Glaze characterization of the glazed pottery from the medieval workshop of Vega (Burgos, Spain)

Sixteen glazes on medieval (14th-16th century AD) pottery fragments from Vega pottery workshop (Burgos, Spain) were characterized to investigate the production technology. To this end, Raman spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) were used. The studied glaze samples correspond to fragments of high-quality glazed ware used by the wealthiest society in the city of Burgos. The most representative types of glazes, corresponding to honey-marble, honey-yellow, bright light green, and dark green types, were analysed. Raman spectroscopy shows lead was used as a fluxing agent in glaze production. SEM-EDX analysis confirms the use of lead oxide and evidences that most of the samples underwent at least two firing processes. No Raman signals of crystalline phases were detected on either glaze surfaces or glaze thin-sections. The exception is the single firing of dark green glazes, which show pseudobrookite and rutile at the clay body/glaze interface measured in thin section. No colouring crystalline phases were identified by Raman spectroscopy either. However, SEM-EDX shows iron was the most-used colouring agent whereas copper was used for bright light green glazes. The polymerization index (I-p) values were used to estimate the firing temperatures. The polymerization index values are typically low for lead glazes and indicate firing temperatures below 700 degrees C.