PYROGALLIC DEVELOPER.—Development with pyrogallol, or pyrogallic acid, as it is erroneously termed, in a solution with an alkali, is more general than with any other. One of the chief advantages with this form of developer is the power of the operator possesses of vary ing it to suit the requirements of the subject. Greater latitude of exposure is also possible with this developer than any other. Pyrogallol is usually kept in the solid state, preventing decom position, but if required in solution it can be dissolved in a solution of sodium sulphite, slightly acidified with citric acid, and of such strength that the solution contains ten per cent. of pyro gallol. The darkening of the solution is thus prevented, as the sodium sulphite suffers oxidation instead of the pyrogallic acid.
Strong liquid ammonia, specific gravity .88o, is mixed with nine times its bulk of water, so that every ten minims contains one minim of the ammonia. The potassium or ammonium bromide should also be made up as a ten per cent, solution. Sodium carbonate, if used, should be purchased in the dry state, and made up in a ten per cent. solution. Potassium carbonate should also be purchased dry, and can be made up into solution as strong as 50 per cent. if
required. In making up these solutions the water must not be measured out but added to the solid until the solution measures ten times the weight of the solid used.
Sodium sulphite is usually made up as a 20 per cent. solution.
The pyro developer can be made up after hundreds of different formulae. Every maker of plates usually recommends a certain one, and it is generally advisable to give this a trial for that kind of plate. These formulae vary very considerably; for instance, one manufacturer of plates recommends Pyro Potassium bromide Ammonia 6 5 and another Pyro Potassium bromide Ammonia 3 7 12 The following is a table of the largest and smallest quantities of chemicals admitted to the pyro developer: Largest. Smallest.
Pyro per ounce to grains 11.4 grains Sodium sulphite 8o grains 5 grains Sodium carbonate 40 grains 11.5 grains Potassium carbonate 21 22.10 5 grains For various formulae for pyrogallic development, see Developer. PYROGALLIN.—See Pyrogallol.