Re: Gum hardening -- top down?

Sorry folks, I've been away and am just now catching up on mail from
last week. It seems a shame to reawaken the top-down controversy
from its present state of quiescence, but there are some things that
merit comment.

On Apr 6, 2006, at 8:34 PM, Judy Seigel wrote:

>
> I'm going to mention two observations that suggest to me that
> hardening in gum isn't top down.
> <snip>

> First is the fact that most of the unexposed dichromate seems to
> come out of the print in the first 10 minutes (in fact some manuals
> point that out & instruct accordingly). ... ...
>
> That dichromate is bright orange -- meaning, as I say, unexposed.
> Why doesn't the pigment come off with it? It's held by the
> darkened, exposed gum in the *lower* layers.
>
> Of course another, or at least partial explanation could be that
> the unexposed gum instantly migrates UP from the LOWER layers and
> washes out into the water. But, if dichromate can instantly migrate
> UP, why not down? Especially when the print is developed face
> down, so that down is up.

I think maybe you're getting a couple of things confused here, if not
up and down. The fact that unreacted dichromate in the print goes
instantly (IME in less than a minute, rather than 10 minutes) into
solution in the developing bath has no particular relevance, AFAIK,
to how the dichromated emulsion behaves as a film, before and during
exposure.

But it's interesting to note that one of Mike's arguments for the
paper-attracts-dichromate theory is the assumption that unreacted
dichromate is very difficult to remove from a gum print. This
assumption is of course simply wrong; as Judy and Chris have already
pointed out, and as my own experience confirms, the remaining
unreacted dichromate runs out immediately when the print encounters
water; it's usually gone before the print itself starts to develop.
But it's interesting that both the supposed unwillingness of
unreacted dichromate to leave the print and the observed hyper-
readiness of unreacted dichromate to leave the print have been
marshalled in support of this bottom-hardening theory.

Katharine
Received on Mon Apr 10 17:54:36 2006