Re: (Gum) Tonal scale

Here's a link to a few scanned gum tests I did yesterday and today which
illustrate varying exposure scales between tests (averaging around 6-7
steps for pigmented gum), different degrees of pigment staining,
dichromate staining and dichromate image, pigment flaking due to high
pigment concentration, and differences in maximum density achieved in a
single coat:

http://my.net-link.net/~jsmigiel/images/technical/gum/gum_test_12_02_05.jpg

Where there is a pigment concentration it is 1gm powdered pigment in 15
ml sensitized gum at 2 parts gum: 1 part saturated potassium dichromate
solution. Exposures were made with a Nu-Arc 26-1K unit and were
extended to ensure blocking up some steps on the darker end. Prints
were autodeveloped 1 hour in 3 changes of water. Paper was Winsor &
Newton 90# CP sized with gelatin and hardened in formalin IIRC. I used
Daniel Smith powdered pigments here to avoid including any unknown
fillers and additives which might be present in tube watercolors.

It should be fairly obvious that one cannot just use a standard weight
or volumetric measurement of pigment across the board. Each pigment
and mixture will have a different tinting strength, speed, and tendency
to stain. Some pigments may possibly interact with the dichromate as
well.

No big surprises here except the curious response of the Bone Black
pigment test (bottom middle). Take a look at the circles around the
numbers and step areas 14 and higher. There is definite pigment stain
in those areas which should be paper white. It is as if there is a sort
of pigment stain solarization effect happening. Steps are
differentiated from about step 6 through step 13 and then the steps
print darker due to pigment stain. I speculating that a very small
amount of exposure has caused steps 12 & 13 to print almost paper white.
 I'm thinking the slightly exposed gum there has reduced pigment
staining which perhaps has occurred during wet processing. IOW, as the
prints autodevelop in water, this particular pigment is released from
all areas of the print to some degree and it muddies the water. Where
an exposure hasn't had any effect at all, the pigment migrates to the
unprotected paper and stains it. This is only happening under the gum
though. Areas outside the coating remain unstained and protected by the
gelatin size. Somehow the emulsion has caused the areas beneath to
stain disproportionately, perhaps by adversely affecting the sizing or
somehow interacting with it and weakening it. Has anyone else seen this
before or have an alternate theory of why it has occurred?

Here are the 5 test conditions and the results:

1) Upper left: Saturated potassium dichromate image only/ 17 steps
differentiated/ very slight dichromate stain

2) Upper right: No pigment/2 parts gum arabic/1 part saturated
potassium dichromate/ 12 steps differentiated/ very slight dichromate
stain

3) Bottom left: 1 gm cobalt violet powdered pigment/10ml gum arabic/5ml
saturated potassium dichromate/ 6 steps differentiated/ very slight
dichromate stain/ no pigment stain

4) Bottom middle: 1 gm bone black powdered pigment/10ml gum arabic/5ml
saturated potassium dichromate/ 7 steps differentiated/ pigment stain

5) Bottom right: 1 gm lampblack powdered pigment/10ml gum arabic/5ml
saturated potassium dichromate/ 7 steps slightly differentiated/lots of
pigment staining and flaking

I'll be running a few more tests with different pigments in the next
several days. After I have them all scanned, I'll treat them in a
potassium metabisulfite solution to see if I can totally get rid of the
dichromate images and dichromate staining.

Joe
Received on Fri Dec 2 19:37:19 2005