Re: Gum hardening -- top down?

On Wed, 5 Apr 2006, Sandy King wrote:

> 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.

Sandy, shame on you. I won't even talk about my own one-coat gum prints
(tho you could check P-F # 6), I mention an entire SCHOOL of one-coat gum
printing, as per Puyo & Demachy, circa 1896. If you read French (& I
daresay you do) there's an entire literature from them on the subject,
"Les Procedes d'Art en Photographie," and also many English translations
(including my own in P-F #1) in which Demachy rather snidely refers to
"les etrangeres" (meaning the Austrian and German school of gum) who
require --- eeewww! feh !!! -- multi-coat!!! printing!!!

I myself usually prefer the variables possible with many coats, also the
color changes... AND the EASE ! But the issue here in any event isn't top
or bottom hardening, it's scale of the coat. Gum doesn't readily hold
enough pigment to give full D-max & long scale, as the bottom tones will
block up -- although it will give a full 21 steps with a very light
pigmenting (or no pigment). So what's that got to do with top or bottom?

Which is to say, you can't possibly have tested this before asserting your
assert. Even so, when you put in enough pigment to give good D-max, you
CAN get a good scale with savvy handling, the right mix, & good balance
of exposure and development... Demachy used some light water abrasion as
well. Others use other devices.... NONE of which indicates top down
exposure, and if you imagine they do... ?????? Tut tut.

I don't do carbon printing, but I gather that the gelatin coat in carbon
is thicker than the gum coat and NOT absorbed into the paper the same way.
And of course there's the VAST difference in behavior between gum and
gelatin.. since one of the things this list is so splendid at is parsing
the nuances of very small differences it's surprising that you assume that
gum and gelatin coats behave the same.

(I'll also add, since the subject came up here last week, that I use a
relatively THIN coat of gum, that is, thin for a GUM coat, not just
thinner than a carbon coat. I add water to my gum arabic, about a third
of the volume, and find the emulsion coats better and prints better that
way -- probably a a LARGE variable in itself. I'd as soon use the thick
emulsion described here as attempt to print with silicon seal.)

As for exposure from the back proving top-down hardening -- surely you
jest --do you expect the hardening to occur on the back of the paper? If
the dichromate is "migrating" to the paper, it doesn't know or care which
side is "up," it only seeks some kind of absorbing or otherwise attracting
material.

Meanwhile, FWIW I have tried that exposure from the back thing for other
purposes... After two hours of exposure NOTHING occurred & I figured the
dichromate would harden from old age -- and me too -- before it "worked,"
and gave up. So when you say "very long," you mean about a week?, in which
case dark reaction could be theorized instead... And it still, given the
givens, doesn't prove "top down" -- the light comes from the bottom, so in
this case the bottom is the top, and yet you say it gives a full scale, so
maybe it "proves" the reverse !?

One other thing hasn't been mentioned.... When the gum coat is too thick,
it will float off... sometimes possibly in strips. Does that prove
hardening from the top down, or simply that the coat is too thick to
expose through. In other words, that circumstances alter cases.

Judy

> 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.
>
Received on Wed Apr 5 18:04:59 2006