From: Sandy King (sanking@hubcap.clemson.edu)
Date: 03/30/01-03:48:29 AM Z
Judy Seigel wrote:
>I found that with concentrations equal, the vaunted differences were
>minimal, including for sodium di. But I ALSO found that the differences
>were different with different colors, so I suspect that with this as with
>so much else, there is no across the board answer. (Tho we do find these
>generalizations -- gum printers know better.)
>
>Judy
This is also true for carbon photography, the fundamentals of which are in
many ways very similar to gum printing. Dave's conditions comparing 30% to
10% concentrations were very extreme. And the color of the pigment also is
an important factor determining the speed of carbon tissue. Tissues that
have blue and magenta tone are much faster than those with brown or yellow
tone.
Also, I believe some gum printers (Dave Rose at the start of this thread)
are using the term "faster" in comparing differences between ammonium and
potassium dichromate (or in comparing different strenghs of these
chemicals), when in fact what they are describing is sensitivity. In my
work with carbon the speed of tissue (as determined by how long it takes to
print the first maximum black) is very similar regardless of which
bichromate I use. However, sensitivity is slightly greater with ammonium
dichromate than potassium, and of course increases significantly with
stronger sensitizers, the practical range going from about 1/2% up to 6-8%.
Sensitivy controls how many steps of the step wedge you can print,
measuring from the first maximum black to the last visible step, and is
there related to contrast.
Sandy King
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