Re: Gum hardening -- top down?

Judy

In a message dated 6/4/06 1:44:44 am, jseigel@panix.com writes:

> But the issue here in any event isn't top
> or bottom hardening, it's scale of the coat. 
>

Surely, it is both.
> Gum doesn't readily hold
> enough pigment to give full D-max & long scale,
>
If you want full gradation, the range of a single exposure is about 0.7 or
just over two stops. One obtains a full range of tone through adding and
overlapping 0.7s. This enables one to vary the colour as well. One can vary the
number of exposures needed by varying the amouints of pigment.

> 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.
>

The very reason why pigment is adjudged to be too much is that the light
cannot get to the lower layers so that remain unhardened. That is why too heavily
pigmented mixtures fall off.
>
>
>
> 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.
>
A gelatine coat in carbon printing is very thin. One can use gelatine to make
the equivalent of a gum print.
>
> (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.)
>
The thinner the gum, the greater danger of the mixture contaminating the
paper. As one controls the tone with the amount of pigment, what is so
aesthetically inferior about using thicker coats?
>
> 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.
>
The reason your paper is absorbing the dichromate is because your mixtures
are too thin.
>
> 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.
>
My first gum print took three days to expose under dull November skies.

> 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 !?
>
If. in exposing through the back, the bottom is the top then the light will
affect the mixture from the paper surface to the surface of the mixture . The
principle is not affected.

> 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.
>
it shows that the light has not reached the lower levels, that is why the
hardened top layers fall or float off the paper .
>
>
>
Terry

Terry King FRPS

RPS Historical Group (Chairman)

www.hands-on-pictures.com/

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Received on Thu Apr 6 06:18:46 2006