From: Katharine Thayer (kthayer@pacifier.com)
Date: 11/10/01-09:40:38 AM Z
Judy Seigel wrote:
>
> I've never read of it done with ammonium dichromate, but I have a hunch
> the am di didn't come into general use until later, and was also more
> expensive,
Not that it makes a never-mind in the larger scheme of things or even in
the larger sense of this discussion, but from what I recall of my
reading on the subject, ammonium dichromate wasn't as widely available
as potassium in those days, was more expensive as you say, and was also
not well regarded because it was believed to be a fire hazard, and for
all those reasons potassium dichromate was the agent of choice.
>and, 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?
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,
and I've noticed that it does go somewhat darker as time goes by, even
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, but Occam's razor suggests that the simpler answer is
probably the right one, and there's no need to resort to more
complicated explanations or, for heavens' sake, to maligning gum
printers, who as far as I can see are no stupider or more careless or
incompetent (or more lacking in knowledge of chemistry, for that
matter) as a group than other alternative process printers.
Katharine Thayer
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