From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/14/03-12:58:27 PM Z
Last report: The test has had all the sun it's going to get, about 5
hours, and now I'd say there is a perceptible, but small, density
difference in both the dichromates. I scanned the test prints to get an
accurate measurement, since I don't own a reflectance densitometer, and
found that the ammonium dichromate started at (mean and median from the
histogram) 128 (where 255 is white) and bleached to 148 in the five
hours. The potassium dichromate started at 165 and bleached to 185. So
the difference is the same, 20 density points, in both cases.
The reason I started out wanting to do this test was to check out two
assertions: (1) that hexavalent chromium will bleach in the sun. That
assertion changed midstream to the assertion that trivalent chromium
will bleach in the sun, a much less interesting assertion. I have proved
to myself here that it does, at least a little bit, but the question
remains: why care?. A properly exposed gum print will not contain enough
reduced chromium to need clearing, and in the odd case that it does,
there are more efficient means of removing it. Which is probably why
only the one person ever made the observation.
(2) the assertion that trivalent chromium that originated from potassium
dichromate will bleach more than trivalent chromium that originated from
ammonium dichromate. I found no support for this assertion in my
observations today.
kt
Katharine Thayer wrote:
>
> Katharine Thayer wrote:
> >
> > Well, it's been two hours, and I'm not seeing a bleaching effect as yet
> > on either dichromate.
>
> Now it's another hour, (3 hours total) and I am noticing a visible
> difference between the stain exposed to sun and the stain covered from
> the sun, although I would call it a difference in color rather than in
> density. The covered stain is a bit brighter and more yellow-brown,
> whereas the uncovered stain is more a dull brown. This is equally true
> for both dichromates. I suspect this has to do with very small amounts
> of hexavalent chromium left in the stain after development, which have
> converted to trivalent chromium with further exposure to light.
>
> (Where else but here could you get a play-by-play account of a
> dichromate fade test, I ask you?)
>
>
> kt
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