From: Katharine Thayer (kthayer@pacifier.com)
Date: 01/13/02-03:04:51 PM Z
BobWicks@aol.com wrote:
> As I stated previously, exposed
> emulsions that have most any organic base sensitized with Bichromate
> will form amino acid chains where it is exposed to light.
All the process does is take chains of material that already exist and
cross-link them, like taking ropes and making a net out of them, which
changes the physical characteristics of the material and makes it
insoluble. In order for amino acid chains to be formed and crosslink,
amino acids would have to exist in the material to start with; the light
cannot create amino acids sui generis. So in gum arabic, which consists
of complex polysaccharides, it is unlikely that the mechanism for
hardening would involve amino acid chains.
Let me explain a
> little further. My very first experience with this was accomplished
> with Knox Gelatin sensitized with Ammonium Bichromate. Where light
> struck the emulsion, it became resistant to being soluble. This same
> area also had a much higher melting point.
I think we all understand this much about the process. Light hardens
colloid in the presence of a strong photosensitive oxidizing agent, such
as the dichromate ion; no dispute and no surprise there.
If some in this group would like
> documentation that is published, I refer them to The Gum Bichromate
> Book by David Scopic, available from Light Impressions, Rochester, NY.
> He gives a rather straight forward explanation of how bichromate
> colloids work.
Scopick contains no information that I can find relating to the chemical
mechanism of the process; all he has is that same simple explanation:
light hardens gum in the presence of dichromate, which is true but not
terribly helpful in the context of this discussion.
Katharine Thayer
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