The Scientific Investigation of Bricks from St. Urban (13th Century Cistercian Monastery, Lucerne, Switzerland)

Abstract

The bricks at St Urban have been the focus of scientific enquiry for rather less than a decade. In 1995 excavations revealed new bricks of extraordinary quality, design and size. The majority of these red coloured bricks are up to 45 centimeters in length, 34 centimeters in depth and 11 centimetres in height, making them extremely unusual as building materials. Similar bricks have been found in other places in Switzerland (Aarau, Basel, Fraubrunnen, Zofingen) and are generally considered to be imports from the St. Urban brick manufactory. The present research is concerned firstly with the chemical and mineralogical composition of the bricks and the establishment of a new reference group. The research also aims at the technical aspects of the manufacture (especially the firing) of these extremely large bricks. The bricks were sampled using a core drill or simple hammer and chisel. In this way we obtained profiles through the brick from rim to core. So far 76 samples including 10 samples of raw materials have been analysed. The mineralogical composition of the bricks analysed by x-ray powder diffraction is quartz, potassium feldspar, plagioclase, hematite. White mica and calcite are sometimes present. The approximate maximum firing temperatures were from 900 to 950 °C. The ambient atmosphere during the firing of the bricks was oxidising, even if partially reducing conditions were observed (black cores). To examine the chemical homogeneity of the ceramic building materials major and trace element analyses were carried out using x-ray fluorescence. This was also performed in order to identify similar ceramics with a different provenance. The results of 46 samples from St. Urban show a relativley homogeneous chemical composition. As well for the 16 samples from Aarau, Basel, Fraubrunnen and Zofingen there is -until now- no evidence which points to a production center other than St. Urban. Raw materials (red clays, coloured marls and sands of the Aquitanian molasse) from an outcrop near St. Urban have been sampled and analysed. The red calcium-poor (less than 1 wt%) clays are composed of illite, chlorite and montmorillonite. The coloured marls additionally contain calcite. The sands (quartz, potassium feldspar, plagioclase, calcite, white mica, chlorite, biotite) are poor in carbonate, but only in the upper levels of the outcrop. The chemical and mineralogical composition of the sands was compared to those of the bricks from St. Urban. For the majority of the bricks a correspondance was found with mixtures of clays and sands, which were used as tempering materials. Thin sections of older and recent brick samples were made in order to compare the textures, grain size distribution and quantity of the tempering material. In addition, experiments on several clay mixtures close to the average brick compositions were carried out to investigate their drying sensitivity and firing behaviour. As it seems difficult to compare firing conditions in small electric kilns with production in the thirteenth century, a kiln has been built to imitate medieval brick making and firing. Firing experiments using this kiln are scheduled for September 1997.