[pure-silver] Re: Kodachrome's swan song
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 23 Jul 2010 10:58:49 -0700
----- Original Message -----
From: "Dana Myers" <dana.myers@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, July 23, 2010 9:52 AM
Subject: [pure-silver] Kodachrome's swan song
Given that Kodachrome is sort-of a pure-silver film (dyes
added in
processing), this may be of interest:
http://www.kansas.com/2010/07/14/1403115/last-kodachrome-roll-processed.html
Dana
An interesting article. Kodachrome differs from "modern"
color film in that the "couplers" or dye formers, are not
incorporated in the emulsion but rather are in the second or
reversal developers. When couplers are incorporated they
must be anchored in some fashion to prevent them, and the
dyes they produce, from migrting from layer to layer. Kodak
evidently could not find a satisfactory method of
accomplishing this, and, I think probably was aware that
AGFA was about to come out with a multi-layer color film.
What they did was to rely on a method of isolating the dyes
to the right layers through the processing method. The
original version of Kodachrome, which was released only as
16mm motion picture film, used a particularly complex
process relying on controlled penetraton of a bleach to
control the color development. The film had to be bleached
twice in baths where the film was floated on the solutions.
The film was dryed between each step to insure the uptake of
solution by the emulsion layers was constant. About two
years later another, and simpler, method of processing was
devised; this one relied on maintaining the color sensitivty
of the bottom layer so that the flashing or re-exposure for
the reversal step would affect only one layer at a time. The
first re-exposure was made using red light from the back of
the film. Only the bottom layer is sensitive to red, the
middle layer is orthochromatic and the top layer is "color
blind" sensitive to only to blue light. When redeveloped in
the cyan developer only the bottom layer would be developed.
The next exposure was to the top layer with blue light. The
yellow filter layer prevents this exposure from affecting
the center layer. The top is then reversed in the yellow
coupler developer. The center layer is then flashed by using
a fogging agent and developed in the magenta developer.
After this all silver, including the silver yellow filter
layer, is bleached and fixed out leaving the reversed color
images. The silver image in multi-layer film is also removed
at the and of processing but the film needs only a single
color development. For negative film this is the same as the
first developer (only one development step) and for reversal
film it is the reversal development.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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