In plaster of paris, the setting time is primarily altered by:
Altering P/L ratio
Altering temperature of mixing water
Speed and length of hand spatulation
Addition of accelerators and retarders
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