Re: Gum hardening -- top down?

On Apr 5, 2006, at 2:13 PM, Yves Gauvreau wrote:

> Katherine,
>
> comments below your text
>
> ----- Original Message -----
> From: "Katharine Thayer" <kthayer@pacifier.com>
> To: <alt-photo-process-l@usask.ca>
> Sent: Wednesday, April 05, 2006 1:47 PM
> Subject: Re: Gum hardening -- top down?
>
>
>
>> Mike Ware's hypothesis, as I understand it, supposes a special
>> affinity of dichromate for paper, resulting in an attraction of the
>> dichromate to the paper. I assume that if his theory were correct,
>> the dichromate would "gravitate" to the paper even if the paper were
>> placed upside down so that the paper is above the coating layer.
>> If the dichromate simply separates and moves faster down through the
>> layer because of its different (weight, density, viscosity, mobility,
>> whatever you want to call it) as Chris seems to be suggesting (and
>> for the record, I've observed something similar at the end of brush
>> strokes) the effect would probably be similar, but it would be a
>> different hypothesis.
>>
>
> As I suggested before, if papers where succeptible to dichromate
> they would
> also be succeptible to other chemicals present in the air that
> surrounds us
> and in time ie. sufficiant exposure to them mostly because of tiny
> concentrations, it would affect the paper and degrade it along with
> the
> image. But let's assume it is the case and that the time required
> for the
> paper to degrade is sufficiently long (centuries) not to be of
> concern. Say
> the paper have an affinity to attrack and bound with dichromates it
> wouldn't
> explain by itself that a sufficient number of dichromate molecules
> would
> react with the paper to change the distribution of those same
> molecules. The
> main reason would be because though chemical attraction is
> relatively strong
> it's effect decays rather rapidely (inverse square law) with
> increasing
> distance. Though I don't have specific numbers I wouldn't be
> surprised that
> the energy tranfered to the molecules by heat alone (room
> temperature) would
> impart them such speed (momemtum) that the chemical attraction and
> actual
> bounding of molecules is more then unlikely. There could be at the
> interface
> say up to a couple of molecules diameter some attraction and
> bounding but
> that would not significantly affect the distribution of dichromate
> accross
> the emulsion. At least this is my thought and maybe it's wrong.

Yves, take it up with Mike Ware; I'm not carrying any water for him,
and in fact it should be clear that I don't find the theory
compelling. But it is a hypothesis that has been proposed, and I was
simply reporting it as a source of the idea that exists in some
quarters here, that hardening occurs at the paper surface rather than
from the top down.
Katharine
Received on Wed Apr 5 17:13:00 2006