#F: Why Djoser’s blue Egyptian faience tiles are not blue?

IXth Egyptology Congress, Grenoble, 2004
Why Djoser’s blue Egyptian faience tiles are not blue?
Manufacturing Djoser’s faience tiles
at temperatures as low as 250°C?
Joseph Davidovits and Ralph Davidovits

30,000 blue faience tiles were found in Djoser’s funerary complex at Saqqarah (3. dynasty). It is generally assumed that the tiles underwent a self-glazing process during firing in the range of 800- 850°C (C. Kieffer and A. Allibert, 1971) or by dipping in a liquid glaze (S. Schiegel, 1988). SEM microanalysis shows the presence of phosphorus in the glaze that suggests the use of the blue mineral turquoise (mafkat), an aluminium-copper phosphate, intensively extracted by pharaoh Djoser in the Sinai mines. Our aim was to replicate the self-glazing process with a soluble silicate binder involving a synthetic turquoise (mafkat) mixture made of pure aluminium phosphate hydrate and copper phosphate hydrate. We were astonished to get a turquoise blue-self-glazed ceramic, stable and identical to Egyptian faience, at a temperature as low as 250°C. Post treatment at 350°C changes the blue colour into grey-black (chemical transformation of blue copper phosphate into black tenorite CuO) that remains stable up to 800°C, where it turns back to blue. Did Djoser’s ceramists use this low temperature process? Apparently yes, if we look at the colours of the tiles. It is striking to notice that in contrary to their labelling, numerous Djoser’s tiles are not blue but grey, black, blue-green and even brown, colours that we have replicated at self-glazing temperatures in the range of 250-350°C.

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