> On Mon, 6 Apr 1998, Wm. Clark wrote:
>
> > A few questions regarding gelatin for alt-photo uses:
> >
> > 1. Specifically for carbon transfer, I am told that "hard" gelatin is
> > better then "soft" gelatin. Is that correct? Why?
>
> I assume that image durability is the primary reason, but don't really
> know. I've posted before looking for information about the different
> kinds of gelatins and received very little useful information.
There is a great deal of information available on gelatin.
Keeping this short and to the point, Knox gelatin is a soft edible
gelatin extracted primarily from pig skins. Hard gelatin is sourced
primarily from cattle hide, bone and cartilage. The extraction methods
vary but fall into either an alkali (liming) or acid processing. The vat
of organic material is processed several times with the first extraction
yielding the highest quality gelatin which relates to a higher bloom
number. Subsequent extractions provide lower and lower gelatin quality
with lower bloom. For your info the hardness or bloom number is a
rigidity measure of a gel at 10 degrees C at 6.67% by weight. Depending
upon the application, increasing the percent of gelatin can compensate for
a lower bloom but be careful, there are other differences between gelatins
which go beyond a simple bloom measurement. Some scientists, say at a
Kodak or Ilford, spend there entire careers studying and experimenting
with gelatin.
A good overview is found in the "Encyclopedia of Polymer Science and
Engineering", Volume 7. The two big users of gelatin are the photographic
and food industries. If you do a library search I think you will easily
find more information than you ever wanted to know about gelatin.
I hope this helps, Sincerely, Art
Art Chakalis
Columbus, Ohio, USA