Most types of plaster of Paris set after 20 to 30 minutes. Some types, such as Herculite Number 2, set faster, in 8 to 12 minutes. Other types, such as Hydrocal FGR95, can take over an hour to solidify.
Setting times can be delayed 10 to 15 minutes by adding a few drops of lemon juice to the mixture. Using hot water to prepare the mix will accelerate the setting time. Plaster of Paris is a powder made out of gypsum. When it is mixed with water, it undergoes a chemical reaction that produces heat and turns the mixture into a solid.