The secret is quartz: technology of production of an eleventh-twelfth century western Mediterranean polychrome glazed ware

A group of a well-known polychrome glazed ceramic, widespread in the western Mediterranean in the eleventh and first half of the twelfth centuries, has been analysed for the first time using Optical Microscopy (OM) and a Field Emission Scanning Electron Microscopy (FE-SEM) with Energy-Dispersive X-ray Spectroscopy (EDS), in order to shed some light on the materials, production technology and provenance, about which there are various hypotheses. This ware is characterised by a perfectly drawn and varied iconography, with often stylised zoomorphic and anthropomorphic and nautical motifs. It was produced in an as yet unidentified workshop in North Africa or the Iberian Peninsula. The pottery analysed was found in an archaeological excavation in the Barrio Andalusi of Almería (south-east of Spain). Technologically, the ceramics are fairly homogeneous, with copper-green and manganese-brown pigments applied over the raw tin glaze filled with large undissolved quartz particles. The use of quartz is consistent with a Fatimid-Zirid contribution from Ifriqiya, the use of tin is consistent with an Andalusi Umayyad-Taifas contribution, and the green and brown colours on a white ground to either Ifriqiya or Andalusi. Our study has shown that the use of quartz on the decorated glazed surface is not related to the need for an opacifier, but rather to the need for a highly viscous melt that limits the spread of the pigments during the firing allowing a finer and more detailed drawing. This fusion of different techniques has been identified for the first time. It is intriguing from the historical point of view of medieval technology, and provides the first insights into understanding the technological transfers and technical solutions that took place in the Mediterranean basin during this period.