Heats of Crystallization of CaSO₄.2H₂O

The heat of crystallisation of Gypsus (CaSO 4 .2H 2 O) has been measured. At room temperature (23°C), from solutions whose initial concentration is greater than 0.4 sole 1 −1 in CaSO 4 ), an exothermic precipitation occurs with an enthalpy of crystallisation of −20.2 ± 0.5 KJ mol −1 . From solutions whose initial CaSO 4 concentration is less than 0.25 mole 1 −1 the crystallisation process is endothermic, the enthalpy of crystallisation being +2.4 ± 0.2KJ mol −1 . Under microscopic examination the exothermically produced crystals appear as very thin needles, while the endothermic crystals are platelet like. The latter are identical to the crystals of gypsum found in nature. Analysis of all precipitates formed below 70°C, corresponded to the formula CaSO 4 .2H 2 O. All precipitates have the same X‐ray powder spectra (Debye ‐ Scherrer). However, Differential Scanning Calorimetry produces distinct scans for the two types of precipitates obtained. The logical interpretation of these results is that there exists two distinct phases of CaSO 4 .2H 2 O. The existence of two phases of gypsum would explain why in some (but not all) tombs in the Valley of the Queens and the Valley of the Kings in Upper Egypt, the gypsum, serving as a base layer for mural paintings, has become dehydrated.