From: Judy Seigel (jseigel@panix.com)
Date: 05/09/01-11:54:18 PM Z
On Thu, 10 May 2001, Sandy King wrote:
> The solubility of sodium dichromate is not the issue as regards its use in
> colloid processes. As far as I know sodium dichromate offers no advantage
> over ammonium and potassium dichromate for colloid processes (gum, carbon,
> direct carbon, etc), but has one major disadvantage: its deliquescence.
> That is, it absorbs moisture from the air, making it complicated and/or
> difficult to mix accurate percentage solutions by weight.
I think the *advantage* may be that the stuff is cheap... and incidentally
for reasons unstated some of the old books give the formulas in sodium.
In the early days, ammonium was (I THINK) not common & more expensive. But
I suspect that carbon printing is done to closer tolerances (mmmm, good
phrase) than gum... Anyway, once you have mixed your sensitizer, even if
it's 10 or 20% off, you're using the same stuff, so you're constant.
Another deliquescer is copper nitrate & potassium thiocyanate, but I got
pretty good results with both just GUESSING about how much the wet
weighed. I won't even say it's not rocket science, because a LOT of rocket
science isn't rocket science (I mean they don't even know metric)
sometimes lethally so. Let's say "brain surgery." Well, hmmm, we do hear
stories about brain surgery. So it's your choice: on the one hand the
comfort of IMAGINING we're in control, on the other leaving ourselves the
out for unexplained effects.
OK, I'll stop now.
Judy
> A number of other chemicals have this quality, including potassium
> carbonate and chromic acid, and the same problems pertain to their use.
>
> Sandy King
>
>
>
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