Gypsum

producing a positive replica of the patients dentition, for designing prosthesis

from calcium sulphate dihydrate. heated up to release water and form calcium sulphate hemihydrate

alpha hemihydrate - heated in an autoclave - forms stone, has good mechanical strength, regular particles and little pores beta hemihydrate - heated in an open vessel, forms plaster, irregular particles and porous - weak properties can be done by adding chemicals - calcium chloride, smoother particles - improved stone - densite

does the reverse of the manufacturer, adds water to powder to produce gypsum

calcium sulphate hemihydrate + water to produce dihydrate. hemihydrate dissolves in water and produces dihydrate crystals, these then precipitate on the impurities of the mix. hemihydrate continues to dissolve until it all has, as dihydrate crystals begin to come into contact with one another - expansion and initial setting begins. this is a semi solid, weak properties but not flowable, can be carved. then final set as the material hardens

mechanical strength - 25-30MPa, low. low hardness - surface can be lost, meaning accuracy is lost. very brittle,surface detail is less than that of impression material, expansion is low

water can become trapped in powder, when the powder sets, water evaporates and a pore is formed. this weakens the properties. the more water added the more pores formed and the worse the properties - plaster requires more water so has reduced mechanical properties

more spatulation - breaks up nuceli of crystallisation so produces more, crystals are more likely to come into contact with one another, set faster same with increasing amount of powder - more powder, more nuclei - increased expansion

stone - 20-35, plaster - 50-60 - theoretical, more probably needed for working