True regarding gypsum bonded investments is:

After dehydration all forms of gypsum shrink between 200-400° C. Between 400 and 700° C, a slight expansion is noted. Lateral shrinkage in gypsum is caused due to the decomposition and the release of sulfur dioxide. It is imperative, that gypsum investment should not be heated above 700° C. Phillip’s 12th edition page 201