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Re: [Rollei] Film advice needed

- ----- Original Message ----- 
From: "Bob Shell" <bob  >
To: <rollei  
Sent: Thursday, January 08, 2004 5:32 PM
Subject: Re: [Rollei] Film advice needed

> Before it is processed and has the dyes injected,
Kodachrome is just
> black and white film.  I see no reason that it ought to be
less stable
> than other black and white films.
> Bob
> On Thursday, January 8, 2004, at 04:26  PM, peter
kotsinadelis wrote:
> > They lied.
> >
> > --- Bob Shell <bob  > wrote:
> >>
> >> On Wednesday, January 7, 2004, at 08:45  PM, peter
> >> kotsinadelis wrote:
> >>
> >>> Kodachrome is especially susceptible to heat as it
> >>> ages. Run a test roll before you us it for
> >> anything
> >>> you really want to keep.
> >>>
> >>
> >> I always was told that Kodachrome was much more
> >> stable than chromogenic
> >> color films.
> >>
> >> Bob
> >>
   I think it is the nature of the dyes rather than the
method of processing that makes the difference. All color
films are essentially just black and white films where the
color image is produced by an ancilliary reaction. The
principle difference between Kodachrome and chromogenic
films is that Kodachrome has no color couplers incorporated
in the emulsions. The term "color coupler" is somewhat
vague, it means the compounds which become dyes on
development. In chromogenic films the couplers for each
color are included in each emulsion layer. The most
difficult thing in devising practical chromogenic film was
to find a way of keeping the dye couplers and the dye they
produce after development from diffusing or "wandering" from
layer to layer or within the layer. Agfa did this c.1935 by
attaching the coupler molecules to very large long chain
molecules which could not move in the gelatin. Kodak found
an alternative method by encapsulating the coupler in an
oily resin which could be penetrated by developer reaction
products but not by the relatively large dye molecules.
Kodak's first film employing this method was Kodacolor,
c.1941. Kodachrome was made without incorporated couplers
because at the time Kodak could not find a method for
anchoring the couplers and wanted to have a practical color
film. This was evidently a pet project of George Eastman,
one he never saw achieved in his lifetime.
  Because the couplers do not have to be in the film a
somewhat greater choice of couplers, and resultant dyes, is
possible for Kodachrome type films. The original processing
method for Kodachrome depended on the controlled penetration
of a bleach. This is the method devised by Mannes and
Godowski, it was used for only about a year and a half. At
that time the processing method was changed to one depending
on controlled color re-exposure for the layers. This is the
method currently used. Kodachrome must be developed four
times, once to develope all three layers to black and white
silver images, and three reversal re-development steps, each
in a developer with the appropriate color couplers. The film
is then fixed out to remove all the silver plus the yellow
filter layer, which is also made of silver, leaving the
three positive dye images.
  Ilford, for a time, made a similar film. However, it
depended on built in layers of fogged silver between the
image recording layers. These layers would become opaque
during the first development, allowing front and back
emulsions to be individually fogged for selective
redevelopment. As in Kodachrome processing the middle layer
was developed in a fogging developer.
- ---
Richard Knoppow
Los Angeles, CA, USA