Gum hardening -- top down?

On Apr 4, 2006, at 9:22 PM, Sandy King wrote:

> Judy,
>
> Regardless of the proof anyone might cite, I am certain that the
> same top to bottom hardening that we see in carbon is also the
> primary mechanism for colloid hardening in gum.

AFAIK, it has hardly been "established" that gum hardening occurs at
the paper surface, as Judy suggested, but it's also interesting that
Sandy is so certain that hardening occurs from top down that he
claims he will remain certain, no matter what proof is offered. :--)
I myself am waiting for further evidence to illuminate the issue.

In the past, the "proof" that has been offered here for hardening at
the paper surface is no proof at all but Mike Ware's speculation that
the dichromate may be strongly absorbed to the paper; he thinks it
may move down through the layer and congregate at the paper
surface, and if this is so, that is where hardening would take
place, because that is where the bulk of the dichromate would be
found. But it's just a hypothesis, and he has offered no evidence
that I know of to support this hypothesis.

But one thing that makes me doubt the top-down theory is the tonal
inversion thing, where it is apparent that some minimal exposure
leaves enough hardened gum to resist pigment stain, even when the
exposure is so inadequate that the gum layer as a whole fails to
harden and dissolves off the paper, seemingly in its entirety.

So, as far as I'm concerned, the jury is still out.

Katharine
Received on Wed Apr 5 08:03:20 2006