In addition to being astounded by the imposing visual dimension of the graduation works, visitors immediately notice the crisp, fresh air and become aware of the taste of salt on their lips. These edifices which, today, give Bad Rothenfelde its status as a spa town were once part of the saltworks or salt factories. The Old Graduation Works date as far back as 1773-1777 the New Graduation Works, 412 metres in length, were constructed between 1818 and 1822. In other words, the original graduation works were by no means intended for human health. Being part of a salt production site, they assumed the function of salt meadows, such as those that exist in the Mediterranean area. In the case of salt meadows, the sea water evaporates across this expanse until the salt concentration is so high that salt crystals form. In the case of the blackthorn walls, the salt water trickles down until the salt crystallises, i.e. only as long as it takes for the time for salt boiling to be reduced to a minimum.
This invention from Lombardy was erected in many areas where there was a brine source with a low salt content, but insufficient surface area for evaporation. Incidentally, with 10,000 square metres available as a trickling surface, Bad Rothenfelde has the longest graduation works in the whole of Western Europe that is in service all the year round. The graduation works are only switched off when temperatures fall below -20°C.
The graduation works were erected using a timber-frame construction made of spruce wood, filled with blackthorn. The fine branching of the blackthorn creates the ideal conditions for an optimum evaporation process. Another advantage is that blackthorn does not lose its bark, even under the impact of salt water. The bizarre petrifactions on the walls are by no means salt, but deposits consisting of iron, gypsum and limestone. In addition to extracting minerals and carbonic acid, the brine transported these particles with it from the depths of the earth, more than 3000 m down. Hence the trickling of the ground source brine is simultaneously a cleaning process. In fact, it takes around 30 to 40 years for the stone walls to become so dense that they have to be replaced with fresh blackthorn. Nowadays, the blackthorn clusters are replaced in part after just 15 years.
The landmarks of Bad Rothenfelde have been used as an open-air inhalatorium since the closure of the Rothenfelde saltworks in 1969. Part of the Old Graduation Works collapsed in 1989. This monument was only rebuilt seven years after the collapse.