The strength of a gypsum investment is dependent
on:
(A) Carbon content (B) Silica content (C) Gypsum (D) Copper
(C) α-hemihydrate of gypsum, and quartz or cristobalite are forms of silica. Most investments contain the α-hemihydrate of gypsum, because of its greater strength. This gypsum product serves as a binder for the other ingredients and to provide rigidity. The strength of the investment is dependent on the amount of binder used. The investment powder may contain 25% to 45% of calcium sulfate hemihydrate. Gypsum shrinks considerably, when it is heated. If the proper forms of silica are employed in the investment, this contraction during heating can be eliminated and changed to an expansion.
Ref: Phillips 12th edition page 201