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The oldest known nanotechnology dates back to the 9th century !

March 22, 2004

The oldest known nanotechnology dates back to the fabrication of the first lustre potteries. Some Abbasid lustre ceramics have a complex and fine decoration and form nano-gratings, the so-called polychrome lustre in which multi-coloured iridescence can be present: a famous example are the Abbasid tiles imported from Syria and placed in the mihrab of the Sidi Oqba Mosque in Kairouan (Tunisia).

The Islamic production technology spread to Egypt, Persia, Andalousia during medieval time (>12th), following the expansion of Islam Kingdoms, and then to Italy (>15th), but the polychrome lustre technology was lost. Lustre preparation was described in the 14th century by Abu al Q'¢sem and in 1557 by Cipriano Piccolpasso in the second book of "Li tre libri dell'arte del vasaio".

When a lustre pottery is illuminated with white light, the colour changes with relative angles between the light source, the pottery surface and the observer, like with opals, butterfly wings or Compact Disks, all materials made from a more or less periodic array of particles with specific optical properties. The regularity of the array is required for a good selection of the "observed" colour. As a matter of fact, any wavelength satisfying the Bragg's equation, will be strongly diffracted and will give a colour in reflection.

On viewing the lustre glaze normally and then moving towards grazing incidence the colour will appear to change toward shorter wavelengths due to the sinq term in Bragg's equation. The nicest Abbasid polychrome lustres offer a colour palette going from blue to red. Because of the imprecision in the precipitate shape and distance between metal particles, the actual range of colour is reduced, as observed in opals showing red to green when tilted, or only one colour, like for monochrome lustre.

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. Rare lustre ware excavated from Egypt/Syria (11-12th centuries) and the Silk Road (14th centuries) have been studied for the first time by Raman scattering, a non-destructive technique. The study shows they associate many layers of different compositions (with or without cassiterite (tin oxide)).

EDS analysis shows 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 Bagdad, 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 form of cassiterite (SnO2) precipitate but as a Ca,K-rich salt. Composition analysis and Raman spectra show all glazes have been processed with rather similar technology.

Distribution of Ag and Cu element 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° (or (Agn)m+ ) nanoclusters 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 evidences and ancient potters reports. Exothermic burning of acetate residus is proposed as the key-step for the preparation of polychrome lustre.
From the old potters reports it is well established that the lustre is obtained by putting a mixture of clay and ochre, silver and copper salts (sulphides and oxides), vinegar and leas in definite places for decoration control of an already lead oxide glazed pottery. The whole system is then heated to dark red (~ 500-600°C) and cooled in a controlled atmosphere.

The mean experimental evidences are: the dispersion of Ag and Cu elements in the near surface glaze and the higher refractarity of the very surface film (less PbO, less SiO2, more Al2O3). Lead oxide is easily dissolved by acetic acid at 80-100°C and, thus, the roles of vinegar and of clay/ochre are clear.

The later medium hinders fast evaporation of acetic acid, allowing a partial dissolution of lead oxide-based glaze. The near surface region of the glaze becomes porous. Some lead acetate is formed and deposited at the surface. Then, silver and copper salts melt on progressive heating and enter the meso-porosity formed by action of acetic acid, and diffuse to some extent in the glass network.

Due to the reducing atmosphere, silver and copper salts are partly reduced. Acetates are rather thermally stable and temperatures of ~ 200-300°C are required to decompose them in carbon-rich residues. But, if a sufficient amount of oxygen is present a glowing phenomenon can be observed: acetates residues burn and the surface temperature of the ceramic can reach 1100°C or more. Such a phenomenon could explain the special composition of the lustre surface: less PbO because its evaporation is very active above 900°C, more Al2O3 because SiO losses are also possible, no metal precipitates because of surface oxidation.

The composition of the mixture (more or less vinegar and leas, Ag/Cu content) controls the surface temperature and embedded metal dispersion and allows one to set the final lustre colour in different places of the same item. The strong temperature gradient arising from the surface residues combustion explains why precipitates located just below the glaze surface have a larger size.

CNRS Institut des Sciences Chimiques Seine-Amont