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, Poland. 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.
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.
- Bad Dürkheim (1736)
- Bad Dürrenberg
- Bad Essen
- Bad Karlshafen (1986)
- Bad Kreuznach (1732)
- Bad Kösen
- Bad Münster am Stein (1729)
- Bad Nauheim
- Bad Oeynhausen
- Bad Orb (1806)
- Bad Rappenau (2008)
- Bad Reichenhall (1911)
- Bad Rothenfelde (1777)
- Bad Salzelmen (part of Schönebeck, 1756)
- Bad Salzhausen (around 1600)
- Bad Salzuflen (18th century)
- Bad Salzungen
- Bad Sassendorf
- Bad Soden (part of Bad Soden-Salmünster, 2006)
- Bad Sooden-Allendorf
- Eibach (part of Dillenburg, 2004)
- Hamm (2008)
- Lüneburg (1907)
- Rheine (Saline Gottesgabe)
- Salzgitter-Bad (2009)
- "Graduation House". www.thermarium.com. Retrieved August 9, 2014.
- Affelt, Waldemar: Wooden masterwork of saline in Ciechocinek, Poland, in: Santiago Huerta (ed.): Proceedings of the First International Congress on Construction History: Madrid, 20th–24th January 2003, Instituto Juan de Herrera, Madrid 2003, ISBN 84-9728-070-9
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