From: Sarah Van Keuren (svk@steuber.com)
Date: 03/29/01-10:29:12 PM Z
> Katharine Thayer wrote:
>
>
>
>
>>I think it's probably best not to try to compare differing results of
>>ammonium and potassion dichromate across different printing processes.
>>In gum printing, my experience is that ammonium dichromate is FASTER,
>>meaning shorter exposure times. In fact, I suspect that the reported
>>observation of some that they get a heavy green-brown dichromate "stain"
>>with ammonium dichromate is because they don't reduce the exposure time
>>enough, relative to their exposure time with potassium dichromate.
>>Katharine Thayer
>
>
>
> If the comparison were between radically different processes such as
> gum and platinum I would agree. However, there is a rather formidable
> amount of research that suggests one can make useful comparisons
> among all of the dichromated colloid processes, specifically
> including carbon(gelatin based) and gum.
>
> As for color, in carbon printing there is no difference in results
> between the ammonium and potassium dichromate, irrespective of
> strength of sensitizer or time of exposure, with the same pigment
> tissue.
>
> Sandy King
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?
I haven't used potassium dichromate to speak of but have found pretty
consistently using ammonium dichromate that 5-6 steps on a 21 step scale
show separation with everything below the first step of separation dropping
out and everything above the 5th or 6th adhering. I try to move that span of
5-6 steps up and down the 21-step scale, placing different colors in
different places by varying exposure times through my continuous tone
pinhole negatives. 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.
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.
Sarah
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