[pure-silver] Re: PMK
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 27 Apr 2006 20:37:52 -0700
----- Original Message -----
From: "Hagner, Andrew" <Andrew_Hagner@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, April 27, 2006 3:39 PM
Subject: [pure-silver] Re: PMK
Other than for use with PMK, what are the advantages (if
any) of an all
alkaline process for film development.
I'm curious, - Andrew.
Supposedly the acid in the stop bath and fixer tends to
clear the stain from Pyro. '
All sorts of claims are made for alkaline processes but
most are based on misunderstandings. About the only one I
can think of is lack of odor from the fixing bath.
Wash rates are faster but even faster wash rates can be
had by using a sulfite wash aid like Kodak Hypo Clearing
Agent.
There are two causes of retarded washing when acid fixer
is used. One is the pH of the emulsion, the other is binding
by white alum hardener when its used.
The effectiveness of fixer is independant of pH, it does
not need to be acid to work. Acid fixers are common for two
reasons: one is to activate a white alum hardener. Alum is
effective only below a certain value of pH and works best in
a fairly narrow window. Modern fixing baths are buffered to
around this value. The other reason is that the acid
prevents carried over developer from continuing to develop.
This is also the reason for using an acid stop bath. While a
plain water rinse will stop development it is not as quick
and must be fairly long to wash out the developer. Some
processes (for instance some reversal processes) require the
use of a water stop bath but it should be in the form of
short wash to insure the developer is carried away quickly.
The gelatin of the emulsion takes on the pH of the last
bath it was in. When an acid fixer is used the emulsion is
left in an acid condition in the wash. Eventually, this will
change to the pH of the wash water but that is slow. The pH
is important to washing because of the electric charges in
the emulsion. When acid, they tend to bind thiosulfate and
complexes since the charges are opposite. By making the
emulsion neutral or alkaline the charges become the same and
repel.
White alum, Potassium aluminum sulfate, (to distinguish
it from chrome alum) has the property of binding or
mordanting thiosulfate and fixer complexes when acid. By
adjusting the pH to neutral the binding action is overcome
but the hardening action is left mostly intact. When made
alkaline the mordanting is also eliminated but so is the
hardening. For that reason Kodak buffers its wash aid to
neutral. Experiments made in the 1930s at Kodak Labs showed
that the wash rates were nearly the same for film fixed in a
non-hardening acid fixer and in a fixer containing chrome
alum (potassium chromium sulfate) even though the chrom alum
requires a much lower pH (around 2.5) to work. The
conclusion was that it was the white alum rather than the
acid which resulted in retarding the washing. The acid still
has an effect as I mentioned above.
When an acid stop bath and fixer are used development is
stopped instantly in the stop bath and is not reactivated in
the fixing bath. Staining is eliminated although the
presense of sulfite in the fixer also prevents staining.
If a sulfite wash aid is used there is no advantage in
wash time for an alkaline processed film or paper.
If no hardener is required in the fixing bath it can be
made much less acid than conventional baths. This can be
sufficient to prevent activation of carried over developer
but will lower the odor considerably.
It should be noted that some color processes require
alkaline or neutral baths to be used to prevent fading of
the dyes. Not all color processes require this.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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