The cast should not be stored above:
If the storage temperature is raised to between 90°C and 110°C (194°F to 230°F), shrinkage occurs, along with loss of strength as the water of crystallization is removed and the dihydrate reverts to the hemihydrate form. As a rule of thumb, it is not safe to store or heat a stone cast in air at a temperature higher than 55°C (130°F). 
Phillips Science of Dental Materials ed 12 pg 192
50° C
55° C
40° C
45° C