gelatin hardeners

In short, I recommend glutaraldehyde, not glyoxal, formaldehyde,
succinaldehyde or other less preferred aldehydes, to harden gelatin.

Until mid 1950's glyoxal was the only dialdehyde readily available, so
this compound gets mentioned in old literature. (Dialdehydes are a
generally preferred class of compounds compared to formaldehyde, which
is also prone to yellowing problem and other troubles.) But more
recent literatures agree that glutaraldehyde is a lot more preferred
gelatin hardener of aldehyde class.

Gelatin yellowing with many hardening agents are common problems. It
is likely a consequence of variety of factors, and what influences
yellowing may be of academic interest to understand why in some cases
glyoxal, for example, worked trouble-free, but I don't think it's very
useful to try to avoid yellowing with hardeners that are already known
to be problematic, especially because better alternatives are
available. I don't remember much about chrome alum hardeners, but this
is certainly one of the fussy hardeners, so I decided to stay away
from from the beginning. There are other organic hardeners other than
aldehyde hardeners in current use (S-triazines, active olefins,
polymeric hardeners) but glutaraldehyde works very well for me, so I
haven't looked any farther.

--
Ryuji Suzuki
"Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)