Re: GUM TESTING

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From: martinm (martinm@SoftHome.net)
Date: 07/15/03-12:57:01 AM Z


A great many issues of your discussion about dichromates might be
enlightened in the context
of "DCG". Have a look at the references at
http://www.xmission.com/~ralcon/dcg-refs.html

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, July 14, 2003 5:43 PM
Subject: Re: GUM TESTING

> 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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