Gesso
Do livro "Dental Materials":
The α-modified hemihydrate is made by boiling gypsum in a 30% aqueous
solution of calcium chloride and magnesium chloride 'I'his process yields the
smoothest, most dense powder particles of the three types, and the powder is
used primarily for dies. The α-hemihydrate is called artificial stone, die stone, or
improved stone.
Depending on the composition oi the gypsum product, this observed linear expansion may be as low as 0.06% or as high as 0.5%.