Salt was hugely important before refrigeration, and one of the ways of getting salt was from the sea or from brine springs. There were a few ways of doing this, which depended on the natural resources available in the area. You could put the saltwater into a large, flat pool and wait on dry air and the sun to do evaporation until there was no water left and you just had the salt, or you could boil saltwater using enormous quantities of fuel to get rid of the water.
But in places where big pools weren't feasible, they did everything in their power to reduce the amount of fuel required for the production of salt, because fuel takes a lot of time and effort to collect and drives up costs.
Enter the graduation tower!
The idea is that you take some source of salty water, pump it up to the top of a wooden tower filled with brushwood (typically blackthorn), then let it trickle down, which greatly increases evaporation by maximizing surface area and exposing the water to the wind along the way. When the saltwater reaches the bottom, it's saltier than it was, and you can send it through again until it's reached the point of saturation. If you do this with ocean water, you can reduce the amount of fuel needed by a factor of ten.
Plus it looks and sounds awesome - these were sometimes called thorn towers.
And at the start of the 20th century, when other forms of salt production had skyrocketed in efficiency, the graduation towers began to be used for healthcare, because as you might imagine, the air next to the graduation tower is very salty, more than it is next to the seaside. From what I can find it seems like the main thing it does is thin mucus, though there are a lot of other health claims.
There are still a few working thorn towers that you can go visit, mostly in Germany or Poland, but they're either historical curiosities keeping a tradition alive, or health and wellness centers, distilling down a brine spring for supposed special properties.
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