Re: Color of "tanned" gum (Was: Re: Gum Sizing

From: Katharine Thayer ^lt;[email protected]>
Date: 09/29/04-04:45:39 AM Z
Message-id: <>

Well, heck, while I'm at it I might as well tell you EVERYTHING I know
about the colors of dichromate stain, and be done with it.

One thing I found that is relevant to the current discussion is that if
I scraped brown dichromate-stained gum off the support while it was
still wet, the resulting sludge was a deep chromium green. As I've
already mentioned, if the brown-stained gum was dried and then scraped,
it was a dark brown color, not green at all. The only thing I can come
up with to explain the difference is that maybe scraping while wet
changes the physical nature of the film, perhaps even changes it from a
film to a solution, since there is quite a lot of water in wet gum; as
we know, dichromated colloids in solution behave differently than
dichromated colloids in film.

The other interesting thing about color is that dichromate-stained gum
which has been cleared in bisulfite looks clear and colorless in the
film, but when the gum is scraped, either wet or dry, the scrapings are
a medium blue, the color of chrome alum. This is consistent with the
idea that the clearing process involves the pairing of chromium with
sulfate ions.

Now IF this blue were the natural color of "tanned" gum, rather than a
by- product of the clearing process, then perhaps I wouldn't have quite
so much trouble with the word "tanned" as a descriptor for crosslinked
gum, since chrome-tanned leather is blue when it comes out of the
tanning bath. But since blue isn't the natural color of crosslinked gum,
and since the process of tanning leather has very little to do with
photoprocesses involving dichromated colloids (though photographers
have mistakenly believed for at least 125 years that the two processes
are closely related) I'm going to suggest that we eliminate the word
"tanned" from our vocabulary.
Katharine Thayer
Received on Thu Oct 14 10:19:07 2004

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