The chief developing substances are pyrogallol, or gyro as the photographer calls it ; hydroquinone ; and methylparamino phenol, which is known under trade names such as "metol" and "elon." All of these developing substances are chemically related to benzene.
It was shown by Lumiere and Seyewetz and by Andresen that in order to be practically useful developers must contain at least two hydroxyl groups or two amino groups or one hydroxyl and one amino group attached to the benzene nucleus in the para or ortho position ; meta compounds have no developing power.
The three developing agents chiefly used are thus hydroquinone, which is para-dihydroxybenzene and is made by the oxidation of aniline and the reduction of the quinone formed; metol or methyl paraminophenol, which can be produced by the methylation of paraminophenol or by various other less direct reactions ; and pyrogallol, trihy droxybenzene, which is produced by the distillation of gallic acid.
In addition to these three, there are a large number of similar compounds which can be used as developing agents and for which advantages are sometimes claimed. In all probability, however, the three compounds given above will supply all the needs of the photographer.
Two compounds which have some wide use, however, are paraminophenol, which is intermediate in its properties between methylparamino phenol and hydroquinone ; and diaminophenol, or amidol, which is used for the development of papers.
With the exception of diaminophenol, these compounds are unable to reduce silver bromide in neutral or acid solution, and in order to make a "developer" alkali must be present in the so lution. On the other hand in the case of the aminophenols the developing agents are used in the form of salts of the bases ; thus, metol is the sulphate of methyl-paraminophenol, and it is usual to employ the chloride of paraminophenol and of diaminophenol. Enough alkali must therefore be added initially to liberate the base itself, which is usually insoluble in water, and to redissolve it as the alkaline phenolate. The most convenient alkali is sodium carbonate, and a typical developer would thus consist of pyrogallol with sodium carbonate added to it. As is well known,
however, alkaline pyrogallol absorbs oxygen from the air very rapidly and would soon darken and lose its developing power. In order to make a developer keep, a small quantity of sodium sul phite is added to the solution. The sulphite protects the devel oper from oxidation and thereby enables it to retain its full developing power, although the sulphite, when used alone, pos sesses no developing power whatever.
The essential constituents of a developer, therefore, are the de veloping agent—pyro, hydroquinone or metol ; the alkali, which is generally sodium carbonate; and the preservative, which is sodium sulphite. In addition, it is usual to add a small amount of alkaline bromide, which will act as a restrainer and prevent the development of unexposed silver bromide, giving rise to what is known as "fog." The various developing agents behave somewhat differently.
Suppose, for instance, that we make up two developers, one with hydroquinone and the other with metol, and start to develop a film in each at the same time. In the metol developer the image will appear very quickly, and will appear all over the film at the same time, the less exposed portions which, of course, were the shadows in the picture, appearing at the same time as the highlights. On the other hand, with the hydroquinone the image will appear more slowly, and the most exposed portions, or the highlights, will appear first, so that by the time the shadows have appeared on the surface of the film the high lights will have acquired considerable density. If development is stopped as soon as the whole image is out, the negative devel oped in metol will be very thin and grey all over, while that developed in hydroquinone will have a good deal of density in the highlights. Thus, of these two developers we may say that metol gives detail first and then slowly builds up density, while with hydroquinone the detail comes only after considerable den sity has been acquired. It is for this reason that these two devel oping agents are used in combination; the hydroquinone gives the density and the metol the detail, and together they make a satisfactory developer.