When Good Chemicals Go Bad; Tips For Maintaining Freshness And Best Image Quality
Many people seem to believe that darkroom chemicals have a fixed capacity,
beyond which they stop working. This is quite a long way from the truth. The
life of most chemical baths used in the darkroom can be divided into four stages.
First, there is the fresh bath, with full vigor. Second, there is the partially
exhausted bath, which still works but takes longer. Third, there is the bath
that appears to work but has a concealed and sometimes fatal flaw. Fourth, there
is the bath that is so exhausted it will not work at all, or at least not within
a realistic length of time.
Knowing how and why the various chemicals die is very useful if you want to get the maximum quality and value out of your darkroom chemicals, so here is a basic guide. Of course you can simply rely on the manufacturers' stated guidelines, but if you do, there are two things worth knowing.
First, the manufacturers assume "average" conditions of use and
storage. A film developer, for example, will exhaust faster if the images are
overexposed or if the pictures are of very light subjects, because there is
more silver to develop. Also, more air means faster oxidation. Make up solutions
with water that has been boiled and allowed to go cold for minimum air content,
and decant concentrates and stock solutions into smaller bottles with no airspace
or otherwise exclude air. Warm weather speeds oxidation, too. Some people refrigerate
their developers, though this can be risky with some formulations if crystals
form and fail to re-dissolve.
Second, some manufacturers' guidelines are considerably more relaxed than others: some quote capacities that reflect no visible change (e.g., the bath is always in its first, vigorous phase) while others assume a small degree of deterioration will be acceptable and can be compensated for by extended processing. Also, the degree of deterioration that is acceptable will depend on the photographer.
Developers die in five ways: through hydrolysis, oxidation, bromide build-up, pH change (loss of alkalinity), and actual exhaustion of the developing agent(s). With print developers, the collapse is fast enough that if you are looking for it, you will see it in the course of a print or two. Contrast falls, and maximum black weakens. If you are developing film, of course, you do not want to take this risk. Stay inside the manufacturers' guidelines, subject to the qualifications given earlier.
Hydrolysis, as its name suggests, is the breaking down of developer components (usually the developing agent itself) in water. Plain phenidone is particularly prone to this but later pyrazolidones, as used in most modern developers, are much more resistant to hydrolysis. The byproducts of hydrolyzed developing agents may be colorless, which explains how sometimes even a brand-new unopened bottle of developer can be water-clear but inexplicably inactive. This is where use-by dates are invaluable.
Oxidation is much more familiar. This is caused by the reaction of the developing
agent with oxygen, and the byproducts are usually brown. Color alone is not
a reliable guide to developer exhaustion, however. You also need to know the
concentration of developing agent(s). If there is very little developing agent,
even a weak tea color may indicate that the developer is oxidized beyond use.
If there is a lot of it (or them), the developer concentrate may still work
perfectly even when it is opaque.