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 Sat May 1 15:44:16 2004