On Dec 3, 2005, at 12:44 PM, Katharine Thayer wrote:
> When I was producing unpigmented hardened gum for analysis, I
> thought maybe I could speed up the process and get more hardened gum
> per sheet by exposing it more, under the hypothesis that more exposure
> might produce more hardened gum, but what I found was that once the
> gum is hardened the way I usually print it, more exposure just
> produced stain and then more stain, without producing any more gum.
This observation was confirmed in the lab by Duncalf & Dunn, who found
in their experiments with dichromated PVA film, that after 4 minutes of
exposure the film was completely insoluble, at which point 58% of the
dichromate had been reduced. After that point, continued exposure
produced further reduction of chromium without adding to the
insolubility of the film. After 10 minutes, 66% of the dichromate had
been reduced, and after 120 minutes 80% of it had been reduced.
Received on Tue Dec 6 21:15:57 2005
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