A very important operation in photography is the washing the negatives and prints in order to remove from them ti chemicals of the fixing bath. If an exhausted fixing bath has bec used, silver compounds will be present during washing, and it most important that they should be removed by complete washin The rate of washing depends largely upon the rate of diffusic of hypo out of the film into the water, provided the water i contact with the material is continuously removed. In washin1 the relation between the time of development and the temperature are in common use in photography. Fig. io shows such a curve supplied for use of X-ray workers giving the time of development for a standard developer at various temperatures.
A negative or print transferred to the fixing bath invariably carries some developer with it, and if this developer accumulated in the bath it would cause stain or fog. It is customary, there fore, to add acid to the fixing bath to neutralize the alkali from the developer and prevent this action. If free acid is added to a thiosulphate solution, it is decomposed forming sulphur. This can be prevented, however, by the addition of sulphite which "preserves" the bath. Weak acids are used so that a relatively large quantity can be employed without making the bath too acid. Acetic acid is generally used. Materials are also added to the fixing bath to prevent softening of the gelatin. The commonest is alum, and a typical fixing bath therefore contains 20% to 3o% of its weight of hypo, about 1% of anhydrous sodium sulphite and of potassium alum, and 3% of 28% acetic acid. The developer alkali carried over by the negatives or prints to the fixing bath tends to neutralize the acetic acid and eventually may cause a sludge of alumina to form in the bath, and decrease its ability to harden. This tendency to sludge can be reduced by adding boric
acid to the fixing bath. This also extends the hardening life of the solution.
The rate of fixing depends upon the concentration of the hypo and on the temperature. It increases to a maximum with about hypo and decreases with stronger solutions owing to a de the amount of hypo remaining in the gelatine is continually halved in the same period of time as the washing proceeds. An average negative, for instance, will give up half its hypo in two minutes, so that at the end of 2 min., half the hypo will be re maining in it ; after 4 min., one-quarter; after 6 min., one-eighth; after 8 min., one-sixteenth; io min., one-thirty-second, etc. It will be seen that in a short time the amount of hypo remaining will be infinitesimal. This, however, assumes that the negative is continually exposed to fresh water, which is the most important matter in arranging the washing of either negatives or prints.
Reduction.—When a negative is too dense, it is sometimes reduced. By reduction in photography is meant the removal of some silver from the image so as to produce a less intense image. Thus, in the case of an over-developed plate there will be too much density and contrast, and the negative may be reduced to lessen this. In the case of an overexposed negative there may not be an excess of contrast but the negative will be too dense all over, and in this case what is required is the removal of the excess density.
All the photographic reducers are oxidizing agents, and almost any strong oxidizing agent will act as a photographic reducer and will remove silver, but various oxidizing agents behave differently in respect to the highlights and shadows of the image. Reducing solutions can be placed in three classes:— A. Cutting reducers B. True scale reducers C. Flattening reducers.
A. The cutting reducers remove an equal amount of silver from all parts of the image and consequently remove a larger propor tion of the image from the shadows than from the highlights of the negative. The typical cutting redacer is that known as "Farmer's" reducer. This consists of a mixture of potassium ferricyanide and hypo, the potassium ferricyanide oxidizing the silver to silver ferrocyanide and the hypo dissolving the latter compound.