Thread: Kodachrome
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Old June 27th 11, 05:26 AM posted to rec.photo.darkroom
Richard Knoppow
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Default Kodachrome


"Geoffrey S. Mendelson" wrote in message
...
Michael wrote:
The last acceptance date at Dwayne's was 12/30 with the
last run set
for 12/31. In fact so much came in at the end that they
did run over
into the new year, but technically that was the same run
and it is now
over.


The process for the older kodachrome is public knowledge,
Kodak made
it for government use long after they discontinued the
consumer film.

Maybe they will release the formula for the last process
and in a few years
some enterprising person will buy up an old kodachrome
processing machine,
whip up a batch of chemicals and offer processing.

Someone might even care enough to reverse engineer the
film, like they are
doing with Polaroid.

Geoff.

It would be interesting to know if the last series of
Kodachrome could be processed acceptably in whatever
formulas may be in the public domain. Processing is complex
but probably doable. Kodachrome film is essentially three
layers of black and white silver emulsion coated over one
another with thin separating layers of plain gelatin plus a
yellow filter made of colloidal silver coated under the top
layer. Its necessary for the first developer to preserve the
color sensitizing of the bottom (red sensitive) layer beause
that is used to separate the second exposures. The process
requires three color development steps with associated
rinses. The three layers are reversed indepently by
selective re-exposure. The bottom layer is panchromatic and
the center layer orthochromatic, so it is _not_ sensitive to
red light. If the film is exposed to red light through the
support the bottom layer only becomes redevelopable and is
developed in a developer containing a coupler for cyan dye.
The top layer is then flashed using blue light. This exposes
the top layer but can not get through the yellow filter
layer so only the top layer will be redeveloped. The
developer for it contains the coupler for yellow dye. Since
all the silver in both top and bottom layers are now
developed no light can get through from either side so the
center layer is developed in a fogging developer containing
the coupler for magenta dye. After the color development is
complete the silver in all three layers and the yellow
filter layer is bleached out leaving the positive color
image composed of dyes. There is a bit more to all this
since the film must also be washed and stabilized.
Kodak came up with this process originally because it
could not find a way to sequester the couplers in the film
layers. The couplers, and thus the color, tended to wander
from layer to layer, so, Kodak decided on a process where
the couplers were in the reversal developers rather than in
the film. AGFA at about the same time, came out with a
coupler incorporated multi-layer film but AFAIK it was never
available in the USA. AGFA's method of sequestering the
couplers was to attach the coupler molecules to long chain
molecules that were large enough so that they could not move
around in the gelatin. This process worked pretty well
although the early AGFA film looked grainy and low
saturation compared to Kodachrome. Kodak later devised
another method of anchoring the couplers: the couplers were
encapsulated in a resin. The resin droplets were too large
to move around in the gelatin. The color developer needed to
have a solvent for the resin, a sort of alcohol being used.
This type of film will look hazy after development until it
dries. I think the surviving method is the AGFA one but am
not sure.
Its very possible that Kodak had access to all the AGFA
information since AGFA-Ansco and its parent General Aniline
and Film Corp, a front for the German I.G.Farben, was siezed
by the government and held by the alien property
administration until some time after the war. Kodak may have
used the AGFA method for some color aerial film but
Kodacolor used the Kodak method as did the post-war
Ektachrome and Ektacolor. Color printing methods were
similar, a color printing material called Kotavachrome was
available for Kodachrome slides and used the same technique.
This material was coated on a translucent plastic material
instead of paper to make the flashing possible.
The original process for developing Kodachrome was
different than the one described above and was used for only
about a year. This was based on the controlled penetration
of bleach into the film layers. The film was first developed
to a negative in all three layers. Then all three layers and
the yellow filter layer were bleached leaving only the
unexposed halide in all three layers. The film was then
flashed and developed in the cyan coupler. This produced a
cyan image in all three layers. The film was then washed and
dried, the drying to control the absorption of the gelatin.
It was then floated in a bath of a bleach which penetrated
the top two layers removing the cyan dye and bleaching the
silver back to halide. The again washed and dried. Now it
was flashed again and this time developed in the magenta
coupler, which produced a magenta image in the top two
layers. Agaain the film was bleached to remove the cyan dye
in the top layer and convert the silver there into halide.
It was then flashed a third time and developed in the yellow
coupler. The film now had a positive silver image in all
three layers along with a positive dye image. The silver was
then bleached in a bleach which preserved the dye image and
then fixed and washed.
Whew, that was a lot of work. Much of the work of the
two Leos was in finding methods of differential penetration
of processing chemicals.
This system was AFAIK used only for the original 16mm
motion picture version of Kodachrome. The differential
re-exposure method was adopted around 1937 and at the same
time Kodachrome began to be available in other formats,
noteably 35mm still film and in sheet film. At one time
sheet Kodachrome up to 11X14 was available. All this film
was processed in Rochester, N.Y. but eventually plants for
motion picture film and 35mm still film were built in other
cities.
For some reason Kodachrome has a remarkable stability
in dark storage although it is _less_ resistant to fading
under exposure to light as in projection. I think part of
the popularity of Kodachrome came from its brilliant
rendition of color. Technicolor decided on a similar look
despite the extreme flexibility of the Technicolor process
which was capable of producing very delicate color when
desired. Brilliant color was what the customers wanted so
its what they got.
Keep in mind that color used to be expensive,
difficult, and remarkable. With the introduction of newer
materials it became cheaper, more common, and, eventually
the rule rather than the exception. These days its
monochrome which is considered exotic.


--
Richard Knoppow
Los Angeles, CA, USA