Re: dichromated colloids

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: dichromated colloids
Date: Fri, 21 May 2004 04:54:48 +0000

> This is what I was trying to say to Sandy a while back and couldn't
> make myself understood, in the context of percent solution of gum. I
> notice in one of the articles I read today that as far back as 1930,
> researchers knew that percent solution of gelatin is not a useful
> measure and that "in order to obtain results truly comparable with
> each other the estimations should be carried out on solutions
> containing equal concentrations of nitrogen." (Galinsky, 1930).

I don't think any single measure is useful in standardizing
gelatin. If it were, we would have standardized photographic
gelatin. We still don't. As far as I know, each gelatin batch is
tested for molecular weight, chemical compositions, rheological
properties, etc. and classified into categories. Despite a great
effort people made for a century to identify what chemical factors are
responsible to influence the photographic properties of gelatin, we
don't have complete understanding of all possible contribution of
photographically active components, and available chemical assay
techniques are often less sensitive than emulsion making process, so
final assessment of gelatin quality is done by making emulsions...
But chemical understanding will at least help in trying to make better
gelatin later. Empirical try-and-error approach will tell you if one
particular batch has such and such good and bad points, but it doesn't
tell you what should be changed in the future.

Perfect bread can be made by measuring if you do it by weight and you
know the fact that flour's moisture content is influenced by humidity
in air, and compensate for it. Also, gluten component varies across
harvesting of wheat, but that's about it. Bread is just too easy to do
it by inspection, and it's not such a huge deal, especially for yeast
breads. Photographic gelatin is not like that. A few ppm of some
unidentified impurity can change the quality drastically in the
context of silver gelatin process. (But I don't think this is very
relevant to dichromate process.)

In old emulsion literature, they tell never to run out of any single
ingredient used to make emulsions. At the point of 1/2 of gelatin,
KBr, AgNO3 or whatever is used, consider the rest to be an emergency
reserve and buy a new stock and test it. One has to find a new stock
that works just as well as the old one before the emergency stock runs
out.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)