Pyrogallol

One of the earliest developing agents first used by Frederick Archer in 1850, pyrogallol (pyro, pyr-ogallic acid, 1,2,3-trihydroxybenzene) is poisonous and is very soluble in water. The image formed by a pyrogallol developer consists not only of silver but also of brownish developer oxidation products which stain and cross-link (tan) the gelatin. Pyrogallol was formerly used in combination with metol in a developing solution without sulphite, for processing negatives known to be severely under-exposed. It is now seldom if ever used for photographic negatives, but has found an important niche as a developer for reflection holograms, where its staining properties reduce scatter and its cross-linking effect on gelatin minimizes emulsion shrinkage.

Of all the developing agents described above, only phenidone, metol and hydroquinone are in wide-spread use today for the development of monochrome materials, while analogues of p-phenylene diamine (4-aminoaniline) are widely used for the development of colour materials. All developing agents should be regarded as being potentially hazardous by skin absorption, and rubber gloves should always be worn when preparing or using developing solutions.

Continue reading here: Restrainers antifoggants

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