New insights into the colour origin of archaeological Egyptian blue and green by XAFS at the CuK-edge

New results on the local chemical environment of Cu2+ in archaeological Egyptian blue and green and also modem Egyptian green were obtained by x-ray absorption fine structure (XAFS) analysis. The information is essential for the understanding of the colouring mechanisms in both pigments. In a previous study, a clear physico-chemical characterisation of Egyptian blue and green was achieved using a complementary analytical approach with ancient and modern synthesized pigments. Electron microscopy (SEM-EDX and TEM), x-ray diffraction and micro-Raman and UV-visible spectroscopy were used to gain information about the conditions of the ancient Egyptian fabrication processes and permitted the clear distinction of both pigments. However, the exact colouring mechanisms could not be elucidated by these methods. Different Cu-bearing amorphous and crystalline phases were found in both pigments. These phases should be at the origin of the blue and turquoise colours. Using XAFS data at the Cu K-edge, new insights into the origin of the colouring mechanisms of both pigments could be obtained from the precision of the Cu speciation. In Egyptian blue, Cu2+ is mainly allocated in a square-planar site in a crystalline cuprorivaite phase, whereas in Egyptian green, Cu2+ is basically situated in a distorted octahedral site in an amorphous phase. Copyright (c) 2006 John Wiley & Sons, Ltd.