The temperatures that various binders used in mould making is important to consider, as once they reach the break down point, they lose their strength and therefore ability to hold the mould together. The following table gives some indication of the characteristics of various binders.
Binders and Break-down Temperatures (degrees C)
Gypsum plaster - 704 - 816
Hydrocal cement - 704 - 816
Hydroperm cement - 760 - 927
Colloidal silica - 1260
Colloidal alumina - 1260
Calcium alumina (ciment fondu) - 1538
These of course, are not the only considerations in mould making but do show why combinations of materials is important. The common plasters and cement break down before the casting temperature of glass, typically 850C.