FIXING, INTENSIFYING, AND REDUCING DEVELOPMENT PRINTS AND TRANSPARENCIES 581. Fixing Developed Images. The action and chemistry of fixing are the same as in the case of negatives (§§ 400 and 401). Most fre quently acid fixing baths will be used, and in warm weather the fixing solutions containing alum already given (§ 409).
Some varieties of paper tend to tint slightly in a fixing bath containing chrome alum, especially if it is acidified with sodium bisulphite only. Before first using a new brand of paper, the following simple test may be made. Take two strips of the unexposed paper, fix one of them in a plain solution of hypo, and the other in a fixing solution containing chrome alum. After washing and drying, these two strips should be compared in a strong light. If the whites of the paper fixed in the solution con taining chrome alum are tinted, however slightly, chrome alum should on no account be used ; the hardening of the gelatine can be ensured, if it is necessary, by corrunon alum.
The use of alum must be entirely avoided with prints intended for Bromoil or any pigment process (§ 698).
It has been sometimes noticed that fixing in an acid bath may be the cause of defective sulphide toning with certain papers (§ 587 et seq.), the whites showing a tendency to become stained ; and more so according to the pro portion of bisulphite in the fixing solution (Moss, 1920). The prints made on such papers which are to be sulphide-toned may with advan tage be fixed in a solution of hypo rendered alkaline with a little sodium carbonate.
It should be added that when an acid fixing solution is used at a relatively high temperature, there is a risk of yellow or brown stains occur ring by local sulphuration of the image or of bromide of silver not yet dissolved.
582. The fixing of prints is carried out prefer ably in deep dishes containing fixing solution of a depth of at least 2 in. A paddle of hard wood should be provided, of broad shape, and with the sharp edges rounded first with a rasp and then with glass-paper. The prints can then be
completely immersed without having to put the fingers in the solution.
Certain papers float on a solution which contains 25 per cent of hypo, an average strength for fixing baths for negatives. In such cases it is necessary to dilute the bath sufficiently to reduce its density until the prints tend to sink to the bottom of the dish. It is well known that silver images are reduced during fixing by the free access of atmospheric oxygen to their sur faces (§ 399). Moreover, parts of the prints protruding from the solution may continue to develop, if they have not been through an acid bath between development and fixing ; or yellow staining of the same character as dichroic fog may result 433). Similar defects may arise with prints fixed face downwards if air-bubbles have been imprisoned under them.
The duration of fixation in a new bath of 20 per cent strength, used at 65° F., is about to 2 minutes. The immersion should not be prolonged beyond 5 minutes. The fixing bath is being constantly diluted by the liquid im pregnating the prints placed in it, and its strength is reduced to about one-half after 350 quarter plate prints have passed through a pint (20 oz.) of bath. Never try to fix more than zoo quarter plate sheets in a pint if only one fixing bath is being used. Excessive acidity (which may be due to leaving the prints too long in a very acid stop bath) retards fixation and favours the formation of the insoluble complex salt.
Whenever possible, fixing should be done in two successive baths (§ 406).
Prints should always be introduced into the bath singly, and moved every now and then, the lowest print being brought to the top, and so with all in succession. The prints should also be taken from the bath one by one to be trans ferred either to the second fixing solution or to the first washing water.
With bromide papers only, an increase of contrast is sometimes noticed when the prints are placed in the fixing bath, either neutral or acid.