Re: Colloid photosensitivity

Regarding your question about light sensitivity of a pure gelatin layer, I
gathered gelatin was sensitive to UV radiation around 260nm but still
required a huge amount of energy. Moreover, given the high absorption in
most optical materials, exposures in the far-UV seem very unpractical.
As for gum things might be - again - quite different. The relatively high
absorption of blue-violet light seems to suggest reasonable chances for UV-A
initiated crosslinking.

To come back to gelatin, it may be simpler to add a cationic dye + an
electron donor to the gelatin.
E.g. you could make a mixture of gelatin + methylene blue + triethanolamine
to get a red sensitive layer...

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, April 28, 2004 6:12 PM
Subject: Re: Colloid photosensitivity

> Katharine Thayer wrote:
> >
> > Hi All,
> but recently I've come
> > across this assertion (in a book about the chemistry of chromium) "Many
> > colloids such as albumen and gelatin can be insolubulized by ultraviolet
> > light."
> >
> > I suspect that what they really meant is that colloids can be
> > insolubulized by ultraviolet light in the presence of dichromate, and
> > they inadvertently omitted the crucial phrase. But just to be sure, I
> > spread some pigmented gum on a piece of paper and set it out in direct
> > sun for 3 hours and then put it in water, and of course it dissolved
> > totally in a few seconds; in other words no insolubulization whatever.
> >
>
> Since I sent the above, I have come across three more citations
> reporting the observation that colloids, or at least that gelatin, can
> be insolubulized using nothing but ultraviolet. I thought, well, with
> all these citations, and in scientific literature too, (not just
> ramblings by well-meant amateur photo-scientists) so maybe there really
> is something to this.
>
> Wouldn't that make hardening your gelatin size easy if true -- just hang
> the sized paper out in the sun for a while and there you are! hardened
> gelatin!
>
> But today I ran the same experiment with gelatin that I ran the other
> day with gum (gelatin spread on paper, dried, set out in direct sun for
> 3 hours) ran hot water over (since gelatin doesn't dissolve in cold),
> and there went the gelatin, just like that. (I colored the gelatin with
> powdered pigment so I could tell if it was there or not.) In other
> words, no insolubilization whatever, just as with the gum.
>
> THEN, I went back to the citations and traced sources back, and sure
> enough, all four citations can be traced back to one source, Brintzinger
> and Maurer, 1927. Get out the banner Judy, it's time to march for the
> cause --- NO CITATION WITHOUT EXPERIMENTATION! Obviously this is just
> one of those things that's been passed on without being tested. If it
> were a replicable observation, more than one person would have noticed
> it in 150 years, for heavens' sake.
>
> Sign me Annoyed in the Northwest,
>
> kt
Received on Sat May 1 04:32:56 2004