[pure-silver] Re: Selenium toning + Agfa Sistan

----- Original Message ----- 
From: "Koch, Gerald" <gkoch02@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, November 30, 2004 7:20 AM
Subject: [pure-silver] Re: Selenium toning + Agfa Sistan


>I believe that the ideo behind Sistan came from the 
>observation that gold toners
> containing thiocyanate offered protection even when the 
> gold salt was eliminated
> from the toner.
>
> Jerry
>

   That was a result of the research IPI did regarding 
Selenium toning. They found that _some_ of the protection 
from Gold toners came from incidental sulfiding of the 
image. However, I think Sistan is the result of adapting a 
chemical used in the stabilization process to fixed and 
washed negatives. Several chemicals were used as stabilizers 
in the stabilizing process but Thiocyanate was one of the 
most frequent. Since this process has been out of use for 
some time I should explain that it was a rapid access 
process where the prints were not fixed or washed. The 
prints were developed and then treated in the stabilizer. 
This converted the undeveloped halides into a colorless form 
which would remain stable for quite some time. The prints 
could be made permanent by fixing and washing them. 
Stabilization processing has been replaced by rapid access 
machines using RC paper with about the same dry to dry time 
and relatively permanent results.
   I don't know how Sistan works. It might work by creating 
some sulfiding of the silver crystals, or it might have a 
more complex function.
   About 1961 T. H. James, of Kodak Research Labs, 
discovered that small amounts of thiosulfate remaining in 
the emulsion had a stabilizing effect on the images. It 
protects them against oxidation from atmospheric polution. 
While not as effective as toning emulsions with some 
thiosulfate in them are much less vulnerable to oxidation 
and "silvering out" than those which have been washed 
completely free of thiosulfate. This was such heresy at the 
time that James was reluctant to publish. However, within 
the year a researcher at Fuji made the same discovery and 
published it, so James did as well. This discovery changed 
all the previous recommendations for washing of negatives 
and prints. It turns out you _can_ overwash and prints that 
had been washed heroically to make them permanent turned out 
to be less stable than those receiving standard commercial 
processing. I rather think the function of Sistan is to 
produce a similar effect in a controllable way.
   According to Doug Nishimura, of IPI, Gold toning is still 
the bench mark for protection of microfilm. Its an expensive 
process so the use of other toners is prefered when 
possible.

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


 

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