From: Katharine Thayer (kthayer@pacifier.com)
Date: 11/18/02-05:52:52 AM Z
Judy Seigel wrote:
>
>
> No, the "important point" is that a beginner doesn't know about "maximum
> concentrations" or difference in solubility and takes the descriptions as
> characteristic of the chemicals themselves....
The difference in how much of the compound makes a saturated solution in
distilled water at a given temperature, and therefore how many
dichromate ions are available in the solution, IS a chemical property,
or characteristic if you will, of the particular compound, as is the pH
of the resulting solution; ammonium dichromate in solution has a lower
pH than potassium dichromate in solution. Both the number of ions
available and the pH affect sensitivity, both are responsible for the
fact that ammonium dichromate at saturation is more sensitive than
potassium dichromate, and both are properties of the chemical.
Having never had reason to test the relative sensitivies of the two
dichromates at equal % solution, I accepted at face value the assertion
that the sensitivity is the same at equal concentrations, but actually,
it turns out that chemists who have studied this report that to achieve
equal sensitivies, you have to use less ammonium dichromate than the
potassium, between 70-85% of the amount you use of potassium in the
same amount of water (the range depending on temperature and other
variables).
As to what beginners know, I would sort of expect beginners to know
something about what a saturated solution is, since most gum printing
instructions spell it out quite nicely (Livick being the lone exception
of the sources on my shelves) and all recommend starting with a
saturated solution of whatever dichromate is recommended or chosen.
I was going to defend the PF kit by saying that the instructions spell
out clearly what a saturated solution is and reassure the nervous
student that it's no big deal if some of it precipitates out, but I've
just learned that the PF kit has at some time, unbeknownst to me, been
totally redone and now uses Livick's instructions rather than Anchell's,
so I agree that anyone using that kit probably doesn't know anything
about solutions, and from now on I will no longer be recommending the
Photographer's Formulary kit.
> It's also useful to know that varying strength (by dilution) of dichromate
> in an emulsion is yet another gum control.>
That's just what beginners need: another variable to make things less
boring! My recommendation to gum beginners is to start with a saturated
solution of dichromate and then after they have a handle on the process,
they can experiment with changing the concentration if they like. On the
other hand, it should be reassuring to those who have found that
ammonium dichromate at saturated strength is too fast for their light,
to know that they can slow it down by using it in a lower concentration.
However, I suspect from my conversations with people over the last
couple of years that many people have been influenced by Livick's
assertion that "ammonium dichromate is no good for gum printing" which
of course is nonsense, but if you believe it you would be more likely to
discard the ammonium dichromate and replace it with potassium than to
accept that simply changing the concentration will take care of the
speed problem.
That leaves the question of contrast, whether the contrast is the same
with similar % solutions, and since we (I mean gum printers as a whole)
can't agree on how the contrast compares between the two compounds at
saturation strength, I don't have a lot of hope whether this other
question can be resolved any time soon.
Katharine Thayer
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