The effect of modification on the thermal expansion and contraction character of hardened gypsum body has been investigated. It was shown that the obvious shrinkage of gypsum on heating within the range 350

4

00C

i

e.at the stage of III CaSO4 II CaSO4 was reduced greatly by the addition of modifier

Thus

the dimensional stability of piaster mould was improved.The mechanism of modification couH b’ consi lered as that th?- allotro-pic transformations of gypsum at different tempera!: ires were changed adding the modifier.by adding the modifier.