The materials most suited to this are natural stone, timber (wooden barrels) and terracotta. Perhaps more than any other material, terracotta has been used for this purpose for millennia. Terracotta exhibits a porosity particularly well-suited to purposes of water storage. This is because it enables a very small percentage of the contained water to evaporate via the vessel walls. Evaporation is always associated with cooling (vaporization, however, with heat) and, according to Walter Schauberger (Viktor's physicist son), if the porosity is correct, then for every 600th part of the contents evaporated, the contents will be cooled by 1oC (1.8oF), thus approaching a temperature of +4oC (+39.2oF).