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More black silt questions



 
 
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Old October 2nd 09, 06:58 PM posted to rec.photo.darkroom
Richard Knoppow
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Posts: 751
Default More black silt questions


"Lew" wrote in message
...
This may be related to the other 'black silt' thread on
this forum,
maybe not. It is not a question about the efficacy of
adding rapid
selenium to film fixer for archival purposes, it's a
question about
why the black silt is appearing.

In the past, I've added Kodak rapid selenium to my fixer
with no
untoward results. These were typical non-hardening, acid
fixers. To
simplify my processing I've recently switched to Formulary
T-4, a
positive ph fixer, (no stop bath, just a rinse between
steps). Now,
however, I get a black percipitate in the stored fixer,
although,
initially, it still seems to clear the film ok.

I assume that adding the selenium to a positive ph fixer
is a no-no
and that this is the cause of the percipitate (which I
also assume is
the silver that accumulates in partially used fixer).
While I intend
to drop this practice, I'd appreciate comments from anyone
who can
shed light on what's happening. Also, is it possible that
Jerry's
students are introducing toner into the fixer they're
using?


Where did you get the idea of adding the toner to the
fixer? While KRST does contain a considerable amount of
ammonium thiosulfate it should not be added to a fixing
bath, particularly an adid one. The acid will decompose the
toner causing the selenium to be precipitated. Prints which
are too acid may develop a peach colored stain in the toner.
In the past Kodak recommended mixing KRST with Kodak
Hypo Clearing Agent at 1 part toner to 19 parts diluted wash
aid as a combination method of decreasing wash time and
protecting the image. It was discovered nearly twenty years
ago that this method failed on microfilm because the low
density areas of the film were not toned enough to protect
them from oxidation. The method was never recommended for
stronger dilutions of KRST.
KRST will provide full image protection but must be used
at a dilution of no greater than 1:9 and for about a minimum
of three minutes. With most paper this will cause a change
in image color or density.
Prints to be toned should be treated in a sulfite wash
aid such as KHCA and washed. Then toned and washed again.
Since there is no acid in the toner the use of a wash aid
treatment after toning is probably not necessary.
I am also skeptacle of the use of alkaline fixing baths.
Since sulfite wash aid gives even shorter wash times and
does not have the disadvantage of allowing carried over
developer to continue working in the fixing bath.
The current recommendation for _image protection_ is to
use a polysulfide toner, like Kodak Brown Toner, for just
enough time to cause a slight change in image color (some
papers will not show this). KBT and similar toners tone all
densities at the same rate so relatively slight tonign will
give some protection. However, the criteria for microfilm is
to obtain a minimum of 60 percent toning as determined by
bleaching the silver image and measuring the density of the
toned image.
This is based on work done by the Image Permanence
Institute of the Rochester Institute of Technology and at
least some of it is available on the web.
Very good image permanence can be gotten with quite
conventional processing. Mostly one should be careful that
the fixer is fresh (use a two bath fixing method) and that
washing is thorough (although you _can_ wash too much). The
use of a toner will help prevent oxidation of the image
silver.
_Complete_ toning in any sulfiding toner, selenium
toner, or in a gold toner will provide very good image
protection. Where the color change is acceptable (or
desired) complete toning provides the best image protection.

--
Richard Knoppow
Los Angeles, CA, USA



 




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