Beyond Vitruvius: New Insight in the Technology of Egyptian Blue and Green Frits

Archeological campaigns along the track of the Domitian road in Cuma resulted in the recovery of a large amount of potshards and crucible fragments (both open and closed forms), covered with residues of blue and green frits, respectively. Typological analysis of the crucibles combined with mineralogical and petrological analyses on the ceramic body and frit residues revealed that the forms are intimately related to the technological data. Newly formed calcium silicates indicate high firing treatments of crucibles during pigment making (not lower than 950°C). In particular, different working temperatures for the two types of crucibles have been estimated by the presence/absence of sodalite-nosean feldspathoid. This mineral formed after the reaction of Si-Al-rich material (the sintered ceramic body) and alkalis transferred to ceramics via chemical diffusion during the pigment synthesis. Thus, the estimated working temperatures are higher for closed forms (>1100°C) and lower for the open ones (950-1050°C). This different thermal treatment perfectly fits with the temperatures of green and blue pigment synthesis; higher temperatures for green pigments allowed the formation of abundant Cu-bearing glassy phase, whereas, blue pigment is prone to the formation and thermal stability of cuprorivaite crystals (950-1050°C). Moreover, the two frits showed similar recipes (quartz-feldspar-calcite-rich sand) with the exception of more abundant Cu-bearing colorant agent in blue hue and higher proportion of alkaline flux in green frit. The obtained data suggested that crucibles were a fundamental tool for pigment manufacturing, likely one of the best example of ancient technical ceramics, as they permitted controlling the temperatures along with the fuel and the treatment time. Combining analytical and archeological data, the production and the technology of the two colored compounds identified as Egyptian Blue and Green frits in ancient Cuma has been inferred. Finally, considering the Vitruvius excerpt that identified an Egyptian Blue production in Puteoli, the area of production can be widen up to the northern Phlegraean Fields, also including Cumae and Liternum.