Re: Gum hardening -- top down?

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

Both are phenomena of development. They may of course have other
explanations which I don't think of now... but...

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). Many ecologically minded printers do these
initial soaks in the same tray so the dichromate is concentrated and more
readily sequesterd for disposal.

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.

Another possibility of course is that the gum just takes longer to
dissolve... so now we watch the development process itself.

Experienced gum printers generally develop prints face down, with
occasional turning to check progress and for possible air bubbles ... But
if you watch development face up, with occasional gentle lavings of water
to remove the melted-off emulsion, you'll probably see the clearing of top
layers of gum/pigment, leaving more substantial (ie more exposed, or
"hardened") areas of gum pigment beneath them -- surely an indication of
hardening NOT top down -- otherwise, once the top dissolved off, the
underneath would float off even quicker.

These are two aspects of the process that come to mind -- the only data I
can think of to "explain them away" is chanting "colloids harden from
the top down."

I may have mentioned, or thought of other observations, but -- enough!

Judy
Received on Thu Apr 6 21:34:49 2006