A single raw material, clay, reveals an almost unbelievable range of forms, colours and decorations. All of this is ceramics.
Ceramics can be divided into two large groups:
- densely fired (sintered) ceramics: stoneware and porcelain
- undensely fired (unsintered) ceramics: earthenware, terracotta, faience, majolika, Raku.
After the mining, the clay was soaked to make it workable. It was fermented in pits, then treaded and stamped. Later on the workbench the clay lumps were cut, milled and beaten in order to remove remaining impurities and air bubbles. Now the clay was ready for use.
The Rhenish potters did not use a potter's disc but a potter's wheel. This wheel was put into motion using a wooden stick and could turn up to one hour.
A ceramic vessel is created in several stages. After being centered on the disc, the clay lump is broken up and is shaped into a cylinder. The final form is then created out of this cylinder.
Aid in firing
In the kiln, the piled-up vessels had to be separated from each other by firing aids. These were small clay tiles called "Krätzchen" in Raeren. Strewn with sand they prevented the jugs from gluing together during the firing.
These firing aids could only be used once since they could not endure the heat in the kiln a second time. Therefore they were later used to tile the floor as can be seen in the lecture hall of the museum and in the reconstruction of a pottery workshop.
During the firing of stoneware a lot of things could go wrong due to the power of the fire. Around 30% of a kiln's contents showed stains from the firing, a flawed glaze, were deformed or wrongly dyed. Sometimes whole piles of jugs collapsed and destroyed big parts of the contents.
The strict quality control entailed that only perfect pieces were exported. Jugs showing small faults could be sold at reduced prices in the surrounding area. Defective pieces however were thrown into a shard ditch next to the kiln and were destroyed. Nowadays these fragments are found during excavations all around Raeren.
The burning heat of hell
Only from the 16th century onwards, the potters were able to control the course of fire in the kiln. A stoneware firing lasted several days and the kiln had to be fired night and day. The temperature needed to rose to at least 1250°C - it was a real hell fire.
For glazing a kiln's contents 400 kilo of sodium chloride (NaCl) were scattered through venting holes into the kiln. While the chloride escaped in thick white clouds, the soda reacted with the clay and formed the glaze on the blazing ceramics.