1) The test really is a waste of time because
2) the addition of dichromate reduces the viscosity of the emulsion causing
3) a longer setting time which allows
4) the pigment to migrate to the paper and stain it.
The viscosity of the emulsion seems to be the key as far as I have explored.
I've been able to add much larger amounts of pigment (powdered not premixed)
without increasing stain. Too much pigment results in flaking (presumably due
to underexposure by increasing the opacity of the mix), not staining.
As a result of this observation and recent related posts, I've decided to run
a few simple tests by increasing the viscosity of the mix. I've decided to
add some powdered gum to the 14B stuff. I won't know the Baume of it, but
I'll be able to tell if it matters. Having just received a new gallon of 14B
gum from Daniel Smith, I decided to use it instead of the old stuff from a
print supply house I had around. Upon opening the new bottle a measuring out
100ml to add the powdered gum to, it was apparent to me that this new gum out
of the bottle was more viscous than the old gum. Hmmm. Apparently all 14B
gums aren't created equal. (You learn something new everyday.) I'll give it
a try and report back if anything significant comes from my experiment.
In the meantime, I have a question for the gumbies/chemists out there. I know
the saturation values of the various dichromates differs, that ammonium
dichromate saturates around 28% (I think) in solution. I've always thought
that this salt was better in terms of printing speed (don't know where I read
that one), but isn't potassium dichromate more concentrated in a saturated
solution, and if so, wouldn't this salt be better in terms of maintaining a
higher viscosity emulsion and thus minimizing the stain problem? Maybe I've
been using the wrong salt all along. Is potassium dichromate best suited for
the process?
Thanks for any info which could lead to more testing ad infinitum....
Joe