Graduation tower technology was perfected in the salt industry in the 16th and 17th centuries following the invention of the principle of grading. It was soon realised that blackthorn brushwood was ideally suited for equipping the walls with the trickling brine. The wood has a low acid content, is finely structured and does not lose its bark. The bundles of blackthorn were superimposed on one another, which led to the emergence of the well-known graduation walls.
Of course, one could not simply place the bundles over one another and then direct brine from above through the piled thorns. A wooden construction needed to be built that served to hold everything together. This is how various construction methods came to be developed in Europe in the course of time; these were named after their designers or origin. The aim pursued was to erect buildings that were as effective as possible.
Coniferous wood is usually chosen as building material for graduation works. Due to the constant feeding of the beam structure with brine, the wood is conserved over the years and can last for hundreds of years. Only in the upper section of the graduation works is an occasional replacement of the construction wood necessary, since this part is constantly exposed to atmospheric weather conditions. The wood used here is that of the larch tree, which is rich in resin and therefore more resistant to the impact of rain water. Nevertheless, some wood types need renewing after only a few years.
The technology of brine-feeding devices and pumping systems was continually refined. The biggest problem was that of generating energy for the operation of the pumps needed to pump the brine into the graduation works. Water wheels or wind turbines, at the time called ‘wind and water arts’, served this purpose in most cases. As a rule, ‘water arts’ were preferred, given that wind is an insecure medium. But then, streams or rivers needed for the use of water wheels were not always available.
Here in Bad Rothenfelde, we used to have a water wheel in the 16th century, at the time when the “Old Graduation Works” were being built; it was that water wheel, located opposite what is today’s Sparkasse (local savings bank), that provided the power used to drive the pumps. The energy generated with the water wheel was transferred to the graduation works by means of a so-called “flatrod” system.
The flatrod and the water wheel have, unfortunately, not been preserved. Occasionally, there was a windmill of Dutch design opposite the Sparkasse with which the pumps were driven.
Another option, though an unreliable one, is the use of wind energy to operate the pumps. In 1818, when Salineninspektor (‘Saltworks Inspector’) Carl Ludwig Schloenbach was planning to construct the New Graduation Works, he thought of using Mühlenbach, a stream roughly two kilometres away, as a source of energy generation. Unfortunately, he had problems with the proprietors of the stream, which made him abandon his plan. Mr. Schloenbach therefore decided to install three windmills on top of the New Graduation Works to drive the pumps.
He defied all sceptics and realised his project, proving that it was possible to successfully run the graduation works with the help of wind power. Schloenbach had found a simple formula: “Along with the winds comes the weather for the graduation works and nothing else will stop the windmills from doing their job.” He was right. Wind contributes to good evaporation. Without wind, water evaporates much more poorly. The elderly ones amongst us know this from our experience in those days when there were no electrically driven laundry driers yet. Wind made it easier for the housewife to dry her laundry.
The brine pumped in this way into the graduation works was collected, and is still collected, in a large box-shaped brine collector laid across the entire length of the graduation works. Spur lines, which can be regulated by means of wooden taps, lead from this central collection basin to the trickling channels from which the brine drips onto the blackthorn wall.
The brine-feeding facility needs to be constantly controlled. Occasionally, the taps are blocked, resulting in an interruption of brine flow. All lines linking to the brine-feeding facility are made of wood. Metal, with the exception of stainless steel, would rust and thus contaminate the brine. Apart from salt, there are other minerals in brine which are partly also left behind in the tubes and taps. However, they are largely separated during the brine dripping process, thus constituting the ‘thornstone’ that settles on the blackthorn branches of the wall of the graduation works.
The blackthorn wall has to be renewed at intervals of approximately 20 years. In this period, so much precipitation settles on the blackthorn branches that a flawless dripping of the brine becomes impossible. In this case, the brine runs down the walls but the drops do not burst to ensure the healthy climate required at the graduation works. The graduation works have long ceased to be used for industrial salt production. With only one exception (as of 2014), there are no longer any pan salt works in Germany.
Today, graduation works serve to enrich the air with salt particles in health resorts such as Bad Rothenfelde. Over 200 years ago, it was discovered that the air around graduation works, like sea air, is good for the health of the human respiratory system. New graduation tower buildings have therefore been erected in the meantime to serve precisely this purpose.
A completely new approach to the utilisation of the graduation tower walls has been evolving for eight years now. It has been realised that with present-day technology, projections can be made on the walls of graduation works. This idea has given rise to the lichtsicht Projection Biennial, which takes place in Bad Rothenfelde every two years, attracting hundreds of thousands of visitors.