Hello Ed & all,
I think two topics are being conflated here -- the gelatine, like Knox's,
mixed by us and coated onto artists paper as a size for gum printing, and
the already super-tough gelatine of factory made paper.
There seems to have been a consensus for some time that putting
"hardener" in your fixer for factory paper makes it harder to get an
archival wash. However, I would strongly advise adding hardener to your fix
for film, as it does make it more resistant to scratches.
On the other hand, if you don't harden your Knox's (or whatever) gelatine
coated by you, it will be more susceptible to mold, will wash off quicker,
and worse, my tests show, will not resist staining as well as many papers
with no added size at all.
Since formaldehyde got hard to get (& is nasty to use) some "authorities"
skip that step. Not a good idea. Hence the discussion of alum.
Judy