Physical Processes Involved in Production of the Ancient Pigment, Egyptian Blue Trinitat Pradell Dept. Fı´sica i Enginyeria Nuclear, ESAB, Campus del Baix Llobregat, 08860 Castelldefels (Barcelona), Spain Nativitat Salvado Dept. d’Enginyeria Quı´mica, EPSEVG, 08800 Vilanova i la Geltru´ (Barcelona), Spain Gareth D. Hatton and Michael S. Tite w Research Laboratory for Archaeology and the History of Art, Oxford OX1 3QJ, UK Egyptian blue, which was the first synthetic pigment to be used in antiquity, consists of crystals of calcium-copper tetrasilicate (i.e. cuprorivaite (CaCuSi 4 O 10 )). The physical processes asso- ciated with the formation of Egyptian blue were investigated by high-temperature X-ray diffraction measurements on synthetic mixtures of quartz, malachite, and calcium carbonate. The high- brilliance, high-energy radiation ID15B beamline at the Euro- pean Synchrotron Radiation Facility was necessary to ensure good time/temperature resolution, penetration, and high-quality data. The results established that the Egyptian blue crystals are formed through nucleation and growth within a liquid or glass phase, even for mixtures with an alkali content as low as 0.3 wt% soda. Furthermore, the microstructures observed in a scan- ning electron microscope indicated the ancient Egyptian blue pigments were produced from mixtures containing several weight percent of alkali. I. Introduction E GYPTIAN blue was first used as a pigment on tomb paintings in Egypt from around 2300 BC, and during the subsequent 3000 years, its use as a pigment and in the production of small objects spread throughout the Near East and Eastern Mediter- ranean and to the limits of the Roman Empire. Egyptian blue was both the first synthetic pigment and one of the first materials from antiquity to be examined by modern scientific methods. A small pot containing the pigment that was found during the excavations at Pompeii in 1814 was examined by Sir Humphrey Davy, and analysis by Fouque´ 1 identified the compound as the calcium-copper tetrasilicate CaCuSi 4 O 10 . Sub- sequently, using X-ray diffraction analysis (XRD), Pabst 2 and Mazzi and Pabst 3 confirmed the identification of Egyptian blue as CaCuSi 4 O 10 , and established that Egyptian blue and the rare natural mineral cuprorivaite are the same material. Examination of Egyptian blue samples in cross-section in a scanning electron microscope (SEM) revealed that they consist of an intimate mixture of Egyptian blue (i.e., CaCuSi 4 O 10 ) crystals and par- tially reacted quartz particles together with varying amounts of glass phase and only occasional unreacted calcium- and copper- rich phases. 4 At this stage it should be emphasized that, in the literature, the term Egyptian blue tends to be used to describe both crystals of calcium-copper tetrasilicate and the bulk poly- crystalline material that is used as the pigment and is sometimes referred to as frit. In this paper, the suffix ‘‘crystal’’ or ‘‘mineral’’ will be added when the former meaning applies, and the suffix ‘‘pigment,’’ ‘‘sample,’’ or ‘‘frit’’ will be added when the latter meaning applies. The first systematic laboratory replications of the Egyptian blue pigment were undertaken by Laurie et al. 5 These and sub- sequent laboratory replications by Chase, 6 Ullrich, 7 and Tite et al 4 established that Egyptian blue pigment can be readily produced by firing a mixture of quartz, copper oxide, and lime together with a small amount of alkali (typically 0.2–5 wt% Na 2 O) at a temperature in the range of 9001–10001C. The result of such firings is typically a coarse-textured, friable block, slab, or ball of polycrystalline Egyptian blue frit, which would then have been ground to a powder for use as a pigment. Alterna- tively, the ground powder could have been molded to shape and refired to produce small objects of Egyptian blue frit. Delamare 8,9 suggested that two different physical processes could have been involved in the formation of Egyptian blue crystals depending on the amount of alkali included in the mixture. In the first, when alkali was added at the few percent level, a liquid or glass phase was formed from which the Egyp- tian blue crystals nucleated and grew. In contrast, when the al- kali content was insufficient to produce significant liquid or glass phase, the Egyptian blue crystals were formed by surface diffusion between the three components, that is, by solid-state sintering. Delamare went on to argue that, in this latter case, as a result of partial diffusion and interaction between the three components, one would expect to find non-stoichiometric Egyp- tian blue crystals. In the research reported in the current paper, the physical processes associated with the formation of Egyptian blue crys- tals in the presence of different amounts of alkali have been further investigated using high-temperature XRD analysis. The implication of these results for understanding the methods of Egyptian blue production used in antiquity is then considered. The primary aims of the XRD measurements were to identify the phase transformations occurring during heating and cooling, and to determine the temperature ranges in which these trans- formations occurred. The high photon flux provided by a syn- chrotron radiation source was therefore crucial for enabling the rapid collection of XRD patterns so that they could be obtained at temperature intervals of less than 251C. This high flux was also essential for obtaining the high-quality XRD data required because of the large number of co-existing phases and the fact that several were present in only small amounts. J ournal J. Am. Ceram. Soc., 89 [4] 1426–1431 (2006) DOI: 10.1111/j.1551-2916.2005.00904.x r 2006 The American Ceramic Society 1426 R. Snyder—contributing editor This work was funded by ESRF project ME-453. Trinitat Pradell also received financial support under Ministerio de Ciencia y Tecnologia (Spain) Grant MAT2004-01214 and Generalitat de Catalunya grant 2001SGR00190. w Author to whom correspondence should be addressed. e-mail: michael.tite@ rlaha.ox.ac.uk Manuscript No. 20924. Received August 26, 2005; approved November 22, 2005.