[pure-silver] Re: PMK
- From: Ryuji Suzuki <rs@xxxxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 28 Apr 2006 14:42:34 -0400 (EDT)
From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Subject: [pure-silver] Re: PMK
Date: Fri, 28 Apr 2006 03:05:35 -0700
> Washing is fastes when there is a continuous, rapid
> change of water at the surface of the film. The emulsion
> washes out by a diffusion process, the rate of diffusion
> depends on the concentration of hypo in the emulsion vs: the
> concentration in the wash water. Washing is very rapid at
> first and slows exponentially. Thats why its important to
> use fresh water toward the end of the wash.
This is not really the case, as I have pointed out several times in
the past.
Toward the end of washing, the amount of thiosulfate contained in the
film decreases exponentially, but so is that in the washing water. The
marginal benefit of frequently replacing water is very small.
Continuous agitation and occasional change of water is sufficient and
more water-efficient in washing films and prints. This is
particularly true if more water is filled in the washing vessel for
given amount of film or paper. That is, if you soak a plain paper in
washing water more than halfway into the washing cycle, and dry that
paper, you won't have anything like 10mg/square meter of thiosulfate
in it. Far less. So replacing water too frequently in the later part
of washing cycle is nothing but waste of water. It is just that most
film and print washing apparatuses are designed to keep sending fresh
water, perhaps because this design is cheaper and simpler. If you use
a fish tank pump or something, you can save a lot of water in washing
process. Inverting tanks, dipping and dunking, or rocking trays is a
low-tech version of the same concept that works equally well.
With my fixer and Ilford fill-and-dump washing cycles, my films
contain less than 10mg/square meter of thiosulfate within a couple of
minutes, and 5 min is already on the safer side.
I use Paterson tank for 120/220 processing (and these are my main
format) so I connect hose from temperature regularted line to the tank
to wash, but I only run water continuously for first minute or so. I
switch to occasional burst of water and/or manual agitation (dip and dunk).
Vigorous dip and dunk in Paterson tank is very easy and effective.
> The use of a sulfite wash aid substantially increases
> wash rate regardless of fixer type and is important where
> economy of water use is important.
Sulfite wash aid doesn't really change the diffusion part of the
equation. Sulfite only helps desorption of silver-thiosulfate complex
and thiosulfate itself from silver image surface, gelatin, sizing
material, paper fiber, etc. These are more important for fiber based
paper prints, and these are relatively non-issue to RC and films fixed
in non-hardening fixer, particularly if the fixer pH is above
6.5. Sulfite washing aid is also useful if wash water is cold. Cold
water is ineffective in desorbing these harmful compounds but sulfite
can desorb them and cold water can still diffuse them out of the
material.
Even with sulfite wash aid, alum hardening fixer takes longer washing
time than non-hardening fix. Do a simple comparison test and you'll see.
Of course, even with neutral or alkaline rapid fixer, sulfite wash aid
is useful for fiber based prints. The washing time can be still
shortened.
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