[pure-silver] Re: How do you wash fiber paper?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 6 Jul 2005 14:38:16 -0700
----- Original Message -----
From: "Mike" <bw.zone@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, July 06, 2005 1:10 PM
Subject: [pure-silver] Re: How do you wash fiber paper?
I would think that the window screens would be fine as long
as you
batch wash.I would stay away from copper and just use the
PVC. I
thought I read that copper in the process is a bad thing, or
was just
in the developer tray? Also I would think that fiberglass
screens
would be your best choice.
Making this washer looks like it would be cheap enough to
build a few,
in different sizes. Start small and test it, if it works
then build
the next size up. One thing that has always bothered me
about the
large plexi washer is my prints seemed to want to stick to
the plexi
dividers. This washer would seem to solve that.
Mike
Copper is a problem for developers because it can cause
fogging in even trace amounts. Its not a problem for
washing. The main thing you want in a washer is a fast
change of water at the surfaces of the paper. Volume can be
small because the washing action takes place in a thin layer
right at the surface. Because washing is primarily a
diffusion process the rate depends on the ratio of the
concentration of hypo in the paper to that in the water. At
first, when the concentration in the paper is very high, the
hypo diffuses out quickly even if the water is not changing
rapidly. However, as washing progresses it slows down
exponentially. If paper is washed in still water the hypo
forms a cloud at the surface of the water. While equilibrium
is never reached it is approached so at some point the
washing action effectively stops. This is why long soaks are
illusory, getting the last of the hypo out requires fresh
water at the surfaces.
The idea of a bottom drain is based on two mistakes: one
is that the specific gravity of hypo laden water will cause
it to sink to the bottom, the other is that there is no
circulation. Since hypo is highly soluble it forms a
solution with the water. In still water the hypo forms a
cloud at the surface of the paper which continues to diffuse
out into the body of the water. At some point it will attain
equilibrium with the concentration of hypo being the same
everywhere, including in the paper. There might be some
circlulation at the surface due to convexion but this will
simply increase the diffusion rate into the water. Local
action like this accounts for directional bromide streaks on
films that are stand developed while vertical. The bromide
is heavier than the developer so it moves downward along the
film. However, it does not become concentrated at the bottom
of the developing vessel, it goes into solution.
Motion picture processing machines often use sprays to
wash the film. I've seen attempts at print washers that used
sprays of one sort or another. This should be effective and
low volume, but there doesn't seem to have ever been a
successful commercial washer based on this.
If you batch wash the important thing is to insure access
of the water to both surfaces of the paper (at least for
fiber). A washer that would allow insertion of sheets at
random times would really have to be several separate
washers so that the hypo from the fresh sheets would not
contaminate the overall washer allowing it to get back into
almost washed sheets. The only advantage of so called
archival washers over a tray with a tray siphon is that they
keep the sheets well separated. My old Zone VI washer
resorts to a very heavy and complex structure to do this. I
am quite sure that a simpler and lighter washer could be
made. Also, the Zone VI takes too long to change the water.
I've tested this using dye (actually beet juice), one can
see the circulation pattern and also see how long it takes
for the dye to be eliminated. To get reasonable wash rates
with this device I must increase the flow rate to the point
where it overflows at the top. I operate mine in the
bathtub.
One can test the effectiveness of a washer using one of
the methods of testing for residual hypo. The simplest is
the silver nitrate test but a more sensitive test is the
Methylene Blue test. When testing a washer design whole
sheets should be tested to investigate the uniformity of
washing. Some washers may be very efficient in the center
but leave hypo in the margins, etc. I have not conducted
tests of this sort mainly because the test solutions become
expensive in the quantities needed.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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