From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/14/03-09:43:04 AM Z
Well, it's been two hours, and I'm not seeing a bleaching effect as yet
on either dichromate. However, the sun here is not very bright, so maybe
it has to be a certain intensity before it works. At any rate, I'll
leave them out as long as the sun shines today.
My initial reaction of surprise and bafflement was to the assertion
that the sun will bleach out residual hexavalent chromium, (yellow)
which was baffling to me since it's well established that the
photoreduction of hexavalent chromium to trivalent chromium in the
presence of UV radiation is fundamental to the gum printing process;
for hexavalent chromium to do the opposite would be surprising
indeed.
Now it seems that what we're testing is not that assertion after all,
but the assertion that the sun will bleach out reduced (trivalent)
chromium, and my reaction to that assertion is, so what? What would
that mean to us? Trivalent chromium is a waste product of the gum
printing process. It's quite stable and it would be no surprise to find
that further UV didn't darken it further; whereas it would be a huge
surprise to find that further UV didn't darken hexavalent chromium. That
reduced chromium bleaches in UV radiation, if it does, would be
interesting, but irrelevant to gum practice except as another reason to
clear reduced chromium out of your print, although if you had that much
dichromate stain in your print you would want it out anyway, since it
would muddy the colors in your print.
The details of my test, FYI:
21-step wedges printed on unsized Fabriano Uno coated with saturated
ammonium dichromate and saturated potassium dichromate. My goal was to
print in such a way as to get a definite dichromate stain in the borders
but not to grossly overexpose, and to get a similar stain with both. As
I said earlier, I didn't quite get the stains the same, judging by eye.
I exposed in direct sun, which is the only reliable way I know of
producing dichromate stains. I was watching the stain not the clock, so
can't give exact times of exposure, but I'd say the ammonium dichromate
exposure was 30 seconds or less and the potassium dichromate exposure
probably not more than a minute. The step wedge printed (in dichromate,
remember, not gum) a very nice well-separated and nicely graduated scale
in the ammonium dichromate, from steps 1 to 10, the higher steps were
all paper-white. The scale for the potassium dichromate was shorter and
flatter, from steps 1 to 6, without clear gradations between steps 3-6.
Again, paper-white above step 6. These readings after development, of
course. I developed and dried the prints, then covered half of each,
lengthwise, with double thickness of black Arches cover, well taped
down. Will report again tonight with final results.
kt
Katharine Thayer wrote:
>
> Christina Z. Anderson wrote:
> >
>
> >
> > So you could actually just duplicate this with test strips exposed under
> > plain dichromate--my exposures were UV BL 5 minutes.
> > Chris
>
> Okay, the sun came out, and I've got a test going. I tried to stick with
> replicating your results as above for now, with one change. I used
> saturated dichromates, but rather than giving them the same exposure, I
> tried to expose them so the dichromate stain in the borders (I made
> really big borders) was the same shade of brown on both. Unfortunately I
> missed and exposed the ammonium too long, even though its exposure was
> significantly shorter than the time for the potassium. So the ammonium
> dichromate stain is a bit darker than the potassium dichromate stain
> and so any difference I see in result between the dichromates will be
> uninterpretable. At any rate this is really just a quick and dirty test
> anyway to see if I can get the same bleaching you got, even with the
> potassium.
>
> Too sick to do more than this today; I'll report back with my results,
> hopefully before you've left for APIS.
> kt
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