A graduation tower (occasionally referred to as a thorn house[1]) is a structure, used in the production of salt, that 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) that have to be changed every five to ten 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 as having beneficial health effects similar to that of breathing in sea air.
Large graduation tower complexes are located in Ciechocinek and Inowrocław, Poland.[2] Ciechocinek's entirely wooden construction was erected in the 19th century by Stanisław Staszic. The complex consists of three towers, with a total length of over 2 km. Many tourists visit it for health reasons.
With years of initial construction where available. Does not include modern indoor facilities found in some spas.