Pyrogallol (sometimes miscalled pyrogallic acid) is a white, fluffy solid which stains any object with which it remains in contact for any length of time.
The chief characteristic of the pyro developer is the stain image which is formed as the same time as the regular silver image. The stain image is usually denser with a used, partially oxidized developer than with a fresh solution. Since the stain image stops light just as does the silver, the printing density of the negative often tends to increase as the solution is used. The stain formation is sometimes used to build up density on underexposed negatives be immersing them in old, oxidized pyro developer until a uniform stain has been built up so that satisfactory prints can be made.
Pyro is one of the oldest known developing agents and has been widely praised for the gradation and printing quality of its negatives. The tendency for staining can be eliminated with some formulas.
by Edmund W. Lowe, “What You Want to Know About Developers Fine Grain and Otherwise”, p. 51