Re: Gum hardening -- top down?

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.

>
> At any rate, for whatever it's worth, Jeremy, I think it's the other
> way around. If you leave a coating mix sitting in a dish, you'll
> usually find (at least I find) the pigment settling to the bottom
> and needing to be stirred back in before coating, rather than the
> dichromate settling to the bottom. But it may behave differently
> when it's a film.

The pigment sinking to the bottom of the emultion is another story, these
are not molecules, they are rather a relatively large number of molecules
bunch together and there mass is large enough to be affected by gravity.
Pigment have a relatively large mass and practically no speed (momentum)
almost a direct opposite to molecules in a liquid where there mass is
practically null compare to there speed (momentum 1/2 mv^2)

Regards
Yves

> Katharine
>
>
>
> On Apr 5, 2006, at 10:19 AM, Jeremy Moore wrote:
>
> > I would think that if the relative density of the dichromate is
> > much higher than the other chemicals in the solution you would see
> > a definite sinking. This happens very quickly in solution. Then
> > again, I have no idea what the relative densities are of the
> > chemicals in the solutions so take with a grain of salt.
> >
> > -Jeremy who knows naught of science
Received on Wed Apr 5 15:11:51 2006