Re: Gum variables

Sam Wang (stwang@CLEMSON.EDU)
Wed, 10 Jun 1998 11:14:56 -0400

>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

Pardon me if this has been discussed before, but why is it that we must
stick to MAXIMUM (saturated) dichromate, either ammonium or potassium?

I remember posting about using MINIMUM dichromate to do the job, a couple
of years ago with the discussion of whether dumping dichromate down our
sewer systems was a good thing. How much dichromate does it take to
sensitize gum? Has anyone tested it?

Not that I am necessarily doing it any better, but my method is quite
different from what "the books" suggest: I dilute my gum with water first,
add pigment till I get the colors I want, THEN add enough dichromate in
powder form to the mixture. The amount I use is from what Franklin Enos
taught me, which is about a quarter of what it takes to get a saturated
solution.

If we were talking about carbon, that would be a different story. The
amount of dichromate determines contrast in carbon printing. In gum, my
understanding is that it does not have the same effect, i.e. more
dichromate may make the emulsion slightly more light sensitive, but that's
about it. I would love to hear from someone who have found otherwise.

Sam

PS: If we keep posting serious dialogs about alt.photo, perhaps the flame
battles will be drowned out. If we freeze with the first gun fires, the
flame wars overwhelm the list and end up wasting everyone's energy.