I have never experienced Fixodent hardening. Nor can I imagine Fixodent hardening because of it's ingredients. Maybe if you put it on dentures and don't put them in your mouth or in liquid for 5 years. Even then, I can't imagine it. I mean, it's water soluble so unless you live on the moon. Geeze. I'm going to put a squeeze on the table and leave it there and see how long it takes to get hard. And then I'm going to put it in water and see how long it takes to dissolve but I'm thinking 30 seconds to dissolve after being left out a month. Experiment on.