From: Judy Seigel (jseigel@panix.com)
Date: 11/11/01-12:44:05 AM Z
On Sat, 10 Nov 2001, Katharine Thayer wrote:
>
> >and,[ammonium dichromate] being stronger, you might not use a pound in
a lifetime.>
>
> Sorry, this part doesn't make sense to me. Since it takes twice as much
> ammonium dichromate as potassium dichromate to make a saturated solution
> in the same amount of water, it seems to me you would go through a pound
> of ammonium dichromate twice as fast as you'd go through a pound of
> potassium dichromate for the same amount of work. No?
Oops, you're right -- I was adding apples and oranges -- I calculate
emulsion ratios in terms of the concentration of the dichromate... that
is, tho my stock solution is 26%, I dilute with water to use as low as 10%
in printing... in maybe equal parts with gum. Writing the above, I was
thinking in terms of going through stock solution, not the dry chemical.
In fact then, as an extremely rough guess (in this matter of cosmic
import), I'd probably go through about the same poundage in one as the
other.
> No doubt the occasional printer might not use a pound in a lifetime; I
> use a pound every couple of years.
>
> But back to the question of whether a dichromate solution will change in
> sensitivity as it ages: I'm inclined to think, on the basis of this
> discussion, that it has something to do with the partially empty
> container; perhaps the air in the container ages the solution and
> changes its properties, as is true of many photographic chemicals. The
> time I threw the stuff out because it wasn't printing right, it had
> turned a dark brown color rather than its usual bright yellow-orange,
Maybe the cap wasn't on tight?
But this discussion drove me to find an old bottle of K di on the shelf,
dated 1995. It's about 75 ml in the bottom of the jar, with crud floating
around like inhabitants of a miniature Sea World, the color not too far
off the norm, however.
I'll mix up some new and compare...
> when I use it within a few weeks. This surprised me since I'd always
> gone on the understanding that once mixed it keeps indefinitely. Since
> Sandy says he keeps his container full and sees no change in
> sensitivity, and Judy and I keep a partially-filled container and see a
> difference in sensitivity over time, I think that's the most likely
> answer. There are other possible explanations, such as carbon and gum
> being different processes, acting somewhat the same but also somewhat
> different, since the molecular structure of the two colloids is
> different,
Presumably the air in the jar plays a role, but the 2 processes are
different in so many other ways, I could argue that all they have in
common is, like compost and chicken soup, a lot of same ingredients, but
made and function differently...
Judy
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