Differential thermal analysis was used to study the timehydration reactions of gaged plaster of Paris in contact with its mixing water. The study corroborates the theory that a colloidal mechanism is responsible for initial set. Hardness is developed by the intergrowth of a cryst. hydration product. Small amts. of cations (inorg. salts) or mech. vibration acceler ate the set. Alc., gelatin, and dispersing agents retard the set. -- AATA