From: Keith Gerling (keithgerling@att.net)
Date: 02/14/02-08:45:22 AM Z
Lately, I've learned quite a bit about these phenomena in gum printing.
Most of my prior gum printing has been with RGB or CMYK-produced negatives
with the goal of producing a color-accurate print. Achieving this requires
an element of measured consistency with regard to the mixture or pigment,
gum, and dichromate. Recently I have been working on a series of duotones
produced from black and white negatives. My goals here are somewhat
different. Color accuracy is no longer important, and what I hope to
achieve is a variety of interesting color effects from the use of a
combination of different pigments, applied in a manner that will afford me a
variety of contrast. Accepting serendipitous results, I have been willing
to experiment with some "wild" mixture combinations, at least when compared
to my, otherwise, more controlled process. I have gone back to pigments and
papers that have been problematic in the past.
What I have learned from this is that, contrary to my previous notion,
staining and flaking are not so much a factor of the pigment itself, but of
the concentration of pigment or dichromate in the emulsion mixture. In
efforts to achieve density, I have occasionally mixed gross amounts of
pigment into the mixture (quadruple what I would have used in the past).
This usually results in flaking. (My understanding of "flaking" is when the
emulsion comes of the print in areas where the printer does not desire it to
wash off). This usually manifests itself in great densities sticking to the
paper in the shadows, nothing sticking in the highlight, and high level of
graininess in the midtones. There seems to be an "all or nothing" effect".
I attribute this to a masking effect by the great level of pigment: the
pigment blocks the light, and the emulsion cannot absorb enough to become
hardened. Likewise, when levels of normally-flaking pigments (which for me
include lamp black and certain indigos) are reduced to small amounts, no
flaking occurs. In no case have I ever observed staining of the paper when
using high levels of pigment. There is always a "paper white" where the
flaking occurs.
In cases where I have hoped to achieve either an extreme low contrast, or
wanting to apply a "tint" over the print, I have occasionally increased the
amount of dichromate to levels I haven't used before, or diluted the
emulsion with water. In these cases, the risk of staining is very high. I
have determined that regardless of the pigment used, any time I reach a
point where the combination of pigment and gum arabic is less that a third
of the total mix, I will likely get some degree of staining. I attribute
this to the fact that the viscosity of the emulsion has been lowered to the
point where it can easily penetrate the gelatin. For the record, I use a
concentrated Potassium Dichromate, Daniel Smith gum, a variety of pigments,
and various gelatin-sized papers.
This archive was generated by hypermail 2b30 : 03/08/02-09:45:21 AM Z CST