RE: Gum hardening: top down experiment

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

>From: Katharine Thayer <kthayer@pacifier.com>
>Reply-To: alt-photo-process-l@usask.ca
>To: alt photo <alt-photo-process-l@sask.usask.ca>
>Subject: Gum hardening: top down?
>Date: Mon, 10 Apr 2006 11:33:03 -0700
>
>Okay, I've coated a very thick, very heavily-pigmented gum emulsion on
>mylar and printed it from the front and from the back. A couple of
>comments before I give you the URL:
>
>(1) though the emulsion was very heavily pigmented, two things resulted in
>not a very deep DMax: (a) the fact that I used ivory black, a transparent
>pigment (if I were to do it again, I'd use lamp black) and (b) the fact
>that it's printed on a transparent material and was scanned as a
>transparency, with the light shining through it. But the thing to note is,
>be that as it may, the DMax is about the same in both prints.
>
>(2) there's a light brown pigment stain (ivory black is a brownish black)
>in both prints that is probably a function of the heavy pigmentation. It
>hardly shows in the prints themselves, but for some reason was
>accentuated in the scanning.
>
>(3) I don't honestly know what to make of the results. If you look just at
>the prints on mylar, you'd have to conclude that back- printing is much
>superior to front-printing, at least for a thick coat on mylar. But if you
>compare the back-printed print on mylar to the regular front-printed gum
>print (using a less heavily-pigmented emulsion) on paper (at the bottom of
>the page), it's hard to claim that the back-printed print is superior. But
>since they are on different materials, it's apples and oranges.
>
>So I guess if I were forced to draw a conclusion from this rather
>inconclusive test, I'd say that if you are going to print on mylar using
>a very thick and heavily pigmented emulsion, then you'll probably do
>better printing from the back. But if you're printing on paper, you can
>get fine results printing from the front with a less pigmented emulsion.
>
>http://www.pacifier.com/~kthayer/html/topdown.html
>
>Katharine
>
Received on Tue Apr 11 08:41:37 2006