Technology transfer in the bronze age: The case of a faience-like blue glaze produced at bread-oven temperatures

Abstract

How familiar were the pre-Homeric Greeks with Egyptian technology? Accounts of apparent mythological nature and archaeological evidence indicate cultural contacts. To what extent did Greek craftsmen learn their art from Egyptians? At excavations in Djoser’s pyramid (3rd dynasty, ca. 2750 BC) some 36000 glazed tiles were found, most with a turqoise-blue glaze coating but also many with other colours. The glazing process has been interpreted as a self-glazing process involving alkali salts, copper oxide (CuO) and firing temperatures of around 850°C. One investigation (Schiegel, 1988) indicated that this glaze was composed mainly of quartz, tenorite and water with a minor amount of alkalis, iron, phosphorus and chlorine. The interpretation of the data as indicative of the presence of chemically bound water is rather puzzling if such high temperatures were indeed used to produce the glaze on these tiles, and in such large quantities. An alternative explanation has been proposed (Davidovits, 2005) according to which what is needed in order to replicate this self-glazing process is soluble silicate SiO2, K2O, Na2O+synthetic turquoise (mafkat), aluminium phosphate hydrate + copper phosphate hydrate and a firing temperature of 250°C. The resulting glaze can be described as a geopolymer-type material where the vitrified matrix is (K, Na)-Poly(Sialate-decaSiloxo), (Si-O-Al-O)-(Si-O-)10. The process can be controlled to produce blue (K+) or green (Na+) at 250°C or black at 350°C in the presence of Cu-phosphate, Cu-silicate and tenorite, or beige in the presence of Fe-salts (phosphate, arsenate). The process results in a vitreous matrix with the appearance of a faience-type material and with a composition corresponding to the starting materials. We have used a number of techniques to characterise this turquoise-blue “geopolymer glaze” produced experimentally by Davidovits. Synchrotron-XRD of small flakes from the outer blue surface layer and the white inner core was used to determine the %weight of the diffracting phases as well as the amorphous component which constitutes some 87% of the scattering mass. SEM/EDS was employed to study the vitreous matrix morphology as well as the local elemental composition. Raman spectroscopy was used to correlate the other observations with the molecular composition of the areas examined. It is an appealing thought, that ancient craftsmen were as ingenious in matters of solid state chemistry and metallurgy as present-day industrial materials scientists. Could similar skills have been used for decorating metal objects such as mentioned in the Homeric epics, particularly when referring to Hephaestian arts? Why not? What else could the Greeks have learned from the wise-old Egyptians, a mature and technologically advanced civilisation well before Homeric times? The great pyramids at Giza had been standing for some 1500 years before artful Odysseus and his bronze-clad Achaean warrior friends sacked the holy citadel of Troy.