[pure-silver] Re: Old paper andDefendre 58-D
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 28 Feb 2008 22:10:48 -0800
----- Original Message -----
From: "Don Sweet" <don@xxxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, February 28, 2008 8:53 PM
Subject: [pure-silver] Re: Old paper andDefendre 58-D
Thanks again for all contributions on this and the
previous thread.
My understanding is that age-related fog in paper is due
to chemical changes
caused by some form of radiation that penetrates the
packaging and partially
exposes the paper. Is that right?
Does it have the same effect as pre-development flashing,
i.e. nudging
everything up the curve a bit? And if that is right, I
suppose bleaching
brings it back down the curve, provided you use a cutting
bleach rather than
a proportional one - OK?
Is there any sort of developer formulation (or additive)
that does the same
thing at the development stage?
Is this Rlibersky chap on to something?
Don Sweet
Age fog has little to do with radiation although that
may be a factor with fast film. Mostly, it is a continuation
of a process called ripening which takes place as a part of
the formation of sensitive particles when the emulsion is
made. Ripening is a very complex process. As it continues
the emulsion becomes more sensitive to light but also
sensitivity centers are produced which result in fog.
Chemicals are added to the emulsion mixture at various times
to suppress the formation fo these centers while ripening is
continued. Other chemicals are added when the emulsion is
ready for coating to stop the ripening process. These last
are not completely effective so some chemical changes in the
emulsion continue with time. One of the main symptoms of
this process is the generation of centers in the halide
crystals that result in development when teh crystals have
not been exposed to light, in other words, fog. There are
other additons made to preserve the emulsion and stop
oxidation which can also result in fog formation. Until
fairly recently cadmium was added especially to warm tone
papers as a preservative. Because of environmental
considerations the cadmium addition was stopped so the
storage life of paper has become much shorter than it was
perhaps fifteen years ago. There may be other changes in
emulsion formulation as well due to the additives being
environemental polutants.
The fog in old emulsion can be partly suppressed by
additions made to the developer. The nature of the fog
centers and development centers produced by light is not
quite the same so the fog centers can be to some degree
suppressed without affecting the light centers. Common
anti-foggants like potassium bromide and benzotriazole work
this way. The organic anti-foggants, like benzotriazole, are
more effective in suppressing fog without destroying the
image centers. All anti-foggants are sensitive to the pH of
the developer so the optimum anti-foggant varies with the
developer.
There are other causes of fog. Developing agents,
properly called reducing agents, are not equal in their
ability to discriminate between exposed halide crystals and
those that are not. For instance, under some conditions,
mostly pH, Phenidone is more likely to develop some
unexposed crystals than other developers. Because bromide is
relatively innefective for Phenidone its usual to find
Benzotriazole used as the anti-foggant in develpers
containing it. This kind of fog is different than the age
fog discussed above because it is a function of the
developer and exists with even fresh emulsion. The age fog
produces development centers which the developer can not
discriminate from those caused by light. However,
anti-foggant of the right sort will help reduced both
effects.
I should mention that there are other causes of fog
than the above or being light struck. Certain developers can
cause fog as the result of oxidation. These should be
avoided where the emulsion is developed in a way that
exposes it to a lot of air. An example of a developer which
resistant to this is Pyro. Even though Pyro is itself
sensitive to being oxidized its reaction products tend to
prevent fogging from this source.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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