Catechol Pyrocatechin, also known as Pyrocatechin and Pyrocatechol, is used as a developing agent for film or paper.
It gives warm black images, with rapid development and high contrast. Pyrocatechin also acts as a tanning developer, which stains (actually fogs) the negative and also hardens the emulsion protecting it from reticulation.
Pyrocatechin is easily soluble in warm water. It is chemically akin to hydroquinone but with special properties, notably the fact that it oxidizes readily and its oxidation products have a tanning effect on the gelatin
When used without sulfite, or a very low sulfite content, it gives a heavily stained image and tans the gelatin in proportion to the density of the image.
This property has led to its use for high-definition and tanning developers (Windisch Extreme Compensating Developer). Used with caustic alkali it is one of the best compensating developers (Maxim Muir's Pyrocatechin Compensating Developer).
All developers containing pyrocatechin should be mixed with distilled water, whether or not the formula calls for it. Pyrocatechin is poisonous and should be handled with care, use protective gloves, a respirator, and amble ventilation.