Re: (Gum) Tonal scale

Joe, you've illustrated my point about how pigment and pigment
concentration can affect tonal scale very nicely, thanks.

I'm not sure how the dichromate stains relate, but as I said, I'm not
getting all the mail, so I've probably missed what this relates to.

IME, dichromate stain is not a necessarily result of the process, but
simply an occasional artifact. When I was producing unpigmented
hardened gum for analysis, I thought maybe I could speed up the
process and get more hardened gum per sheet by exposing it more, under
the hypothesis that more exposure might produce more hardened gum, but
what I found was that once the gum is hardened the way I usually print
it, more exposure just produced stain and then more stain, without
producing any more gum. So I went on printing the gum for analysis in
the same way that I print gum for printing, which produces hardened gum
that is clear and colorless. And of course, the fact that if stain
occurs, one can remove the dichromate stain without affecting the
hardened gum, is another indication that the stain is superfluous, not
essential. So whether you will get stain when printing unpigmented gum
is a function of printing practice (IMO the light source is a suspect
variable, as I almost never see dichromate stain with my photoflood)
and isn't a foregone conclusion.
Katharine

On Dec 2, 2005, at 5:42 PM, Joe Smigiel wrote:

> 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 Sat Dec 3 14:45:09 2005