Graduation tower

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 as having beneficial health effects similar to that of breathing in sea air.
A large complex of graduation towers is located in Ciechocinek and Inowrocław, Poland. This entirely wooden construction in Ciechocinek was erected in the 19th century by Stanisław Staszic. The complex consists of three graduation towers with a total length of over 2 km. Many tourists visit it for health reasons.
Partial list of towns and cities with graduation towers.
With years of initial construction where available. Does not include modern indoor facilities found in some spas.