Thanks, PE!

"Hardening (of gelatin) is slowed by acidic pH and is sped by alkaline pH."

Interesting...this is the opposite of what I would have imagined -- not doubting you, just surprised. In carbon printing we can put the developed carbon print in an alkaline bath to soften the gelatin and get it to release some of the pigment -- to lighten the print if over-exposed. Also the Part B of Kodak Rapid Fixer is the hardener -- and is very acidic, I believe.