Some workers advocate a second fixing bath rather than a prolonged soaking in one bath, but if the first bath is fresh, properly mixed, and of the correct strength, a second fixing bath should be quite unnecessary. If the fixing bath is so weak or so loaded with impurities that it is unable to perform its work, it should be discarded, as it is in a condition to do harm a second bath cannot correct.
Where a variety of developers are in use, and if no care is taken to wash the plates thoroughly free from developer before placing them in the fixing bath, there is a possibility of trouble arising if the same bath is in use for all plates ; but there is quite as much risk of having the transparency of the gelatine impaired by failure to wash off the developer even when a freshly made bath is used.
A common mistake is to employ " hypo " fixing baths too strong. The silver bromide is less soluble in, say, a 5o per cent. solution than in a 25 per cent. solution. The latter, 4 oz. of " hypo " to 16 oz. of water, is a suitable strength for films and plates, but for papers half or even a quarter strength will serve. When a very weak " hypo " bath is used, the double salt referred to is likely to remain in the film, because there is no excess of " hypo " to act upon it. The bath should not be weaker than i in 5 for plates and / in io for prints. A fixing bath made of the usual strength, the ingredients being properly weighed or measured and not taken by guesswork, and used in sufficient quantity to cover the plate, will contain an excess of " hypo " that will act upon the double salt if time enough be given.
A solution of " hypo " will not attack the actual image which has been developed so long as the plate is well covered with the solution, but when a negative wet with the fixing bath is exposed to the air, the " hypo " in solution, in conjunction with the oxygen of the air, does attack the developed (reduced) silver which forms the image, with the result that the nega tive becomes thinner. This action is not very rapid, and no appreciable harm is done when the negative is taken out of the bath for examina tion, and a reasonable time may elapse before it is placed in the washing water ; but irrepar able damage is done when the plate is left in the fixing bath, or in the washing water after fixing, only partly covered with the liquid. The covering of the plate with a solution of " hypo " and glycerine, and leaving it exposed to the air, is, in fact, a little-used method of reducing.
As regards the exhaustion of the fixing bath, Messrs. A. and I,. Lumiere carried out some experiments (published in February, /907), and found that, to avoid subsequent yellowing of negatives on modern gelatine plates, it is advis able (1) to fix not more than one hundred 9 by 12 cm. plates in one litre of i 5 per cent
solution of " hypo " (this is roughly equal to about 120 quarter-plates in 35 oz.) ; (2) to fix not more than fifty of such plates (sixty quarter plates) in one litre of a /5 per cent. fixing bath, plus 1•5 per cent. of bisulphite ; (3) to fix not more than seventy-five plates (ninety quarter plates) in one litre of a 15 per cent. fixing bath plus 1.5 per cent. bisulphite and •5 per cent. chrome alum. The moment when the fixing bath is used up and should be thrown away can be determined by placing a drop of the bath on white paper, and exposing for some time to light and air ; if the spot turns brown, the bath is exhausted.
The function of the fixing solution is the same for prints as for negatives, although the silver salts may differ chemically and may be attacked under different conditions. The porous paper allows the " hypo " to act on both sides of the film, but this is more than counterbalanced by the larger amount of water that must be dis placed before the " hypo " can begin its work of dissolving the silver bromide.
While, on the whole, a plain solution of " hypo " is sufficient for the fixing of negatives and silver prints, several additions have from time to time been recommended, some with a view to hardening the film and so prevent frilling and blistering, others to prevent or remove stains from the film, and still others to keep the solu tion itself from being discoloured in cases where it is employed over and over again. The most harmful, and at the same time the most frequently recommended, addition is common alum, which decomposes the " hypo," liberating sulphur, and forming injurious compounds that may possibly interfere with the fixing, and lead to degradation of the negative or print.
As the result of a series of experiments, Messrs. Haddon and Grundy have stated that the hest strength at which to use a " hypo " bath is io per cent., and that such a bath will at a normal temperature completely fix a print in ten min utes. As to the exact amount of " hypo " to be used for each print, this depends on many circumstances, but on an average 2 oz. of " hypo " dissolved in 20 oz. of water will thoroughly fix 42o sq. in. of print—that is, one sheet of paper 24 in. by rye in., equal to about thirty quarter plate prints or about ten or eleven half-plate prints. Otherwise stated, it requires approxi mately from 8o grs. to 90 grs. of " hypo " in the form of a io per cent. solution to fix a half-plate print at normal temperature, immersing the print for ten minutes.
In process work, wet collodion negatives are invariably fixed in potassium cyanide, the usual formula being i oz. of 3o per cent. cyanide in 20 oz. of water.