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.
Chemically pure sodium nitrate can be obtained by repeated recrystallization of Chile saltpetre or by synthesis.
They are rarely metamorphosed to the point of recrystallization, though locally shales are altered to roofing slates, sandstones are indurated, limestones slightly marblized, and coals, originally bituminous, are changed to anthracite in northern Pennsylvania, and to graphite in Rhode Island.
The recrystallization induced by pressure is probably of still greater importance.
It may be purified by recrystallization from hot water.
By recrystallization from hot benzene, the a form is obtained in large prisms which melt at 157° C., and at their boiling-point decompose into hydrochloric acid and trichlorbenzene.
It may be prepared by keeping moist and exposed to the air for from four to six weeks, at a temperature of 20° to 25° C., a paste of powdered gall-nuts and water, and removing from time to time the mould which forms on its surface; the paste is then boiled with water, the hot solution filtered, allowed to cool, the separated gallic acid drained, and purified by dissolving in boiling water, recrystallization at about 27° C., and washing of the crystals with ice-cold water.