[pure-silver] Re: Old paper andDefendre 58-D
----- 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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