Copper and silver nanocrystals in lustre lead glazes: Development and optical properties

In the early 9th century AD ancient potters of Iraq discovered that after firing some copper oxides and silver salts with clay, iron oxides and some sulphur compounds applied on a ceramic glaze produced a beautiful layer with a wide range of colours, from reddish to yellowish or even greenish, and some with a characteristic metallic copper or purplish shine. Modern studies of these layers showed that they are formed by nanocrystals of copper and silver embedded in a glass matrix. Some attempts have been performed to understand ancient lustre coloration and characteristic gloss but have failed to give a clear correlation between chemical composition and colour, and generally make some assumptions on the shape and the size of the nanoparticles and the lustre nanostructure. The aim of this paper is to establish a basis for understanding lustre nanostructure linked to its optical properties from a sequence of lustre reproductions on traditional lead glazed tiles. These modern lustre decorations have been studied by means of optical microscopy, transmission electron microscopy and electron energy loss spectroscopy, UV–vis spectroscopy, low irradiation angle X-ray diffraction, synchrotron radiation X-ray diffraction and electron microprobe analysis. These results show that changes in the lustre nanostructure affect the glaze colour and shine during the lustre formation process. Lustre nanostructure showed crystal size range as a function of depth, that subsequently disappeared followed by an increase of nanoparticles mean diameter and reduction of the interparticle distances. Consequently, the dipole plasmon coupling between copper nanoparticles appeared, and seems to be responsible for the metallic shine and copper metal like coloration of the copper lustre. However, colour from the glaze surface differs when calculated for diffuse or reflected light. Diffuse coloration appears strongly affected by the copper nanocrystals, while specular coloration is not only affected by copper but also by the presence of an inhomogeneous distribution of silver nanocrystals which gives the lustre a characteristic purplish shine.