From: Katharine Thayer (kthayer@pacifier.com)
Date: 02/10/02-05:19:20 PM Z
Cactus Cowboy wrote:
>
> I've used 30% ammonium dichromate and 10% potassium dichromate solutions
> extensively. Using the same pigment/gum mixture, I've made identical test
> exposures with both AD 30% and PD 10% respectively. The results? AD yields
> muddier, lower contrast prints. PD will produce deeper shadows, more
> contrast, and a shorter scale. Even after adjusting exposure to negate the
> difference in emulsion speed, AD will create a very slight apparent increase
> in pigment stain as compared to using PD. But pigment/gum ratio has a far
> bigger impact on staining than sensitizer does.
>
My observations partly coincide with and partly differ from Dave's. I've
tried twice to prove the assertion that ammonium dichromate prints
flatter and muddier than potassium dichromate, and have been
unsuccessful. What I found was that if I used the same pigment
concentration that produces a good print with ammonium dichromate (nice
maximum density, good tonal separation, good tonal scale) with potassium
dichromate, I got a flat, muddy print with very poor maximum density.
Only when I increased the pigment concentration considerably with
potassium dichromate did I get a good maximum density, but also (as Dave
reports) a sharply truncated scale. Neither have I been able to prove
that ammonium dichromate promotes pigment stain more than potassium
dichromate. When I've inquired further into assertions by others that
ammonium dichromate stains, it has turned out that the stains being
complained of were dichromate stains rather than pigment stains, which
is another issue entirely, although people often confuse them.
Raising the issue of dichromate saturation as a source of staining
strikes me as a curious suggestion; since most printers choose a
dichromate and stick with their choice unless they find a compelling
reason to change, and since most printers use their dichromate as a
saturated solution, I'm not sure what purpose it serves to demonstrate
that a less saturated solution might stain less. It seems more to the
point to observe that, given a saturated solution of whatever dichromate
is chosen, there will be a range of pigment concentration for any given
pigment within which pigment can be increased or decreased (to increase
or decrease maximum density and contrast) without risking pigment stain;
in other words, as Dave said above, the pigment/gum ratio is more
important than the dichromate.
In my experience, pigment stain is one of the most common problems
beginning gum printers face; I'd say it's the question I most often get
from people seeking gum printing advice. In my opinion the problem comes
from not understanding that the amount of pigment that produces a good
printing range varies widely from pigment to pigment, and indeed from
brand to brand with the same pigment. Once people see that a
surprisingly small amount of some pigments will produce a good print
with deep maximum density and a good tonal scale (which is what my
version of the pigment test taught me) they are well on their way to
conquering pigment stain.
My 2cents
Katharine Thayer
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