In plaster of paris, the setting time is primarily altered by:
Chemical modifiers have been used extensively to increase or decrease the setting time of gypsum products; they are called retarders and accelerators respectively. The manufacturers use both types of compounds to formulate gypsum products with specific setting times.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 186
Altering P/L ratio
Altering temperature of mixing water
Speed and length of hand spatulation
Addition of accelerators and retarders