A graduation tower (occasionally referred to as a thorn house[1]) 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.[2] 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.
A large complex of graduation towers is located in Ciechocinek, Poland.[2] This entirely wooden construction 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.