Identification of overglaze and underglaze cobalt decoration of ceramics from Valencia (Spain) by portable EDXRF spectrometry

The purpose of this work is the application of non-destructive analysis to differentiate underglaze and overglaze techniques used in the decoration of glazed ceramics from Valencia (Spain). This study was carried out by portable EDXRF spectrometry using an X-ray tube and a thermoelectrically cooled semiconductor detector. From the EDXRF spectra we have identified the main elements present in the tin-opacified lead glaze and in the cobalt pigments. The identification of the underglaze and overglaze decoration is based on the different ratios between the fluorescence line of the main component of the glaze (Pb-La line) and the fluorescence lines of the main components of the pigment (Co-Ka, Mn-Ka, Ni-Kα,... lines) when the incident angle of the X-ray beam varies. If the position of the detector is fixed, these ratios (Pb/Co, Pb/Mn, Pb/Ni,...) increase with the angle for underglaze decoration, and decrease for overglaze decoration.