[pure-silver] Re: Developing Portra and Kodachrome in B+W chemistry
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 31 Mar 2012 16:52:07 -0700
----- Original Message -----
From: "K W Hart" <kwhart1@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, March 31, 2012 4:17 PM
Subject: [pure-silver] Re: Developing Portra and Kodachrome
in B+W chemistry
Silver is silver, and developer is developer, it's just a
matter of what else is (or isn't) in the film and developer.
I've developed C41 film in B&W developer in the past. The
resulting negs are low in contrast and have an orange mask,
making them difficult to print.
I've never tried K-chrome in B&W developer. My limited
knowledge of Kchrome is that it is actually three B&W negs
separated by masks. The colors are added in processing. You
shuld be able to develope Kchrome to B&W negs, but the
internal mask(s) may be a problem. If you needed to get
Kchrome processed, you could try Film Rescue International
in Canada.
If you would like to have your Portra processed as color
negatives, I could do that for you, just as Kodak did a
hundred years ago: shoot the film, pack up the film holders
and the camera in its case, and ship it to me. I'll develope
the negs and send them back to you!
Ken Hart
kwhart1@xxxxxxxxxxxx
FWIW, Kodachrome has only one filter. The color
separation is achieved by tailoring the color sensitivity of
the layers. the top layer is blue sensitive only. Under it
is a yellow filter made of very finely divided silver. The
second layer is orthochromatic and records the green light.
Since the yellow filter removes blue light and layer is not
sensitive to red light it records only the blue. The bottom
layer (closest to the support) is a panchromatic layer but
uses a combination of sensitizing dyes that leaves it blind
to green light. Since there is no blue coming through the
filter layer it records only red light. This red sensitivity
is preserved during the first development by choice of a
developer which does not destroy it. That sensitivity is
used to divide the re-development so that each layer is
developed in a solution containing the correct coupler to
produce the desired color. After the first development,
which produces a negative image, the film is flashed from
the back which makes the bottom layer developable since none
of the other layers is sensitive to red. The film is then
redeveloped in the first bath with a coupler for cyan dye.
Next the top is exposed to blue light. Since the yellow
filter layer remains intact, the blue can not penetrate it
and expose the center layer. The film is now redeveloped
with a coupler for yellow dye. Now, since both top and
bottom layers are completely converted to silver, the center
layer is treated in a reversal developer containing a
chemical fogging agent and the coupler for magenta dye.
After this development the film is treated in a bleach which
converts the silver to halide, including the filter layer,
and fixed to remove the halide, after which is it washed.
The film now contains three dye layers in subtractive colors
to form the image. I suspect the reason the bottom layer is
redeveloped first is that the reversal developers may
destroy the color sensitizing dyes and make it impossible to
separate the exposure of the bottom layer from the center
layer.
BTW, for a time Ilford used a variation on this system.
In the Ilford film a layer of fogged silver was included
between each image layer. The first development left these
as dense silver layers. The top and bottom layer could then
be fogged with light coming from that side of the film and
separately reversal developed, a fogging developer being
used for the center layer as in Kodachrome. The final
bleaching and fixing would remove the fogged layers as well
as the filter layer. I no longer remember the trade-name of
this film but think it was not made for very long. The
fogged silver layers may have introduced difficulties in
maintaining sharpness and maybe other problems.
The reason for the method used for Kodachrome is that
Kodak was having difficulty in finding a method of
sequestering the dye couplers in the various layers so they
would not wander and was under the gun to devise a practical
color film. They may have been aware of the work being done
at AGFA. In any case, AGFA announced a multiple layer color
film with incorporated couplers at about the same time
Kodachrome went on the market. Due to conditions in Germany
it was not brought to the US market. It was the system used
in the later Anscocolor and Anscochrome. Kodak eventually
devised another method of sequestering the dyes which was
used for a long time. Both systems were in use for decades.
I am not sure which has survived but think its the old AGFA
system.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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