RODINAL A one-solution developer consisting of a con centrated solution of para-amido-phenol introduced by Andresen ; it needs only the addition of water to make a working solution. Formerly it was obtainable in powder form, under the name of " unal." The following is one of the formul given by the Chemische Zeitung for making rodinal :— Potass. metabisulphite or sodium sulphite . . . . 3 parts Para-amido-phenol chlorohydrate i part Sodium hydrate (sat. sol.) . q.s.
Hot distilled water . . io parts When the first two are dissolved in the water, a saturated solution of sodium hydrate (caustic soda) is added very gradually until the precipi tate first formed is dissolved and the solution quite clear. Few photographers attempt to make it, however, as the commercial article is so cheap and good. It keeps well, darkening in colour with age. When diluted with water, however, it assumes a reddish tinge and gradu ally loses its developing powers, but if diluted with a 5 to io per cent. solution of pure sodium sulphite instead of water it will keep quite well. For normal exposures, use rodinal i part and water 20 parts. In cases of over-exposure, use less water, and add a few drops of a to per cent. solution of potassium bromide. In cases of under-exposure use from 3o to 4o parts of water. The more dilute the rodinal the softer will be the negative, and vice versa. When the extent of the exposure is not known, it is well to begin development with <1 t in 3o solution, and then, if necessary, to correct over-exposure by adding, drop by drop, a solution composed of 3 parts each of rodinal and water and t part of potas sium bromide. The solution should be added
to the developer in a measuring glass, and not direct into the developing dish. It is better, in a case of known over-exposure, to add the bromide to the developer before it is applied to the plate.
Many fail to get density with rodinal simply because they do not develop long enough. The image appears very quickly, and there is a temptation to remove at once the negative from the developer instead of giving time for density to be attained. Negatives developed with rodinal appear to lose much of their density during fixing.
For bromide and gaslight papers rodinal should be used in the proportion oft part to 20 to 3o parts of water, adding 2 drops of a to per cent. solution of potassium bromide to each ounce of developer. The tone or colour of the print varies according to the exposure and strength of the developer, strong solutions giving blue-black and weak solutions grey ; with too little bromide the high lights may not be clear, while with too much bromide there will be greenish blacks. These remarks apply also to lantern plates. For stand development the strength usually employed is z part of rodinal to too to 200 parts of water. Rodinal may be mixed with other developers, as explained under the heading " Developers, Mixed or Combined."