Re: (Gum) Tonal scale

dec 6 2005 @ 11:48 pm

Katharine,

I'll try to say this as best as I can and please correct me if I'm wrong.

Below you quote this "after 4 minutes of exposure the film was completely
insoluble" and that "at which point 58% of the dichromate had been reduced".
Assuming all this is correct ie 4 minutes => all the stuff is insoluble but
only 58% of the dichromate have been ("used") reduced. I think right away
that any further exposure wont make any more of the stuff (PVA) insoluble
since all of it already is.

Since this is your reply to "more exposure just produced stain and then more
stain", does this mean that the dichromate that does not participate in
transforming the soluble gum into insoluble gum might or is the causes of
these stains???

Is this the right way to read this or I miss the point again??

Thanks
Yves

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Sunday, December 04, 2005 10:43 AM
Subject: Re: (Gum) Tonal scale

> 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.
> kt
Received on Wed Dec 7 03:28:12 2005