This thread has made me think more about stop baths and fixer than ever before in my life! In my previous post I mentioned the 1942 Ilford Manual, curiosity drove me to consult this book again. Some may care to read all of the extract below, others may like to simply consider the recommendations of
a) acid stop bath plus 2-bath acid fix for permanency
b) plain hypo fix to retain maximum pyro stain.
Regards,
David
CHAPTER IX
FIXING, WASHING, AND DRYING
After development the plate or film is fixed, but it is bad practice to transfer directly from the developing bath to the fixing solution. Commonly, a short rinse is given in clean water and this is often all that is necessary, but a better technique involves the use of an intermediate acid stop bath, the function of which is to neutralize the alkali in the developer carried over in the emulsion layer. The action of the stop bath is to arrest development and it also tends to prolong the life of the fixing bath. It is of particular value when a highly alkaline developer, such as caustic hydroquinone, has been used. Although the bath must be acid it must not cause precipitation of sulphur from the fixing bath, and this rules out the stronger acids. Solutions of potassium metabisulphite (2 1/2 per cent.) or acetic acid (1 per cent.) are commonly used.
Occasionally a hardening stop bath containing 2 1/2 - 3 per cent. chrome alum is employed. This is particularly desirable when processing is being carried out at high temperatures, since the gelatin is hardened before it has had time to swell excessively.
The developed negative bears the silver image, but it is not yet in such a condition that it may be brought into the daylight or be used in the further operation of making the positive print. The silver halide which was not affected by exposure and which has not undergone reduction by the developer still remains in place, lowering the contrast of the negative and increasing the density of the image. This silver halide is still light sensitive and would gradually print out, changing colour and masking the image to a greater and greater extent as time went on. The remaining silver salt must therefore be removed and this is done in the fixing bath, which is in fact a solvent bath for silver halides. Of the possible solvents, sodium thiosulphate (hypo) is the one in general use-all the others have disadvantages of one kind or another. The alkali cyanides and thiocyanates, for example, although more rapid in action than hypo, exert a softening action on the gelatin and have a fairly considerable solvent action on the silver image. The cyanides have the additional disadvantage of being highly poisonous. They have been and still are generally used, however, in the Collodion process.
Hypo itself has a weak solvent action on the silver image and while its action is negligible during the time required for fixation, prolonged immersion in the fixing bath results in considerable reduction in density, and the effect may be very marked where fine. grain negative emulsions and printing papers are concerned.
The fixing bath removes the residual silver halides by transforming them into soluble double salts of sodium and silver which must themselves be removed from the emulsion. Some of them are not particularly soluble and, in addition, tend to break down to form silver sulphide. This is one reason why fixing baths should not be worked to exhaustion point-such baths would contain a certain amount of these unstable compounds and although they might be capable of dissolving silver halide they would leave the film with a very high concentration of the soluble double salts, including the unstable ones from the effect of which it might not be possible to free the film by subsequent washing. The practice of using two fixing baths, the last being relatively fresh, has much to recommend it.
For rapid fixing the optimum concentration of hypo crystals is approximately 40 per cent. (8 oz. per 20 oz. solution). Speed of fixation also increases with temperature, but wherever possible it should not be allowed to exceed 68°F. It is obvious that in tropical countries this condition cannot be complied with, but for such circumstances special processing instructions have been provided and no undue difficulty will be met with if the directions given are followed exactly (see page 187). The addition of ammonium chloride to hypo baths to speed up fixing has been advised but the procedure is of doubtful value.
The hypo should be dissolved in hot water, since the formation of the solution is accompanied by a considerable fall in temperature. If cold water is used to begin with the crystals will dissolve only very slowly and the bath will have to be warmed again before use. Where large quantities are involved the hypo may conveniently be placed in a muslin bag suspended just below the surface of the water. This method hastens solution and obviates any need for stirring.
Plain Hypo Baths
These are seldom used, since unless a satisfactory stop bath has been employed there is very considerable danger of the gelatin becoming stained due to the oxidation of developer carried over in the emulsion layer. It is more common to use an acid fixing bath which may be regarded as a combined stop bath and fixing bath, or an acid hardening-fixing bath, which is still more satisfactory. Such baths are described in the following pages. A plain hypo bath is, however, used when it is desired to get the maximum amount of staining with a Pyro developer (and also for certain kinds of paper). With plain hypo baths the white light must not be turned up until fixation is complete.
Acid Fixing Baths
The addition of a suitable acid to the hypo solution provides a more satisfactory fixing bath, since it prevents any danger of stain from the oxidation of developer carried over into the fixing bath. At the same time it arrests development immediately. Under these conditions white light may be turned up shortly after the sensitive material has been placed in the fixing bath. Acid fixing baths should be discarded as soon as they show any sign of depositing sludge on the film or become excessively turbid. ....
