[rollei_list] Re: was Rollei Retro Film now is Rodinal pin holes

  • From: Dennis Purdy <dlp4777@xxxxxxxxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Mon, 25 Feb 2008 07:00:24 -0800

sometimes my grammar is appalling.   Should have read "opinions that 
stop bath DOESN'T really cause it."
sorry DP
On Feb 25, 2008, at 06:50, Dennis Purdy wrote:

> Richard, or anyone,  do you know what exactly is the cause of pin holes
> in film?  I have tried to research it and find a lot of recommendations
> to avoid stop bath and then I read opinions that stop bath don't really
> do it.  I have just processed some ACROS in Rodinal 1:50 and it is
> beautiful but has lots of tiny pin holes.  I use this film a lot and
> have never encountered pin holes before, processing in XTOL or Beutlers
> or R09.  Are pin holes somehow created in development?
> thanks if you know and even if you don't
> Dennis
>
>
>
>
>>
>>      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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