Digital BW, The Print

Hi Mantinieri -
In reading your message below, something didn't sound quite correct,  
so I looked up processing formulae in
my father's Handbook of Chemistry and Physics (c 1936). It was  
amazing to find almost all of the modern developing and
fixing methods listed that far back, including 3 or 4 Pyro solutions.  
It seems that potassium bromide, and sodium sulfate was used in  
almost every
developer, both to limit over development and often to increase  
emulsion sensitivity. It was also used in many fixer formulae,
but not to increase shelf life, since in those days, all the  
photographic chemistry was mixed from raw ingredients. Rather, the  
sodium
sulfate, as much as I can figure, helped to control the pH of the  
fixer (which needs to remain acidic). Traces of developer
get into it, as all developers are alkaline. This increased the life  
of the fixers considerably.

Fixers with emulsion hardeners always contain chrome alum or some  
other compounds as the hardening ingredient, as well as both sodium  
sulfate and sodium bisulfate, also to buffer the acidity . But  
virtually all the fixers use Sodium Thiosulfate as the active  
ingredient to clear the
film down to the silver image. It seems logical, but not conclusive,  
in my mind, that the anti-halation coating is made soluble during
the development stage, and is then dissolved away in the fixer -  
whether thiosulfate alone or with hardener added.

I believe you are mistaken about sodium sulfate being the ingredient  
that removed some of the silver, since it's found in both the developers
and fixers. However one could find out easily enough by an experiment  
using both types of fixer solutions. With my darkroom and chemistry
about ten years gone, you'll have to be the one to do the test:-))).  
Let us know!

An interesting aside to some of this info is that, years ago, I read  
about some household chemicals like cleaning supplies,
Clorox beach, vinegar (for stop bath), baking soda, and other items  
to develop film in emergency situations, where standard chemistry
wasn't available. (it may have be published during WWII). One wonders  
what was used in place of the developing agents like Metol, Elon,
Hydroquinone. Most are very toxic, but quinone was used in medicines  
(for Maleria?) and some mixed drinks I think:-). Anyway, there's a  
huge world
of chemistry all around us, to which we seldom pay much attention.

Clay Price



  "Mantinieri" mantinieri@... wrote:

 >>>To my knowledge, it is sodium sulphite (contained in commercial  
fixing to prolong shelf life) rather than sodium thiosulfate that  
wears out the unexposed silver when using too strong fixing or too  
long fixing times. Therefore, soaking the film in a pure sodium  
thiosulfate solution (240 gr/l).
As a matter of fact, pure thiosulfate is what I use for fixing in  
order to maximize shadow details and acutance of my film. When  
performed adequately, it is perfectly able to dissolve the anti- 
halation layers of the film I use, even the t-grain versions (Fomapan  
is not among them). Of course, it is a 1-shoot bath.>>>




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