1 There is never any necessity to use formaldehyde.
2 There are plenty of safer ways to achieve the same effect i e hardening.
3 As the strength of the chrome alum solution needed to harden gelatine enough
but not too much, is probably less than one tenth of a gram to a litre, the
risks are hardly high.
4. I now only harden the gelatine when I am making oil pigment prints.
5. As a light hardened dichromated gelatine can provide a better first size than
gelatine, neither alum nor formaldehyde are needed to gum print, (remember that
this known as a kingjudytype).
Terry