Richard Knoppow (dickburk@ix.netcom.com)
Fri, 22 Oct 1999 17:16:38 -0700
At 04:31 PM 10/22/1999 -0600, you wrote:
>On Fri, 22 Oct 1999, Richard Knoppow wrote:
>
>> Actually the backing is not removed, only decolorized. In think the only
>> backing which is physically removed is the Rem-Jet baking on some Kodak
>> motion picture color films and on Kodachrome. You can experiment with a
>> scrap of film. You will find that either developer or fixer will decolorize
>> the anti-halation layer but that nothing is removed from the film.
>> In some cases a plain water soak will decolorize it.
>> Some color films have an anti-halation layer under the emulsion.
>
>I have a sneaking suspicion that we are getting close to discussing
>semantics here. At any rate, the color from the antihalation
>coating needs to be removed. My impression is that this is basically a
>water soluble dye that is washed out of either a layer in the emulsion or
>a coating on the back of the film. Indeed when I'm developing film my
>prewash water and developer tend to become discolored with this dye that
>evidently is removed from the film. Whether or not this means that the
>substrate carrying the dye is removed I don't know.
>
>What I'm curious about, is whether or not the film processing causes the
>dye to become less soluble, or if somehow the developer contains something
>that removes, washes, or decolorizes this dye better?
>
>Another way of formulating the question would be to ask: what is the
>mechanism for the decolorization you talk about?
>
>- Wayde
> (wallen@boulder.nist.gov)
>
Well, I think it is a bit more than semantics. The anti-halation dye is
in a coating of gelatin on the back of the film for most films. It is
sometimes in a coating near the substrate in some color films. The gelatin
is not removed, the dye is either decolorized or washed out, I don't know
which. Its known that some dyes are destroyed by acids and others by
alkaline solutions. That may or may not be what is happening here.
I've found that both developer and fixer will decolorize it. Developer
seems to be much more effective. I tried a solution of sodium carbonate
just now. It worked but was slow. Dektol stock removed the color in a few
seconds. A fairly strong solution of Acetic Acid did not work nor did plain
water and a ten minute soak. Hypo works, but slower than developer and can
leave just a bit of residual color.
I can't cite any literature at this point but will do some reading.
In any case, the reason for making the distinction is that actually
removing a coating wouild require a fair amount of agitation rather than
just exposure to the chemicals.
You can prove the coating is still on the back by gently scraping it off.
Before processing it comes off along with the color. After processing it
comes off clear. The coating is also an anti-curling coating.
----
Richard Knoppow
Los Angeles,Ca.
dickburk@ix.netcom.com
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