Re: Colloid photosensitivity

"Do you have a reference for this, or is this
Brintzinger and Maurer 1927?"

I think it's from Glafkidès (Chimie et physique photographiques, Paris
1967). Having looked into Zelikman & Levi (Making and coating photographic
emulsions, London/New York 1964, p. 38), I found this:
"Colourless grades of gelatin transmit in all region of the visibible
spectrum; they have two absorption bands, at 274 and 254 mu, in the
ultra-violet."

"At any rate, if this is the case, the purported photosensitivity of gelatin
is curious but irrelevant to our concerns, I would say."

That was my point. However, given the large variety of gelatin to be found,
sensitivity to UV might vary from case to case. So it may well be the
possible crosslinking by UV light is actually due to traces of photoactive
"ingredients".

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Saturday, May 01, 2004 11:01 AM
Subject: Re: Colloid photosensitivity

> MARTINM wrote:
> >
> > 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.
>
> That's interesting. Do you have a reference for this, or is this
> Brintzinger and Maurer 1927? At any rate, if this is the case, the
> purported photosensitivity of gelatin is curious but irrelevant to our
> concerns, I would say.
>
> Moreover, given the high absorption in
> > most optical materials, exposures in the far-UV seem very unpractical.
>
> Yes, but this doesn't work even in hours of direct exposure to sunlight
> without any of those optical materials in the way. I think the crucial
> part of your description may be "requires a huge amount of energy," I'd
> guess it requires more than we can generate (or would want to have
> bouncing around in our environment) with our usual light sources.
>
> > 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.
>
> Well, it doesn't happen in 3 hours of direct sunlight, I can tell you
> that for sure. And if it doesn't happen in three hours of sunlight, this
> knowledge is probably not very useful for our purpose.
>
> >
> > 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...
>
> Simpler than what, to do what? I think I've lost track of what the
> question was.
>
> Oh, I remember. I was tracking down references about how the dichromated
> colloid processes works and came across something that said colloids can
> be hardened by exposure to UV alone and then generated some theoretical
> idea about dichromated colloids based solely on that "fact." And I'm,
> like, wait a minute! ;-) Colloids can be hardened by UV? Since when?
>
> If the hardening by UV is real, but is such that it doesn't occur in the
> conditions under which we print pictures, then it shouldn't be invoked
> to explain how our pictures print, was my point.
> Katharine Thayer
Received on Sun May 2 03:57:29 2004