Analyses of the white barbotine decoration of two Roman pottery groups

White barbotine is a rare decoration form in Roman pottery production. It was applied on three main pottery groups from the 2nd to the 4th century AD: on the Samian ware from Rheinzabern, on the black coated ware from central Gaul and on the motto beakers from Trier. This kind of decoration has not yet been analysed with scientific methods. As a part of a research on the black coated ware in Pannonia some motto beakers from Trier and some samples of Samian ware from Rheinzabern were analysed with XRD, petrographic and EDS-EDX methods to get information about the technique and the material of the white decoration. The results show that the raw material of the white barbotine decoration is white clay. Two types of applications can be distinguished: one method was to fire the barbotine together with the vessel at a temperature of about 950 °C. This method is characteristic for the majority of the barbotine decoration on Samian wares from Rheinzabern. In the second method the raw white clay was first fired at a high temperature (about 1100 °C) and then crushed. Water and perhaps potash-rich materials were added to this mixture to give it the proper consistency and plasticity to draw motifs on the surface of the coated vessels. This technology could be observed on some Samian wares from Rheinzabern and on all of the motto beakers from Trier.