Actually Dick, as far as gelatin goes, fog *is* caused by surface
crosslinking.
I'm on thin ice, comparatively speaking, when it gets to the chemistry of
dichromated *gum*, as so little has been published in the scientific
literature about it.
Gelatin, gum, and other colloids, seem to harden more or less the same way
when subjected to dichromate + light. They react differently when simply
subjected to chemicals however. Gelatin is sensitive to a lot of things,
including various pigments. Alcohol hardens albumen. So does steam. Some
people around the turn of the century attempted to market pre-coated gum
papers where the gum was hardened by alum (so that it would not dissolve in
the sensitizing bath) but the only successful attempts, according to Clerc,
where those in which the gum had been mixed in a jelly of gum tragacanth
prepared by boiling, e.g., Hochheimer, 1900.
My book _Gum Dichromate and Other Direct Carbon Processes_ has more info on
the subject.
Luis Nadeau
NADEAUL@NBNET.NB.CA
Fredericton, New Brunswick, Canada
http://www3.nbnet.nb.ca/nadeaul/