[pure-silver] Re: Chemical Blues
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 3 Feb 2009 14:14:47 -0800
----- Original Message -----
From: Adrienne Moumin
To: Pure Silver
Sent: Tuesday, February 03, 2009 10:41 AM
Subject: [pure-silver] Re: Chemical Blues
Thanks so much for that definition, Jonathan!
I did in fact buy the anhydrous form of sodium sulifte; I
was so paranoid since I'd never bought it in bulk before,
that I shot a test roll to process before using it on
anything I cared about.
I spoke to BKA yesterday, they were in fact very nice &
promised to make sure I get some fresh chemistry.
On another strange chemical note, I just got an e-mail from
B&H about a shipment of three orders. I track the order
info to find they are sending me 11 bottles of Agfa Sistan,
which I ordered from them in 2005 and which kept arriving
here opened and leaking. So now, in 2009, they will be
sending me more.....??? Curious thing, I will wait to see
what arrives!
Adrienne Moumin
The problem with Sistan is that, AFAIK, there has never
been any valid research published showing that it works. A
very long time ago I talked to Dr. Nishimura at IPI and he
told me that one of his students started to run a test but
it was never completed. Sistan appears to be a solution of
potassium thiocyanate in a wetting agent. Thiocyanate was
used as a stabilizer in the old stablization process, a
rapid access method of processing prints where no fixing
bath is used. It prevents the unfixed halides from
discoloring for a period of time. Thiocyanate is also a
fixing agent. I am not quite sure of the mechanism by which
Sistan is supposed to work. There is no doubt that it does
stabilize the image, but is not as effective as toning in a
sulfide or selenium toner or gold toner. It has the virtue
of not changing the appearance of the image.
The presense of the wetting agent is becuse Sistan is
intended to be used as a final rinse. However, unlike a
straight wetting agent rinse (like a solution of Kodak
Photo-Flo) it must be squeegeed or or swabbed off beause an
excessive amount will cause staining.
The main problem with permanence is oxidation of the
image silver by atmospheric polutants or gasses exuded by
storage materials. Silver is very vulnerable to oxidation,
that's why silver utensils and jewelry tarnish. By
converting some of the silver to silver sulfide, silver
selenide, or a gold compound, the oxidation can be
significantly reduced or eliminated. A secondary cause of
image damage is sulfiding, either by sesidual
sulfur-containing compounds in the emulsion or support, or
from gasses as in oxidation. This form or image damage is
eliminated by insuring complete fixing and washing.
Curiously, it was found (at Kodak and Fuji) that a very
small residue of thiosulfate (fixer) in the emulsion would
provide a certain degree of protection against oxidation. It
is thought that it causes some sulfiding of the image
silver, enough to protect it. Silver sulfide is an extremely
stable compound. The difference between this and lack of
washing is that the latter results in uncontrolled sulfiding
throughout the emulsion and the production of colored sulfur
compounds all over the emulsion.
Lack of complete fixing allows some halide to remain in
an insoluble form so that it does not wash out. Eventually,
this halide will decompose causing staining and the
production of reaction products that attack the silver
image. Although stabilized halide appears to be capable of
normal fixing and washing for some period of time chemical
changes in the redidue halides generally make it impossible
to fix out after about two weeks.
Since display prints are subject to considerable
exposure to the air and to light they should always be
either toned or treated with a stabilizer. While there is no
formal research showing Sistan works there is informal
evidence that it does and is a good choice where the changes
in image density and color produced by toning are
undesirable.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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