From: Sandy King (sanking@clemson.edu)
Date: 11/09/01-09:36:17 PM Z
Judy Seigel wrote:
>
>Eat your words, fella (it that sounds rude, I never said it) -- I've
>tested it a couple of times (tho admittedly after a year, maybe more) and
>each time found a considerable (approximately 25%) drop in speed. The way
>I tested was to mix an emulsion with a new mix and one with the last of
>the old at the same time, matching as closely as possible in all respects.
>(But to think that you think I'd mix ammonium dichromate with TAP WATER --
>even NYC tap water!! -- is a mortal wound.)
>
>This leaves the possibility that things are different
>
>a. with carbon
>b. in S. Carolina
>c. for the anointed ones
>d. ........ whatever
>
>Or, really stretching, looking for explanation, I let my bottle gradually
>empty out. Maybe the bottle you tested was full? Or, no it's not possible
>you merely *read* that, like, for instance in Kosar ????
>
>Judy
In addition to the d ... answer, which I must accept as a
possibility, perhaps two others answers are in order?
e. gum printers are by nature incapable of carrying out
meaningful sensitometric testing,
f. gum printing is a witch's brewthat does not
obey any of the photochemical laws, Kosar or otherwise.
Take your pick. I could accept either or both of these explanations
since they are related in concept!!! The first possibility is
neater, of course.
And more seriously, three points.
First, there has been at least one thread (and I think more) on the
subject of the stability of dichromates, both in solution and in
distilled water, with comments from persons with a lot of knowledge
of chemistry. As I recall the bottom line was that this chemical is
highly stable in both powder form and when mixed with distilled water.
Second, I keep on hand numerous bottles of both ammonium and
potassium dichromate in various strength solutions, all mixed with
distilled water. Some of the bottles are several years old but I know
from extensive testing that solutions several years old give
identical speed and contrast in carbon printing to ones mixed two
weeks ago or yesterday. I do a great deal of testing of materials and
you can take that information to the bank and get a healthy return on
it. I would add that I keep all of my solutions topped off, or almost
so, in storage.
Third, because of the small quantities of solutions (with the
possible percentage of error) that are used in mixing gum emulsions
for gum printing, I suspect that it is difficult, if not impossible,
to carry out any meaningful sensitometric testing that would give you
results with an accuracy of greater than 25%.
So said, one of us probably deserves the "Often Wrong But Never In
Doubt" prize on this one. Fit your hear or mine better?
Sandy
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