> S_IZZO@guvax.acc.georgetown.edu wrote:
> >
> > Exactly 3 years to the day before Sam Wang's latest message (Jun-10-1995),
> > he wrote to the list, "I use 2% dichromate (either ammonium or
> > potassium) for gum. . ." I was just beginning to think about trying
> > gum and being Scotch, using the least amount of chemical seemed the
> > way to go.
>
> HI all,
> Could not let this one go.....one is a Scot, NOT scotch.
Jonathan, are you sure? There is no adjectival form of being of Scottish
descent?
But that's not why I'm writing -- I'm writing about Crawford. Since when
is Crawford the *authority* for gum printing? Didn't we just explode the
whole business about the Anderson gum-pigment ratio test that CRAWFORD was
instrumental in embedding in the modern-day canon? (In fact Schaefer, of
"Ansel Adams book" fame, explicitly credits him for the "tables" of
pigment ratios he lists.)
There are plenty of other mistakes in Crawford, too. The book is
terrific, a landmark, the best of its kind to that date. I have read it
a couple of times... so far. But it is NOT gospel, as we see.
Anyway, I think Dave is right -- depending on which chemistry source you
consult, the solubility of ammonium dichromate ranges from 30 to 33%... I
use 26% for various reasons, including not wanting it to precipitate out
if the room gets cold (who needs ANOTHER variable?). I tend to cut it by
about 1/3 with water, making the original about 17%, half of that would be
8.5%. If you take something away for the paint, maybe it's 6% (assuming
equal parts gum & sensitizer).
So now I suppose I'm going to have to test with Sam's 2.5% (tho I would
NEVER use the dry powder each time.... in fact in my experience if you mix
dichromate fresh & don't let it sit a half hour, it speckles your print.
Suzanne, did I say this last time?)
OK, in the next couple of days .... anybody else?
Judy