This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
When it is desired to lengthen the time of setting after preparing plaster of Paris, dissolve 1 oz. of citric acid in water used for mixing 100 lb. of plaster, and it will retard the setting for about three hours.