The DARKROOM COOKBOOK, Third Edition

Pyrogallol and Pyrocatechin 61
Edward Weston’s Variation The main difference between standard ABC Pyro and Edward Weston’s
variation is the dilution ratio. Weston diluted the formula with 30 parts of water. The result
was a softer working developer with an even fuller than usual tonal scale.
In order to keep the developing time within workable limits and to prevent complete
exhaustion of the developing agent before development was complete he increased the
developer to 3 parts instead of one. His resulting dilution was 3:1:1:30, with a developing
time of 15 to 20 minutes at 70F/21C with continuous agitation in a tray.
Aerial oxidation does not appear to be as serious with Edward Weston’s dilution since
the amount of developing agent has been increased three times, allowing the process to be
complete before the developer reaches a point of exhaustion conducive to staining.
PYROCATECHIN (CATECHOL)
Even though it has been in use since 1880 pyrocatechin (catechol) is one of the most overlooked
developing agents. The working properties of catechol are very much like those of
hydroquinone and it can even be used as a substitute for hydroquinone in many formulae,
though it works more rapidly than hydroquinone.
As a developing agent, catechol tans the surface gelatin of the negative (see Tanning
Developers, below). It is also easy to use catechol to compound formulas that create a desirable
image stain density when low quantities of preservative (e.g., sodium sulfi te) are used
(see Image Stain, below). Both of these properties it shares in common with pyro.
In addition, catechol has a reputation as a useful developing agent for subjects of extreme
contrast. This is because the use of minimal or no preservative with catechol (as stated
above) will produce an image stain that enhances the image silver density in the shadow
regions while the developing agent itself will rapidly oxidize preventing the high values from
overdeveloping (Formulas: Extreme Compensating Developers).
Dilute catechol developers will provide a speed increase with most fi lms.
TANNING DEVELOPERS
As developing agents, both pyro and catechol (pyro/cat) are commonly used to harden
the gelatin of the fi lm’s emulsion. This allows development to take place only on the surface,
thus minimizing the effects of halation and irradiation (Chapter 4: The Anatomy of Film).
Negatives developed by this method, known as tanning, exhibit an extremely high degree of
acutance. The effect is greatest in areas that have achieved the most exposure. The tanning
process is enhanced by a low concentration of sulfi te or its complete absence. In fact, if more
than 2.0 to 3.0 grams of sulfi te per liter are present in the formula the tanning action will
be limited.
There are other developing agents, chief among them hydroquinone, that will produce
a tanned image when used in a formula similar to D-175 (Formulas: Extreme Compensating
Developers: D-175 Tanning Developer). Even so, pyro/cat are the two most commonly used
for this purpose.