[pure-silver] Re: Washing prints
- From: daniel <daniel.bouzard@xxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sat, 29 Dec 2012 18:12:34 +0100
Le 29/12/12 17:50, Richard Knoppow a écrit :
----- Original Message ----- From: "daniel" <daniel.bouzard@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, December 28, 2012 11:03 PM
Subject: [pure-silver] Washing prints
Hello everybody,
There si currently on the french forum Galerie-Photo a debate about
the washing process of silver prints.
To sum up the question is the following: is the washing process of
silver prints an osmosis or a diffusion process?
I think that Richard Knopow should know the right answer as he was
strongly involved in Kodak researches.
Daniel Bouzard
I should make clear that I was never employed by Kodak nor have I
taken part in any research on washing. I did write a couple of
summaries of generally available information on the subject.
FWIW, an enormous amount of research has been done on both fixing
and washing over nearly a century. Much of this work was done by Kodak
Research Laboratories and published in various scientific and
technical journals. The criteria for "archival" or long image life
washing of silver prints was amended significantly following the
discovery by T.H.James, of Kodak, and another researcher at Fuji, that
very small amounts of hypo retained in the emulsion actually protected
the image from attach by peroxides. As a result the previous
recommendations for very long washes were changed.
The washing process for the _emulsion_ appears to be a diffusion
process but for unprotected paper support it is partly diffusion and
partly frictional because the hypo is held in the fibrous structure of
the paper. It was to avoid this that Ilford came up with their
"archival" processing method which uses very short fixing times. The
Ilford process does not work on all paper; some will just not fix out
quickly enough. However, other research shows that conventional
processing, especially when a sulfite wash aid is used, results in
washing down to archival levels with relatively short washes.
The frictional effects of the support and also the adsorption of
hypo and reaction products by the Baryta layer, account for the longer
washing times required for "fiber" paper. According to Dave Valvo, a
former product designer for Kodak, the longer fixing times required by
fiber base paper in comparison to RC is due to penetration of some
halide into the Baryta layer, this also results in extended washing
times.
Kodak research of about the middle to late 1930s was very
thorough; they measured the absorption of hypo in uncoated paper,
paper with only the Baryta layer, emulsion alone and various
combinations.
I don't know what Ctein did but it was probably re-inventing the
wheel.
A note: for best permanence the image must be toned in either a
sulfiding toner or a gold toner. Selenium toner also provides
complete protection if carried out long enough to tone all densities.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
Many thanks Richard for your answer and sorry to have thought you worked
for Kodak.
Daniel Bouzard
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