Local or imported? The origin of the raw material used in manufacturing bricks from castles of the Teutonic knights in north-central Poland and their significance to our understanding of medieval construction techniques

This article presents the results of research on bricks from medieval castles in north-central Poland. They were characterised based on petrographic analyses of thin section and mutually complementary instrumental methods. The bricks came from the oldest parts of the castles preserved to this day, or from castles not preserved but examined and excavated during archaeological research. The data obtained allowed for the identification of building materials that shared similar material and technical properties and that can be associated with different phases of the castles. Based on the results, it was found that the Pleistocene tills commonly found around the castles were not used in the manufacturing of the bricks. Small local deposits of fatty clays were used—Pleistocene varved clays or Miocene variegated clays. The buildings differed in the composition and texture of their bricks, indicating that the local clay raw material was extracted for each building separately. The share of raw material brought in from larger exposures further afield was small. Analyses of brick samples indicate the use of a fatty clay raw material that was improved by the addition of clastic material (quartz sand or a mixture of sand and quartz silt of various fractions).