Re: Gum hardening: top down experiment

Marek may like to reconsider this opinion.

If Katherine's scans show that the gum hardens top down, this hardly adds to
our knowledge.

Sandy has already told us that there are more variables than three.

If you expose from the front and the top and everythng falls of there are a
number of possible reasons,why as Judy pointed out. The two most likely are
that the pigment is too thick so that the light has not reached the bottom, thus
'proving' what we already know , or that the exposure was not long enough
which demobstrates the same thing.

Just try both with less pigment, you only need a lot of pigment if you only
wish to leave the shadows to show. which is when you need a short exposure not
a long one and you want most of the exposure to fall off..

But my own advice would be that the exercise is a waste of time.ubless one
wishes to learn how to make a gum print. But then making gum prints is a better
way of learning.

Terry

Exposing from the back hardens that part of the coating closest to the
substrate so it does not fall off. Again demonstrating what we already know. But if
your exposure is not long enough, the coating is not hard enough and it will
off, or as Marek puts it, continue to develop.

In a message dated 11/4/06 3:43:44 pm, marekmatusz@hotmail.com writes:

> Katharine,
> I was very impressed with your results and thanks for contributing to this
> discussion in a positive and constructive way. Looking at your scans I would
> conclude that heavily pigmented gum layers on unabsorbant substrate do
> harden from the top down, just like other dichromated colloids. I see very
> nice tonal gradations in the print exposed from the bottom. The three
> variables (gum, dichromate and pigment ratios) are not optimised, but at
> this point I am looking for illustration of principles, rather then perfect
> prints.
> I did a similar experiment last night. I coated a heavily pigmented and
> thick layer of gum on a transparency material that I use to print
> diginegatives (HP brand). This brand has a nice sandy feel to it, so I
> though it would help to hold the gum. I exposed coated pieces  for twice my
> usual times, one through top, the other from the bottom. The piece exposed
> from the top flaked off rather quickly leaving no image. The was no image
> that I could see at any point. The piece exposed thorough the bottom once in
> the water started behaving like a carbon print, where the colloid was
> dissolving in water, rather then flaking off. After about 3 minutes I was
> excited to see a full tonality image, with beautiful tonal gradations.
> Unfortunately the image continued to develop even afet I took it out of
> water and hanged it to dry. This morning there was only a faint image left
> on the piece of transparency. I will give it another try with much longer
> exposure and perhaps lower dichromate to get more depth of UV penetration
> and hardening and higher Dmax.
> Marek, Houston
>
>
Received on Tue Apr 11 09:27:13 2006