my 2 cents:

the oils I have worked are all far too soft for using without hardening. That's why I use it.

I buy the magnesium carbonate in the same store where I buy the ink, but I have also in the past tried something ammonium - something (the actual name eludes me).

As I have never tried the aforementioned suggestions, I'll stick (pun intended) to the magnesium....

Clive: one reason to use hardener is, that it can be a means to highten/lower the contrast in the final bromoil..

(the stiffer ink, the higher contrast..)