DEVELOPING SOLUTIONS 358. Organic Developers. The table on page 241 is a list of developers in general use. It gives their common names, the corresponding commercial names, their chemical formulae, dates of introduction for photographic use, the names of their authors, reduction potentials (as stated by A. H. Nietz, 1922) measured by the effect on their speed of development exer cised by potassium bromide' (the developers with the highest potentials being those least susceptible to the action of bromides), and, in the last column, the molecular weight (corresponding with the chemical formula).
In the following paragraphs these developers are dealt with, according to the same classifica tion, as regards their practical properties, with mention of some characteristics by which, if necessary, they may be identified.
359. Polyphenol Developers. Hydroquinone. Small, colourless crystalline needles, melting at 336° F. (169° C.) without decomposition. Very soluble in alcohol, ether, and hot water. Less soluble (6 per cent) in cold water ; insoluble in benzene.
Formerly there was sometimes used a yellow hydroquinone which was a combination of hydro quinone and sulphurous acid. It has no practical advantage over the regular (white) product.
Hydroquinone deteriorates in air, but slowly. It is used with alkali carbonates or caustic alkalis ; these solutions keep fairly well.
They do not stain either the gelatine or the fingers. Owing to its extreme sensitiveness to bromide, hydroquinone gives great density while retaining perfect transparency in the less exposed portions of the image, and is therefore a favourite developer when copying drawings, manuscripts, etc., in black-and-white without intermediate tones. It is also largely used for the development of warm tones.
For developing negatives of subjects including half-tones, hydroquinone is almost always used in admixture with a high-potential developer, generally metol" (§ 349).
Hydroquinone developing solutions are almost inert at low temperatures (55° F. and under).
Pyrocatechin. Colourless crystalline needles or scales, the smell of which is similar to that of pyro, melting at 1o4° C. (2r9° F.) without decomposition, very soluble in alcohol, ether, water, and benzene.
The oxidation products of pyrocatechin give rise, in gelatine containing the silver image, to a tanning effect almost as complete as that due to pyro, at least when the sulphite content is small. From a practical point of view, pyrocate chin does not offer any appreciable advantage over various other products of easier manu facture. It occurs, as a rule, in German formulae.
Pyrogallol. White crystalline powder of ex treme fineness and lightness (t oz. of resublimed pyro fills a 10-oz. size bottle), fairly easily oxidizable, on account of its state of fine division, the dust remaining floating in the air after handling. Crystal pyro is commercially available and is free from these disadvantages. Pyro has a smell ; it melts at 13o° C. F.) ; it is very soluble in alcohol, ether, water (over 25 per cent), and dissolves with difficulty in warm benzene.
Pyre is the developer most usually recom mended for use with plates and films of English and American origin. These emulsions tend sometimes to give images of soft contrast. Hence, the advantage of a developer like pyro, which gives, in addition to the silver image, a brown image of oxidation products (§ 350).
Pyre is certainly one of the best developers known, but, in addition to the staining of the gelatine (which can be almost completely avoided by increasing the amount of sulphite), it has the disadvantage of staining the fingers and nails a dark brown 1 (particularly if used continuously), unless means are taken to keep the hands from contact with the developer. It is the developer preferred by workers who en deavour during development to modify the character of the image by variations in the composition of the developing bath.