[pure-silver] Re: Fwd: Kentona, response from Freestyle Photo

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <pure-silver@xxxxxxxxxxxxx>
  • Date: Sat, 6 Dec 2008 12:44:15 -0800
----- Original Message ----- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, December 05, 2008 4:36 PM
Subject: [pure-silver] Re: Fwd: Kentona, response from Freestyle Photo
A couple of onter notes which did not occur to me when I originally posted:
Dr. Richard Henry tested for loss of brightener during washing and found that it varied with the papers he tested. In no case was the brightener completely washed out. 
    Brighteners are NOT used in tinted stocks.
I have never heard of the tint of a tinted stock washing out. I doubt if dyes are used, probably pigments added to the baryta layer. My sample books of Kodak and AGFA papers, some of which are getting on to seventy years old, do not show any signs of fading or blotching of the tinted stocks.
Colloidal silver, which is extremely finely devided metallic silver, is bright yellow. In fact the yellow filter layer in Kodachrome is made of colloidal silver so that it will be bleached out when the image silver is removed as part of the processing. A deposit of colloidal silver on film results in "dichroic fog" so called because it looks yellow by transmission but pinkish by reflection. Often the reflected fog has a rainbow or oil-slick look because it is very thin and produces interference patterns. Dichroic fog is formed when the fixer has too much silver in it and some is deposited on the surface of the film or paper. It can also be produced where a water rinse rather than an acid stop bath is used and the fixing bath is alkaline or not acid enough to stop the developer activity. Carried over developer can, in that circumstance, remain active enough to reduce some of the silver complexes in the fixing bath to metallic silver which is deposited on the surface of the film or paper.
Silver stains can sometimes be removed by using a bleach made from ammonium thiosulfate fixer with some added citric acid. Kodak recommends using film strength rapid fixer with acid hardener with 15 grams per liter of citric acid added. This solution is capable of bleaching the silver image as well as dichroic fog so material being bleached in it should be watched carefully. This solution might also test whether the stain on the prints under discussion is composed of silver. Since fresh fixer was used it is unlikely that a silver stain due to exhausted fixer is responsible but an emulsion defect could also be the cause. That is one reason a examination of a sample of the paper in white light should be made. Note that the light should not be bright because some photolytic silver will be produced by the exposure and may mask any staining already on the paper. Ultimately the analysis should rest with Ilford. I think the age of the paper is irrelevant since the effect may one which takes place on processed paper as well as raw stock and manufacturers have become sensitive to potential threats to the lifetime of images. In any case, a year is usually within the expected expiration life of B&W paper, or for that matter, color materials. A reasonable shelf life should be several years. 

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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