. Miethe, on the basis of the experi mental results of Haddon and Grundy, recom mended fixation in two successive baths, separated by a brief rinsing. Under these conditions the first bath may be used well beyond the normal limit possible when fixation is carried out by the single-bath method. In fact, this bath will not be discarded until the first phase of fixation, indicated by the dis appearance of the milky layer of silver halides, occupies an abnormally long time. It is true a portion of the silver will remain in the form of the insoluble hyposulphite, but during the second stage of fixation, carried out in a solution of hyposulphite which is almost fresh, the complete solution of the silver salts will be effected. Negatives treated by this process will be fixed as well as if a fresh bath of hyposulphite had been used from the start.
Since the negatives reach the second bath almost completely fixed, there is little more to be done, and thus the practical limit of safety is not attained. When the first bath has to be abandoned, the second is made the first and is itself replaced by a new one.
Negatives are taken from the first bath when there appears to be no more silver bromide to dissolve ; they are then placed in the second bath and allowed to remain for about the same time as in the first bath.
407. Choice of the Best Concentration of Fixer. The amateur, having to develop only one or two negatives, and not wishing to keep the bath for subsequent use, will preferably use a fairly dilute bath, for example, 15 per cent hyposul phite (=- 3 oz. in 20 oz. of water).
The professional or commercial worker, work ing almost continuously and using the two-bath method, will be well-advised to use much more concentrated solutions, which may be almost completely exhausted.
If speed of fixation only be considered, it would appear best to use a 40 per cent solution (§ 4o5), but other factors lead to the use of a less concentrated bath.
For one thing, the sudden change of concen tration on passing from a too concentrated fixing solution to the washing water, especially if the water is comparatively warm, may cause frilling or reticulation with plates and films, or blisters with papers. For another, if white light falls on the sensitive coatings during fixa tion, and if the concentration of the fixer be more than 30 per cent, there may occur in the film a slight insoluble residue which will not disappear, however long the negative or print be left in the fixing bath ; at concentrations above 5o per cent, this insoluble residue may form even in the dark (Welbome-Piper).
For these different reasons fixing baths are usually employed at concentrations between 20 per cent and 30 per cent, i.e. 4 oz. (200 grm.) to 6 oz. (300 grm.) of crystallized hyposulphite of soda per 20 oz. (I,000 c.c.) of bath.
408. Tests for Exhausted Fixer. When fixation is carried out by the single-bath method it is obviously only possible to calculate the area of sensitive material which has been fixed by a given volume of the bath. This calculation is, however, not likely to be of any value, unless data are available concerning the amount of silver in the emulsions used and the volume of the solution removed by the plates, etc., already fixed.