[rollei_list] Re: Rollei Retro Film
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 25 Feb 2008 02:17:54 -0800
----- Original Message -----
From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, February 24, 2008 10:52 PM
Subject: [rollei_list] Re: Rollei Retro Film
> At 01:38 AM 2/25/2008, Richard Knoppow wrote:
>
> > Are you using a hardining fixing bath? If not try one
> >and see if if it helps with the curling. Curling is
> >mostly
> >caused by the differential shrinking of the emulsion vs:
> >the
> >support. Most film, both roll and sheet, has a coating of
> >gelatin on the back to counteract the curling. Usually
> >this
> >back coating also contains the anti-halation dye. The dye
> >is
> >changed to a colorless form by the sulfite in the
> >developer
> >and fixing baths. Not all films have anti-curling
> >coatings.
> >My suggestion about the hardener is that it will tend to
> >reduce the amount of shrinking.
>
> I have been warned for four decades to avoid a=20
> hardening fixer. I never knew WHY hardening=20
> fixers existed. Me? (a Dative of Reference, for=20
> you Indo-European Scholars in our midst) I will=20
> stick to a non-hardening fixer and to Hewes=20
> reels. I also let the film hand for a day or so=20
> before printing it unless I have a rush=20
> commercial job (which I've not had in the past six or
> seven years).
>
> Marc
There seems to be a cult about alkaline processing.
This is at least partially based on a mis-understanding of
how hardeners affect washing.
First of all, hypo does not need to be acid, the acid
is required by the commonly used white alum hardener
(properly potassium aluminum sulfate). Alum works when the
pH of the bath its in is within a certain pH window.
However, hypo, either sodium or ammonium (rapid) type will
work at almost any pH.
Now, why should fixer be acid if its needed only for
the hardener? There are a couple of reasons. One is that
developer can't operate in a strongly acid bath. So, in
order to insure development is stopped at a definite point
one must use an acid stop bath followed by an acid fixing
bath. A water wash does not stop the development right away
and simply rinsing the film in water leaves quite a lot of
developer in the emulsion. For paper on untreated paper
support (so called fiber base paper) the paper base will
soak up a lot of developer and carry it over to the stop
bath and fixing bath. A definite wash step is needed if an
acid stop bath is not used to deactivate the developer.
Actually, there is some chance of staining in a water stop
(not very great) because the developer continues to work
without only the carried over sulfite to prevent reaction
products from forming. In any case an acid stop bath
followed by an acid fixing bath, with or without hardener,
prevents a lot more problems than it causes.
Now, about washing. It was found long ago in research
carried out at Kodak and at Agfa that emulsion treated in an
acid hardening fixing bath washed out much more slowly than
emulsion treated in a neutral or akaline bath. The retarding
of washing rate was especially noticable when a white alum
hardener was used. Kodak experimented with comparing fixing
baths with white alum to fixing baths with chrome alum
(potassium chromium sulfate). Chrome alum (a misnomer since
there is no aluminum in it) must operate at a much lower pH
(much more acid) than white alum. Even so it does not show
the strong restraining effect on washing of white alum. So,
Kodak concluded that there was something specific about
white alum which was causing the effect. In fact, there are
two things happening: one is that white alum, when in an
acid condition, acts to bind thiosulfate and
thiosulfate-silver complexes, the later the result of the
fixing process, to the image silver and to the gelatin. The
bond is quite strong so washing out of these materials is
made much longer. The second effect is from the acid itself.
Gelatin is what is called amphoteric, that is, it has no
definite pH of its own. It takes on the characteristics of
whatever bath it was in last. That is, if treated in an acid
bath it acts like an acid and if treated in an alkaline bath
it behaves like an alkali. However, gelatin _does_ have a
preferred pH. This is called the isoelectric point and has
to do with the way the gelatin was manufactured. For most
photographic gelatin the isoelectric point is just on the
acid side of neutral. The term isoelectric comes from the
fact that the net electric charge on the molecules of the
gelatin is neutral when its at this point. This is important
to washing because the thiosulfate ions and those of the
fixer reaction products carry a charge such that that are
attracted to the gelatin when the gelatin is more acid than
the isoelectric point and repelled from it when the gelatin
is more alkaline.
It also turns out that white alum looses its hardening
effect if the gelatin is made too alkaline but will retain
most of it when its about neutral.
It was discovered in the early 1930's at both Kodak and
Agfa that treating film or paper in an alkaline bath after
fixing but before washing would much accelerate washing.
However it would also destroy the hardening effect. Various
substances were suggested, sodium carbonate (agfa) ammonium
carbonate, Kodalk, even Borax (Kodak) all at around a 2%
concentration. All work but none preserves the hardening
action.
Sometime before 1900 it was discovered that sea water
was more effective in washing than fresh water but the
reason was not known. It was known that a fresh water rinse
had to follow the sea water wash if the images were to be
permanent. When sea water washing became imperative during
WW-2, especially for ship board operation extensive research
was done for find the reason for the effect.
Kodak discovered that it was partly due to pH but mostly
due to the ion exchange properties of some of the salts in
sea water. After considerable research Kodak discovered that
the most effective salt was a sulifite, the cation not being
particularly important. From this Kodak developed Kodak Hypo
Clearing Agent. This consists of about 2% sodium sulfite
buffered to neutral with sodium bisulfite and containing two
sequestering agents. Kodak found that at neutral pH, as
provided by this bath, the binding effect of the acid due to
electrical charge was broken (the pH being below the
isoelectric point), the pH was outside of the window where
white alum has binding action on fixer and its reaction
products but is NOT outside the pH window where the
hardening cross-linking is undone. In addition sulfite has a
specific ion exchange affinity for thiosulfate and its
silver complexes. So, by its use washing is accelerated even
beyond the rate yielded by a simple akaline bath and the
rate does not depend on the pH of the fixing bath or on
whether it has alum hardener or not.
The sequestering agents, sodium citrate and EDTA
tetra-sodium salt, are present for two reasons: one is to
prevent a deposition of aluminum salts from the hardener on
the surface of the film or paper, the other is to hold in
suspension any mineral impurities in the water (mostly the
magnesium and calciumcarbonate consitituting the "hardness"
in water which would also form hard-to-remove deposits).
Since this wash aid is capable of un-binding some
incompletely converted fixer reaction products it also has
the effect of increasing fixer capacity a bit.
In any case, when such a wash aid is used there is
absolutely no advantage of a non-hardening or a non-acid
fixing bath.
Even though some modern B&W films have very hard
emulsions not all do and there is still some advantage to
the hardener in preventing damage to the film when its wet
(the hardener has little or no effect on dry film).
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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