Re: Gum hardening: top down?

On Apr 10, 2006, at 11:33 AM, Katharine Thayer wrote:

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.

Thank you for taking the time to do the test.

Let me remark that in carbon photography, where image formation
should be very similar to your test in exposing from the bottom, just
coating with a heavily pigmented emulsion would not necessarily
result in high Dmax. There must be the correct balance between
negative contrast, thickness of the coating, how heavily the coating
is pigmented, and the strength of the dichromate sensitizer.

Many people find it very difficult to match all of this up when they
first start printing with carbon, primarily because they use a
dichromate sensitizer that is much too strong for this type of
exposure. If they do, the resulting image will be very weak (low
Dmax) because the strength of the sensitizer is so great that all of
the hardening takes place right on the surface of the print (on the
top at the end with carbon because we transfer to another surface,
but at the bottom with your experiments). The result is you have a
very thin layer with relatively little pigment, and you can recognize
this because a very high percentage of the pigmented carbon layer
just washes away during development.

Assuming that there is some kinship between the mechanism of carbon
transfer, and exposing a gum colloid from the bottom (which I
strongly believe there is), you should find that the strength of
dichromate that is used in regular gum printing is much to strong and
will result in a fairly low Dmax when exposing through the bottom. In
carbon, for example a sheet of 8X10 carbon tissue might contain 80ml
of an 8% gelatin solution, but we would need no more than about 10ml
of a 2% ammonium dichromate solution to sensitize. In gum, where
typically you expose from the top, and do not transfer the image, you
use a much higher ratio of dichroamte:colloid. So I would think that
if anyone wants to optimize a gum coating for exposing from the rear
they need to think more in terms of the carbon paradigm that gives
maximum relief and Dmax, moderately pigmented, thick emulsion
sensitized with very dilute dichromate solutions

Sandy
Received on Mon Apr 10 17:57:04 2006