From: Sandy King (sanking@hubcap.clemson.edu)
Date: 03/30/01-09:02:04 PM Z
Sarah Van Keuren wrote:
>Sandy, do you mean here that if you used either ammonium or potassium
>dichromate to achieve a previsualized end result you could do so with both
>forms of dichromate although the exposure may have to be longer with the
>potassium dichromate and even though a saturated solution of dichromate was
>different from a saturated solution of the other?
Yes, that is correct. It is absolutley irrevelant which of the two
dichromates you use, as the end result will be the same in terms of
contrast, color, tone, etc. Ammonium dichromate gives more sensitivity than
potassium dicrhomate at a given percentage solution and that might be of
advantage when printing a negative with very great contrast. Negatives with
DRs of about 1.6 - 2.0 print well with a simple potassium dichromate
solution and speed is approximately the same regardless of the strength of
the sensitizer, *except when using very dilute sensitizer* and then it
drops.
>I imagine that with carbon prints all 21 steps would
>separate with the perfect exposure for the reasons that Wm. Crawford gives
>in The Keepers of Light, i.e. exposure through the back, through the
>uncoated side of the tissue hardening the gum in varying degrees allowing
>the unhardened gum to float off instead of being trapped against the paper.
Yes, it is possible to print all 21 steps with carbon, though no many of us
make negatives with that kind of density range.
>In gum printing the top of the gum bichromate emulsion can harden like a
>layer of ice over a puddle but slough off entirely in the wash because it is
>not firmly attached to the support. I expect that you know what I'm talking
>about even if it seems cryptic to those who don't do gum printing.
Having done a little work in gum and research on other direct carbon
processes I understand this principle.
Sandy
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