[pure-silver] Re: fixer question - wash water xchg rate
- From: ERoustom <eroustom@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Tue, 3 Jun 2008 20:31:10 -0400
Thank you Richard for you generosity here, and in other forums.
Elias
On Jun 3, 2008, at 6:13 PM, Richard Knoppow wrote:
----- Original Message ----- From: <eroustom@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, June 03, 2008 2:27 PM
Subject: [pure-silver] Re: fixer question - wash water xchg rate
What rate of water exchange is sufficient for good washing?
Elias
Kodak's recommendation is that the water in the washing vessel
change completely in five minutes. This is a bit misleading since a
running water tank or tray changes exponentially. What is important
is the rate of change at the surfaces of the paper. The emulsion
washes out by diffusion, an exponential process. The rate at which
the hypo leaves depends on the difference in concentration of hypo
in the emulsion versus the concentration in the wash water at its
surface. The greater the difference the faster the hypo diffuses
into the water. Obviously, even when there is no hypo in the water,
the rate will become slower as the washing progresses. If a print
or film is washed in standing water the hypo will form a cloud at
the surface and eventually reach approximate equilibrium where the
process becomes very slow. The diffusion continues into the body of
the water in the vessel so that the processes is continuous until
it reaches true equalibrium but without agitation it will
essentially stop at some point. The idea that hypo is heavier than
water and will sink is a misunderstanding of the process. There may
be some slight convexion at the surface but nothing more.
The best washing is in a low volume vessel with continuous
change of water at the surface, for instance, the spray washing
found in many motion picture processing machines. Large volume
tanks with relatively low flow rates, such as many "archival"
washers, are less effective than a tray with a Kodak tray syphon in
it, provided that a single print is washed so that both surfaces
are exposed to the water. This is quite practica for RC paper,
which also has a short wash time but may be a problem for fiber and
for large quantity work. The best "archival" type washers, perhaps
"vertical" washer is a better term, are those with the smallest
volume and highest flow rate at the surfaces of the prints.
One way of testing the flow rate of a tank or tray is to get it
going and then put some vegetable dye in it. I've used the juice
from canned beets. See how long it takes before the color fades to
the point where it is visibly gone (again this is exponential). It
should not be more than five minutes. I have a Zone VI 16x20 washer
which has too low a flow rate. I use it by putting it in a bathtub
and allowing it to overflow the top. I also make a practice of
pulling the plug and draining it about halfway through the wash and
starting over with a fresh filling. I also use a 16x20 tray
(actually about 18x24) with a Kodak tray syphon. This will
discharge the dye in about two minutes at a reasonable flow rate.
One can also achieve good washing using the successive bath
method. This is necessary were running water is not available or
where fresh water is at a premium. Both Kodak and Ilford have
instructions for this method. They are slightly different: Kodak
recommends several successive baths of the same length while Ilford
recommends baths which begin by being short and are successivly
longer. This is to take advantage of the exponential rate of
washing. Both are effective but the Ilford method probably uses
less water.
Since the support of fiber paper does not wash out by a
strictly diffusion process it needs more time. The emulsion of
fiber will wash out nearly as quickly as RC paper but the substrate
(Baryta layer) and support take longer even when a wash aid is used.
Paper and film can be tested for residual hypo by using the
silver nitrate test recommended by Kodak. This consists of a
solution of silver nitrate in acetic acid as a preservative. Its
used in the same way as the residual silver test mentioned in an
earlier post. A couple of drops are placed on the wet, but blotted
off, emulsion surface and allowed to stand for about two minutes.
There should be no more than a very slight yellow stain. The stain
will darken with time so the examination should be made
immediately. There is a method of fixing the stain so that a
densitometer can be used to determine the quantity of hypo remaining.
A true answer this question would require knowledge of the
diffusion rate of the emulsion involved and also the rate of
washing of the support of fiber paper. There are some actual rates
mentioned in the literature but they are important only for very
critical work. The Kodak recommendations will result in fixing and
washing sufficient for good permanence.
I try to avoid using the term "archival" because it is really
very vague. True archival use implys that the prints or films will
be stored in highly controlled conditions, certainly not displayed.
We are mostly concerned with making our images in a way that is not
overly sensitive to atmospheric polutants or has residual compounds
contained in it which attack the image. This is not too difficult,
one needs only to follow good conventional practice.
The use of a sulfite wash aid will reduce both paper and film
wash times by a factor of about six times.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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