The "dry" method is based on the heating of gypsum
at 130-150°C under atmospheric pressure. The
product obtained by this method is an inferior form of
hemihydrate, the so-called beta hemihydrate. In this
method the liberated water vapour diffuses through the
crystal layers significantly disturbing the structure of the
crystal lattice, forming an incoherent granular structure
with a rough surface and large surface area. This means
that the following reaction occurs in the solid phase