Re: GUM PRINTING QUESTION

Judy Seigel wrote:
>
  I recall that Sil Horowitz,
> chemist, said that there is no risk to archivality from residual
> dichromate, as it's already oxidized (or like that).

Katharine Thayer wrote:
>
> I think you must have meant
> "already reduced" rather than "already oxidized," but putting that
> aside--
>
 I expect Sil was
> referring to dichromate stain that's actually made of reduced chromium,
> not dichromate stain that's mostly unreduced chromium, and I think it's
> important to make the distinction.

This is why, several years ago, I tried to make the distinction between
the kinds of dichromate stains clearer by calling them by different
names; that could have prevented the confusion that's occurred in this
discussion. My attempt to get people interested in the distinction never
caught on, and I don't think I picked very good names, but still it
seems a useful distinction.

Although some people may never see a yellow stain in practice, others
may see it often, and it would be misleading to give people the
impression that there's only one kind of dichromate stain and that one
can treat them the same. A yellow stain apparently still contains
hexavalent chromium and should be considered still unstable to UV in the
presence of organic materials such as are found in sizing and paper.

The problem is that we really don't know enough about the chemistry of
dichromate stain. I don't remember precisely but my recollection is
that I wanted to call the reduced stain "chromium oxide," because at the
time I believed the conventional alt-photo wisdom about the chemistry of
the process. I know more now (and also less, as is always the case). I'm
not even entirely sure any more that the Cr in the reduced dichromate
stain is actually CrIII; a chemist who has been working on the mechanism
of the gum process thinks perhaps it may be a (or more than one)
transitional compound. But the point for this discussion is that it's
certainly not the same animal as hexavalent dichromate, and the two
should be considered separately.
Katharine
Received on Fri Aug 12 08:09:14 2005