How do teeth harden?

Harden vrs Decay?: Fluoride hardens teeth to be more resistant to decay. Good nutrition including calcium, and avoiding sugars/carbohydrates and sodas also helps prevent decay. Brush your teeth! brittle hard comes with age and drying. Endodontic therapy (root canal) removes the nerve of the tooth, but also the blood circulation drying the tooth.

Tooth calcification: If you mean how teeth harden as a they develop inside of the jaw the answer is calcification of the tooth bud. This is a one time & permanent process. As the teeth grow into the mouth, their surface can pick up Fluoride or calcium form saliva and harden to some degree. Of course, the most important thing is to take steps to prevent "decalcification" of the tooth or "decay" by proper oral hygiene.