According to Wikipedia,

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) [...] 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.

The graduation tower produces a more concentrated brine, saving energy in the salt production down the line.

The process engineering behind these devices is quite straightforward, the twigs provide contact surface and the open construction allows air exchange. This is simple stuff with a modern understanding of the physics of evaporation, and trickle down column reactors are an old hat. I want to understand how someone invented graduation towers without the modern analytical toolkit and knowledge, for this I want to know possible precursors.

According to German Wikipedia, the first graduation tower was built (using straw) in the 16th century. Graduation towers were common in what today is Poland, Austria and Germany. What was the state of the art of salt production in these areas before graduation towers came along? Was there any unit operation/process step before boiling the brine?

  • 1
    Define state of art... also, what kind of salt are we talking about? Salt can be mined, or made from seawater. There are obvious differences between these two.
    – Greg
    Commented Feb 28, 2017 at 0:56
  • You might want to read the book "Salt" by kurlansky. Accessible history of the topic.
    – MCW
    Commented Sep 27, 2017 at 11:17

2 Answers 2


A historically important method of producing salt is the open pan method (or variations on this theme). A salty solution called brine is placed in an open pan, and the pan was evaporated until the salt fell out.

The graduation tower came into use about the 16th century as a means of effecting this evaporation. This made it easier to collect the salt after it evaporated. But before the tower was used to accelerate this process, one could use heat or even "natural" evaporation to effect the necessary evaporation.


I can't find link now, but prior to the first graduation towers some salines would have workers toss the brine against rough walls, to provide evaporation surface.

The first graduation towers would use straw, but this would rot and foul the brine. The technology matured over a few decades. Graduation towers needed (wind powered) pumps & a large investment up front.

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