A graduation tower is a large structure in which bundles of blackthorn brushwood are stacked, over which salt water (brine) slowly trickles down. Because of the large evaporation surface, part of the water evaporates on the way down, so that the salt in the remaining water becomes more concentrated.
Function and origin
Graduation towers were built from the late Middle Ages to extract salt from weak brine springs. The brine is spread at the top and seeps down through the brushwood bundles. Wind and sun cause evaporation; the salt content rises before the water was boiled down to salt. This saved fuel.
The structure consists of a timber frame, often tens of metres long and several metres high, filled with twigs of blackthorn (sloe).
Later use
Later, graduation towers gained a second life at spa resorts. The evaporating brine forms a salty, iodine-rich mist that is considered good for the respiratory tract. Promenades and spa parks grew up around graduation towers.
Construction considerations
- The timber frame is constantly exposed to salt water and needs durable, resistant wood
- The brushwood bundles are replaced periodically due to deposits and ageing
- The structure must be wind-resistant because of its large, open faces
Related Terms
- Timber
- Evaporation
- Historic structure
- Orangery
- Spa architecture
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