Clay source and firing temperatures of Roman ceramics: A case study from Plovdiv, Bulgaria

Abstract

During urgent archaeological excavations in the city of Plovdiv (South Bulgaria), a Roman pottery workshop was found, including several dome kilns, ceramic sherds, and a large clay deposit incorporating yellow and gray clay layers. A set of the ceramic specimens (kitchenwares and tablewares) and raw clay samples were investigated using a portable device for X‐ray fluorescence and laboratory equipment for magnetic measurements, aiming to provide specific information about the origin and firing technology of the pottery. Despite significant difference in calcium content between local raw clay and pottery samples, we conclude that local natural clay sources were used and the initial stages of production involved clay purification, although we cannot rule out the procurement of more distant clay sources. Magnetic susceptibility testing allowed for determination of the maximum firing temperatures, showing that the pottery were fired predominantly at 780°C, with some samples being fired at temperatures up to 860°C. Magnetic analyses show that kitchenwares were fired under more uniform conditions than the tablewares, which undoubtedly reflected in the quality of the vessels produced.

Archaeometric Investigations on Archaeological Findings from Palazzo Corsini Alla Lungara (Rome)

“…The chemical compositions of the fired clay samples were compared with those of the ceramics taken on site. The concentrations of quartz and plagioclase, and in particular, calcite, in all of the clay fragments lead us to believe that these artefacts were not subjected to temperatures higher than 800 • C, as calcite decomposes above this temperature [39,40], although potentially, a minor concentration of calcite may form as a secondary product of lime carbonation. The absence of clay minerals in the fired clay indicates that it was heated at temperatures higher than 600 • C [41].…”