From: Katharine Thayer (kthayer@pacifier.com)
Date: 01/21/02-02:25:52 AM Z
BobWicks@aol.com wrote:
>
> Hi Katharine Thayer:
One of my colleagues Prof Barry Barnhardt did
> some original research on this and wrote a paper on it which I tried
> to send
>
> I will add your name to the list of recipients tomorrow.
Hi All,
This topic has come and gone but for closure for the archives if nothing
else I'll report that Bob has graciously sent me a copy of the paper he
refers to above, "Ammonium Bichromate Hardener in Gelatin
Silk-Screening" by Barry B. Barnhart of Harrisburg Area Community
College. Bob went to quite a lot of trouble trying unsuccessfully to
send me the paper electronically and finally sent it through the mail,
and I really appreciate his efforts; he went a couple extra miles for a
colleague and I thank him for it.
The paper is a two-page summary of the chemistry of the process of using
dichromated colloids to create silkscreen stencils, apparently taken
from other sources since it describes no original research. The
chemical explanation offered is what has been assumed about dichromated
colloids for some time: the hardening of the colloid is accomplished
through the photoreduction of the dichromate ion from +6 to +3, and the
subsequent formation of complexes between the reduced chromium ion and
entities in the colloid; in the case of gelatin those entities are amine
groups.
As I believe Judy said earlier in this thread, comparing the process of
silkscreen stencil-making to the gum bichromate process may be comparing
apples and oranges. There's a tendency to lump together all processes
using dichromated colloids, but the differences between them may be more
important than we thought. Although the basic chemical mechanism may be
the same (and that mechanism is not fully understood; the photoreduction
of the chromium is well worked out but the crosslinking of the colloid
and the role of the chromium ion in the final complex are not) the
differences in chemical composition of the colloids may result in
important differences in how the processes functionally operate.
Katharine Thayer
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