From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/16/03-06:54:03 AM Z
Katharine Thayer wrote:
>
> martinm wrote:
> >
>
> > The bleaching of a Cr3 gum layer by sunlight may also be attributed to
> > interaction with the colloid. Though I am not a chemist - I would look into
> > that direction.
>
> The colloid isn't an issue, because at least in my tests of this, there
> was no colloid involved; this was pure dichromate stain, just
> dichromates brushed on bare (unsized) paper and exposed. No colloid
> whatever.
>
Slight qualification: while there may be a small amount of colloid (most
likely gelatin) present as internal sizing in the paper (although it
doesn't seem to me that Uno has much) there was no colloid in the
coating. At any rate, I've demonstrated to my own satisfaction some time
back that the presence of colloid is not necessary to the photoreduction
of chromium (I needed to prove this to myself because some proponents of
the tanning theory of dichromated colloids do insist that this
photoreduction cannot proceed without the presence of the colloid); I
proved it by coating a piece of clear glass with dichromate and putting
it in the sun; it turned just as brown as dichromate brushed on paper,
so I don't believe that the little colloid in the paper changes the
equation here.
kt
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