Alum work
A special branch of historical mining was called the alum plant .
history
Alum has been used for tanning , dyeing and pickling since ancient times . For this purpose, alunite (alum stone) or alunite-containing earths were mined, as they often occur in volcanic and ancient volcanic areas. In the Middle Ages , the salt had to be imported from the Orient or the Byzantine Empire. With the conquest of Byzantium by the Turks in 1453, the Christian West was largely cut off from the alum trade. But in 1462 Giovanno di Castro discovered rich deposits in Tolfa near Civitavecchia - at the time part of the Papal States - which Pope Pius II (Eneo Silva) had exploited and exploited together with the House of Medici . To this end, they set up the first alum plant in Europe. Up to 6,000 people were employed there to break alum stone, burn it in shaft furnaces, dissolve it in water and then evaporate this solution.
The Church and the Medici maintained the European alum monopoly for a good fifty years. In addition, with his Easter bull of 1463, Pius II had threatened to excommunicate anyone who imported or bought "unchristian alum". But shortly after 1500, alum must also have been extracted from black slate (also called alum slate, although it naturally contains hardly any alum). Black slate is far more common than alum stone. In 1510 the papal monopoly collapsed and numerous new alum works were created in Europe. There they copied the well-known method of extraction from alunite: burning the slate, dissolving it in water and evaporating the solution until the alum precipitated.
Shortly after 1800, the newly emerging chemical industry put a quick end to the alum factories. The now cheaply produced sulfuric acid replaced alum in more and more applications. The salt could now also be produced inexpensively in factories from sulfuric acid. With this, the old alum factories disappeared as an interesting chapter in the history of the mining industry, even before the idea of monument protection arose. Today you can almost only find the mining aspect of the former alum works. Because of the chemical activity of the alum slate and the formation of stalactites from diadochite , these mines are attractive show facilities; for example the fairy grottoes in Saalfeld or the Morassina mine near Schmiedefeld, both in Thuringia. There was also a plant in Thuringia in Thalebra , but there are no more systems here. The Alumwerk Mühlwand near Reichenbach in Saxony, which is also a visitor mine today, still shows remnants of the process technology (roasting stages, burning hold).
Procedure
Despite the external similarity, the processes were basically different: While alunite is dehydrated when burning in the shaft furnace, when roasting the alum slate, the pyrite contained in it oxidizes to sulfuric acid , which dissolves metals such as aluminum, calcium, iron or potassium from the clay minerals that are also involved . Since roasting required large quantities of wood, it was increasingly abandoned and instead the broken slate was left to “weather” in the open air for years. However, this was at the expense of quality, because the now missing wood ash contains an essential component for "good" alum: potassium. The end products often obtained were simple sulphates, called vitriol . The addition of z. Sometimes unsavory auxiliary materials such as rotten urine or from slaughterhouse waste only led to inferior types of alum (ammonium replaced the monovalent metal in it). A variety that we call potassium aluminum alum today was considered “good” alum.
For the people of the Renaissance , however, the chemical relationships were completely inscrutable. The productivity of the individual alum plants remained dependent on numerous coincidences and imponderables, and litigation was seldom as consistent as in the Mühlwand alum plant near Rotschau , today a district of Reichenbach in the Vogtland / Saxony.
literature
- Lexicon of the Renaissance, VEB Bibliographisches Institut Leipzig, 1989
- Müller, Pötsch, "From royal purple to jeans blue", URANIA-Verlag Leipzig / Jena / berlin 1983