A graduation tower (occasionally referred to as a thorn house) is a structure used in the production of salt which removes water from a saline solution by evaporation, increasing its concentration of mineral salts. The tower consists of a wooden wall-like frame stuffed with bundles of brushwood (typically blackthorn) which have to be changed about every 5 to 10 years as they become encrusted with mineral deposits over time. The salt water runs down the tower and partly evaporates; at the same time some minerals from the solution are left behind on the brushwood twigs.
Graduation towers can be found in a number of spa towns, primarily in Germany but also Poland and Austria. The mineral-rich water droplets in the air are regarded[by whom?] as having beneficial health effects similar to that of breathing in sea air.
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