calcium sulphate, i.e. gypsum. As the surface of the stone
dries up, and that process continues without interruption, the
calcium sulphate migrates towards the stone surface, where
it crystallizes. In so doing, it binds two molecules of water,
thus occupying in the surface twice as much space as dissolved
limestone. Those processes lead to the formation of a
hard impervious crust on the surface of stone facades, sculptures
and/or monuments. With time, the crust falls o2, taking
with it details of sculpturing, and the residual sand spills out
from under it. The process of deterioration recurs, exposing
deeper and deeper layers of stone (Strzelczyk, 1981b).