HARDENERS Solutions used for hardening the films of gelatine negatives and prints with the object of preventing frilling and blistering. Their use is not so necessary now as it was in the early days of gelatine plates and papers, as means of hard ening the gelatine during its manufacture are now employed. As, however, the melting point of wet gelatine is about 9o° F. (about 32° C.), hardening solutions are still necessary in the tropics, and in colder parts of the world when the films are to be heated to beyond the normal tem perature. Chemicals which possess the property of hardening gelatine may be mixed with the " hypo " bath or used separately. Baths which fix and harden at the same time will be found under " Fixing-hardening Baths," but fre quently it would be fatal to leave the hardening until fixing, as the harm may be done in develop ing or toning. Formaline, ordinary alum, and chrome alum are the commonest hardeners, others being potassium nitrate, aluminium chloride, and aluminium sulphate. Formaline is by far the most satisfactory, 1 oz. of formaline being added to io oz. of water ; this requires about five minutes for complete action, but may be used weaker if desired—say, one part in twenty—in which case fifteen minutes' immersion will be necessary. The stronger bath is the less safe of the two, and may cause the film to become horn-like, crack, and leave the support. The formaline solution should be distinctly alkaline, inasmuch as acid and neutral solutions have but very little hardening effect upon gelatine ; also care should be taken that the hardening action goes right through the film and not merely half way.
Chrome alum is next best ; a suitable strength is I oz. to 15 oz. of water. Its tanning action is greater than that of ordinary white alum, and although it has a deep rich colour, it does not stain the film. Namias recommends the following chrome alum mixture, and states that it has great hardening action : Dissolve oz. of chrome alum in pint of cold water,
and add liquor ammoniw slowly until a pale green precipitate is plainly visible. Then add pint of a io per cent. solution of ordinary alum, and boil the whole for about three minutes ; when cool it is ready for use. Immerse negative or print from ten to twenty minutes. Ordinary alum (sodium, potassium, or ammonium alum) was at one time very popular as a film hardener, but it is not considered to be as safe as forma line or chrome alum. The proper strength is oz. to the pint of water. The plate or print should be immersed for from ten to twenty minutes. Any of the above hardeners may be used either before or after developing, toning, etc., but it is always necessary to wash well before and after treatment, more particularly when ordinary alum is used after developing and before fixing ; otherwise, ugly scum-like markings appear on the negative, and these cannot be removed. Alum markings are due to alkali from the developer remaining in the film, combining with the alum and precipitating aluminium hydroxide. When hardeners are used after development the alkali from the developer can be destroyed by rinsing the developed plate in a weak solution of citric acid, In process work, hardening solutions are useful. In the Paynetype process the plate is immersed in a 5 per cent, solution of potassium bichrom ate for three minutes, which hardens the gela tine image to such an extent that it can be developed like a carbon print. In the enamel process on zinc it is recommended that, after developing thoroughly, the plate should be placed for three minutes in a bath of— Ammonium bichromate . 2 oz. 44 g.
Chromic acid . . . „ x „ Methylated spirit . . 5 „ ioo ccs.
Water . . . . 5o „ 1,000 „ Wash, dry, and burn in. The image then resists the acid better. (See also " Fixing-hardening Baths.")