Non-destructive Raman study of the glazing technique in lustre potteries and faience (9-14th centuries): silver ions, nanoclusters,, microstructure and processing

The oldest known nanotechnology dates back to the fabrication of the first lustre potteries. A lustre is a thin film formed just below the surface of medieval Islamic glazed potteries which contains silver and/or copper in the metallic and ionic form. Raman studies of the lustre films of different ceramics excavated from Fustat (near Cairo, Egypt, 11-12th century) or from the Silk Road (Termez, 13-14th centuries) showed that they associate many layers of different compositions (with or without cassiterite). Energy-dispersive spectroscopic analysis shows that all studied glazes are Ca- (and K)-rich, nearly free of Al silicates, with some addition of lead. Comparison is made with a copy of three-colour Tang ceramics made in Bassorah or Baghdad, in the 9th century, which is among the first known 'faiences', i.e. ceramics enamelled with an Sn-containing glaze. Surprisingly, Sn is not present in the form of a cassiterite (SnO2) precipitate but as a Ca,K-rich salt. Composition analysis and Raman spectra show that all glazes have been processed with similar technology. The distribution of elemental Ag and Cu is very heterogeneous in the lustre decor. The main Raman signature (50-100 cm(-1) peaks) of the lustre film is assigned to Ag+ ions. The additional low-wavenumber features could be due to the Ag-0 [or (Ag-n)(m+)] nanocluster modes. It is clear that the lustre colour arises from the combination of iridescence (diffraction) and absorption/diffusion. Raman criteria are proposed for a sample classification as a function of processing (cassiterite content, processing temperature). The glazing technique is discussed on the basis of experimental evidence and ancient potters' reports. Exothermic burning of acetate residus is proposed as the key step for the preparation of polychrome lustre. Copyright (C) 2004 John Wiley Sons, Ltd.