PYRO, PYROGALLIC ACID, PYRO GALLOL, OR TRIHYDROXYBEN ZENE (Pr., Acide pyrogallique ; Ger., Pyrogallol, Pyrogallussdure) Molecular weight, 126. It occurs in fine white feathery crystals produced by sub limation, and in heavy prismatic crystals of a more compact form (see " Pyraxe "). It is sold generally in blue bottles containing 1 oz. It is easily soluble in water, alcohol, and ether. It is not actually an acid, being neutral to litmus. It was introduced as a developer by P. Scott Archer, in 1851, at which time it was very expensive, the price, six years later, being one shilling a drain. For many years it was the principal developer used in the earlier processes, and later for dry plates. In its earlier days it was generally used with ammonia, which was gradually superseded by sodium carbonate, and to only a slight extent by potassium carbonate. As it readily oxidises when exposed to the air, the action being still more rapid in solution, it is necessary to use a preservative, which is generally potassium metabisulphite or sodium sulphite ; in the latter case the solution should be acidified with citric, sulphuric or sulphurous acid. The most convenient way of keeping pyro is in a 10 per cent. solution, an average formula
for which is :— Pyro . . OZ. no g.
Potass. metabisulphite 36 grs. 8 Water . . . 10 oz. 1,000 ccs.
The pyro is added last. Every io drops will contain approximately I gr. of pyro. Should other preservatives be preferred the following formula may be used : Pyro . . . I OZ. no g.
Sodium sulphite . 4 „ ,, Citric acid . . 3o grs. 7 ff Water (warm) to . 10 oz. 1,000 ccs.
The citric acid may be replaced by io mins. of sulphuric acid or 6o mins. of sulphurous acid. Dissolve the sodium sulphite in about 7 oz. of water, add the acid, then the pyro, and finally the remainder of the water. This will also con tain about 1 grain of pyro per To drops, so that any formula containing pyro can be wade up from it. Pyro combines with ammonia, soda, potash, metol, acetone, etc. It may stain negatives, hands and linen badly. Pyro developed negatives have a more or less strongly pronounced yellow or greenish-yellow stain, and therefore print more slowly and give greater contrast than clean negatives in which the image consists of plain developed silver, although the density may appear the same.