Is it ensure, insure, or assure?
mid-15c.; originally it was made from the extensive gypsum deposits of Montmartre in Paris.
plaster of Paris plaster of Par·is (pār'ĭs)
Any of a group of gypsum cements, essentially hemihydrated calcium sulfate, a white powder that forms a paste when mixed with water and hardens into a solid, used in making casts and molds.