[LargeFormat] A better way

[Richard Knoppow]

At 09:39 AM 08/08/2002 -0700, you wrote:
>
>>   The anti-halation backing on most B&W film is a dye in the anti-curl
>> gelatin coated on the back of the support. It is not removed but
>> decolorized by the developer. The fixer usually will 
>> decolorize it too, but
>> more slowly. 
>
>AH!  This might explain something I found curious this year.  A friend had
>shot some Fuji Neopan 1600 (35mm), had a lab process the negs, and brought
>it over to my darkroom to make some prints.  I was somewhat surprised to see
>that the base of this film was very clear, not the usual grey.
>
>Six months later, I had a reason to shoot some of this film for the first
>time, and was startled to find that when I processed it (in ID-11), it had
>the usual grey colour in the unexposed areas.  Could it be that the lab had
>chosen a developer/fixer combination that produces that clear-base look?
>
>This could be a potentially useful bit of knowledge. There are some projects
>in the back of my mind for making b&w slides from negs, using a slide
>copier.  Knowing how to pick a developer to remove that colour cast would be
>great!
>
>So, if you have more details on this, Richard, I'd be delighted to hear
>about it.
>
>Cheers,
>\donw in Victoria
>
   I'm going to try to respond to three posts here.
  First, Its possible the anti-halation dye also washes out to some degree.
I've never tried soaking some film in water and then adding a little
carbonate to see if it clears the color. 
   The pre soak color might also be residual sensitizing dye. 
  The color left in T-Max film is sensitizing dye which is bound to the
silver halide. It doesn't come out in the developer because the halide is
still there. It should come out in the fixing bath but even extended fixing
in fresh fixer doesn't seem to completely remove it. Sulfite wash aid does
remove it, very effectively. Why, I'm not certain, but sulfite wash aid
does make soluble some otherwise insoluble silver complexes and it may be
that the dye is bound to them. 
  Many 35mm B&W negative films have a gray pigment in the support itself.
This is not removed by processing. In fact, there is no practical way to
remove it. It meant to prevent light-piping, the conduction of light
through the support. Light piping is the same phenomonon that is the
principal of fiber optics. 
  Since one end of 35mm film is often exposed to the light the pigmented
support prevents fogging of adjacent film. 
  It also has some anti-halation properties since image light must pass
through it twice. Combined with a good anti-halation backing this results
in very low halation.  
  Color films, and some modern B&W films use an anti-halation layer under
the emulsion. this, of course, also prevents fogging from light-piping, so
these films are perfectly clear after processing. 
  I believe Agfa uses this system on its APX films. 
  I don't know what to make of the Fuji film other than it sounds like the
gray may have been fogging which was prevented by whatever the lab used to
develop the film. 
  Its easy enough to see if a film has a pigmented support, just remove the
emulsion and backing (if any). You can scrape it off or put a scrap of the
film in household ammonia. That will take the emulsion off in a few
seconds. 
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com