Re: (Gum) Tonal scale

On Dec 6, 2005, at 8:48 PM, Yves Gauvreau wrote:

> 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??
>

Yes, you've read me right. I should clarify that it's a tentative
conclusion drawn from an observation compared with a finding, but yes,
that's what I was speculating anyway. Thanks for checking,
Katharine

> ----- 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 13:40:20 2005