Re: Gum hardening -- top down?

If the jury is still out on whether hardening of gum on exposure to
UV light takes place from the top to the bottom of the emulsion
layer, or from the bottom to the top, I suggest someone go to the
jury room and check on the condition of the jury. They may have left
the building.

Were there any gum system that would result in hardening of the gum
emulsion from the surface of the paper up to the top of the emulsion
it would be possible to make gum prints with one coating and exposure
that have both high Dmax (log 1.80 or above as is possible with
carbon), and a complete range of tones from the shadows to the
highlights. No one has ever been able to do that in gum printing, and
unless the laws of photochemical reactions are reversed, they never
will.

I have suggested a very simple test that will clearly prove that
hardening of a gum emulsion on exposure to UV light is a top to
bottom phenomenon, same as it is in carbon printing. Just expose the
gum print from the back through the surface of the paper. If you do
this you will find that it is possible to get very high Dmax prints
with a complete range of tones, though printing times will be very
long and some detail will be lost from the texture of the paper.

Sandy

>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 11:22:03 2006