Amateur Photographer & Cinematographer (1933)

October 18th', 1933 The AMATEUR PHOTOGRAPHER p 6 CINEMATOGRAPHER s more appropriate by certain simple additions. These additions and the purposes for which they are intended are dealt with in the following article. THE primary purpose of hypo, whether employed as the sole or only the principal ingredient of a fixing-bath, is to remove every trace of unchanged emulsion from the developed image. This it does first by rendering the silver salts soluble, and then taking them up into its own solution. It will thus be seen that from the earliest moment of its use the hypo solution is prone to become con¬ taminated, and the more it becomes contaminated the less likely is it to perform its work cleanly and efficiently. For this reason, steps are fre¬ quently taken to preserve the active life of the solution concerned (a) by avoiding contamination as much as possible, and (b) by the addition of certain other substances which either neutralise contaminations at the source or render the hypo itself more able to do its work cleanly and without giving rise to stains. No general ruling can, however, be given as to the life of any hypo bath — whether plain or compounded. This depends entirely upon the work it has to do ; and this, in turn, depends upon the very variable amount of unchanged developer and emulsion that always accompanies both negatives and prints under¬ going treatment. Hypo and Rinsing. The photographer cannot, of course, avoid bringing over the unchanged emulsion (represented by the half-light and shadow portions of negatives and by the high-lights of prints), but he can restrict the introduction of surplus developer left adhering to the surfaces of the materials concerned. If this be continually brought over into the fixing-bath, it will rapidly alkalise the solution, with the result that the bath will be unable to check the progress of development im¬ mediately the materials are trans¬ ferred from the pyro, M.O., or amidol trough. Excessive and uneven develop¬ ment, together with certain obstinate stains, are likely to result from this ; hence it is very necessary in the first place to prevent the transference of this alkaline developer. This can be done by carefully rinsing the developed materials im¬ mediately before placing them into the hypo bath. In cases where a common tank is employed for both processes, it is sufficient to run off the initial developer and then to flush out the tank with plain water preparatory to introducing the fixing solution. Metabisulphite . Even this treatment, however, will not long keep the simple hypo solution free from alkali and a murky discoloration. So it is cus¬ tomary to add certain small quan¬ tities of a suitable acid substance to the original plain bath. Strong acids, such as hydrochloric, are not suitable. For this reason, recourse is always had to one of the milder organic acids, or to an inorganic substance which is not too strongly acid. Usually, metabisulphite of potas¬ sium is favoured. The proportions in which the substance is added vary somewhat according to the formulas of different authorities who have taken the trouble to write them down ; but for most practical purposes they may be accepted as one ounce of the metabisulphite crystals to 40 oz. of the familiar hypo solution (con¬ taining 8 oz. of hypo pea-crystals). Alkalinity . Gold-toning papers are not, of course, intended to be treated like plates or films or even like ordinary silver-halide printing papers. To them acidity is fatal ; and every precaution must be taken to prevent the introduction of active acids. In this case, it is advisable to make the plain solution somewhat weaker than that which would 348 suffice for negatives, and prints on bromide papers, and then to add a mild alkali. One very suitable solution can be made by dissolving 6 oz. of hypo in 40 oz. of warm water, and when solution is complete adding one fluid oz. of the “ strong solution of ammonia, B.P.” (‘88o Sp. Gr.). Perhaps a more convenient method of alkalising the bath is to add 1 oz. of sodium bicarbonate to the 40 oz. of fixer. Nor should any bath employed for the fixing of gold-toning papers ‘ be used more than once or for any one batch of prints. Failure to observe this precaution may lead to stains and a loss of permanency. Rapid Fixing. There are certain other substances which may be added to hypo solu¬ tions and made to exert appropriate influences upon the materials con¬ cerned. Thus, most photographers are familiar with hypo contain¬ ing toners, reducers and emulsion¬ hardening baths. Unfortunately, with most of these, the hypo tends to lose its own identity ; it is no longer regarded as the principal ingredient in the mixture, and in some cases it acts merely as a vehicle for some other constituent. The rapid-fixing mixture cannot, however, be numbered amongst these. It is at once a boon and a friend to most Press photographers, for it enables negatives to be cleared in a fraction of the time usually taken by plain hypo employed in highly-concentrated solutions. The principle employed in all rapid fixers is that of chemical acceleration or catalysis. That is to say, a substance is added which bestirs the hypo to do its work in an unusually short time. ‘ In this case, the sub¬ stance added is ordinary sal am¬ moniac, 6 oz. of a 40 per cent solution of which is slowly stirred into a solution of hypo (8 oz.) in water (40 oz.). Papers, of course, should not be treated with the hyposal ammoniac mixture. 7