The crystals are purified by recrystallization from water.
The potassium salt, after recrystallization from warm water, separates in large tabular crystals.
Schists in the common acceptance of that term are really highly crystalline rocks; fissile slates, shales or sandstones, in which the original sedimentary structures are little modified by recrystallization, are not included in this group by English petrologists, though the French schistes and the German Schiefer are used to designate also rocks of these types.
The precipitated alumina is filtered off, the filtrate evaporated and the ammonium salt of the acid purified by recrystallization.
Chemically pure sodium nitrate can be obtained by repeated recrystallization of Chile saltpetre or by synthesis.