Mining alum shale in the Saalfeld region began as early as around 1550.

Producing the fascinating white alum crystals from black alum shale was labor-intensive and often yielded only small amounts. Today, alum is hardly known. For many centuries, however, it was among Europe’s most sought-after raw materials.

Even in antiquity, alum was used as a flame retardant. The Romans also valued it as a mordant, which made it possible to fix dyes permanently in fabrics—such as their togas. Until just over a hundred years ago, alum could be found in many households and craft workshops. In papermaking, it long served as an important additive. Since the Middle Ages, it was indispensable especially for tanners, who used it to preserve and keep goatskin and sheepskin supple. Alum also played an important role in medicine: among other things, it was used to treat skin and intestinal diseases and to stop bleeding.

Although alum is a mineral, it occurs only rarely in nature in pure form. Before alum shale was mined in Europe in the Middle Ages, the sought-after raw material came from the Middle East, from Egypt and from Constantinople. There it was obtained from alum stone, also called alunite.

After the Ottoman conquest of Constantinople in 1453, the trade routes in the eastern Mediterranean came under Turkish control. As a result, the price of alum in Europe rose sharply. Only a few years later, however, large alunite deposits were discovered at Tolfa near Rome, in what was then the Papal States.

The Pope quickly recognized the economic value of this raw material. By 1510, the Papal States effectively held a monopoly on alum production. Buying alum from Muslim-controlled territories was forbidden, as it was considered support for the enemy. Anyone who defied this even had to reckon with excommunication.

All the more significant was the discovery that alum could also be obtained from black alum shale. Production took place in several steps: roasting, leaching and crystallizing. Despite the great effort involved, yields were often low, so many alum works were abandoned again after just a few years.

White crystals from black rock

First, the crushed alum shale and large quantities of wood were piled up into massive heaps and set alight. This controlled smoldering fire required a lot of oxygen, but had to proceed slowly and evenly. For this, the experience of a so-called roasting master was indispensable.

After roasting, the material was moistened with water and exposed to the weather for a longer period of time.

Then the salts contained in the rock were dissolved by leaching the material with water in large wooden vats. Only at this stage of the work did the sought-after alum form. The solution obtained was then taken to the boiling house.

There it was boiled down in large lead pans over an open fire. This process was repeated several times so that the alum solution became increasingly concentrated.

Finally, rods or strings were suspended in the hot liquid. Gradually, the white alum crystals formed on them. After drying, they were stored in clay or wooden containers and then transported.