Structural investigations of thin lustre layers developed by ion-exchange mechanism on lead alkali glazes

Abstract

Unique ceramic objects with a range of rich under glaze decoration were produced between the 15th and the 17th century in the times of Otttoman Empire in İznik. These ceramic objects had high quartz content in their structure and they also went through difficult processing steps. Traditional Iznik tiles contain four different layers known as: body, engobe, decor and glaze. Typically Iznik ceramic bodies were produced from a mixture of quartz, clay and frit. Iznik glazes contain both lead and alkali fluxes in their structure and they are very pure containing little amount of potash, lime, alumina, magnesia and iron oxide. These glazes are more distinct than other glazes because they are transparent enough to reveal the underglaze decoration. Lustre was first produced in early Islamic times during the 9th century AD in Iraq. This decoration spread throughtout the Mediterranean basin together with the expansion of the Islamic culture. In the 10th century the technique used in Egypt. Between the 11th and the 12th century potteries with lustre decoration were produced in Syria. During the 14th and the 15th century lustre technique was used in Spanish ceramics and this technique reached its maximum development. Between the 15th and the 16th century lustre was used to decorate the polychrome pottery of the Italian Renaissance. Lustre is a metal-glass nanocomposite thin layer (100 nm to 1 µm) with 2-50 nm sized metallic silver and/or copper nanoparticles in its structure. These metallic nanoparticles give metallic shine (golden, coppery, bluish, purplish, pinkish) and coloured iridescence to the lustred object. The most striking property of thin lustre layer is to reflect light like a continuous metallic surface. Several different lustred objects were produced during the medieval times. Physical and chemical properties of lustred objects vary according to the region of production. Lead glazes, high alkali glazes, mixed alkali glazes were used in ancient times for ceramic glazing. Different lustre paste compositions were applied over the ceramic objects with these different glazes. Ion-exchange mechanism is responsible to obtain a thin lustre layer over glazed ceramic objects. During ion-exchange mechanism, silver and/or copper ions (Ag+ and/or Cu+) diffuse into the glaze from the lustre paste. On the other side, sodium and/or potassium ions (Na+ and/or K+) diffuse out from the glaze. Following that mechanism, a reducing atmosphere is introduced in the furnace and the silver and copper ions (Ag+ and Cu+) are reduced to their metallic state. Since the decor layer in Iznik tiles has its own colour, there is no need and demand for lustre decoration over Iznik tile surfaces. In this dissertation, lustre technique was applied over reproduced Iznik tile surfaces without the decor layer in laboratory-controlled conditions and advanced characterization techniques were carried out on the lustred samples. Firstly, the physical and chemical compositions of the raw materials that compose the body, engobe and two different glaze layers were determined. The production of Iznik tiles involves grinding of raw materials, preparation of the body mixture, shaping the body in the form of plates using dry press and drying, preparation and mixing the engobe, applying the engobe mixture to the dried body, firing of the body and engobe layer together (bisque firing), preparation and mixing the glaze, glazing the previously fired body and glost firing steps. Glazes which were used in this dissertation comprise different amounts of alkalis and lead (as PbO) as 30.5 wt% and 63.2 wt%), respectively. To obtain thin lustre layers by ion-exchange mechanism on the surfaces of the Iznik tiles, six different lustre pastes consisting of silver compounds (silver sulphate, silver chloride, silver carbonate and silver nitrate) and copper compound (copper sulphate) in a clay carrier media (kaolinite and ochre) with some amount of water were applied by brushing. Kaolinite and ochre were calcined to remove molecular bound water and ground before their addition to the lustre paste. After application of lustre pastes over glazes, lustre firing was performed at the different firing temperatures. The furnace was heated slowly to previosly determined temperatures, where the glaze partly softened. During cooling down strong reducing atmosphere was introduced into the furnace. The effects of glaze compositions, lustre paste compositions and lustre firing conditions were investigated using several advanced characterization techniques. The characterization of the thin lustre layers were carried out using scanning electron microscopy, wavelength dispersive X-ray fluorescence, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, high resolution transmission electron microscopy-electron dispersive spectrometry and Raman spectroscopy. Glaze composition, lustre firing regimes, lustre paste compositions are important process parameters to obtain thin lustre layers. By changing these parameters physical and chemical properties of the final lustre layer also change. In other words, process parameters effect formation and final state of the nanoparticles in the thin lustre layer. Changing the process parameters in a controlled manner provides to obtain thin lustre layers with diverse physical, chemical and optical properties. To investigate the lustre firing regime effect on the thin lustre layer formation, different lustre paste compositions and firing regimes were used. Since reducing silver is easier than copper in the reducing atmosphere during the lustre firing, silver was found in metallic state but copper was dissolved in the glaze in ionic form. In addition, cuprite phase was observed. Increasing the lustre firing temperature caused increase in nanoparticle sizes. Nanoparticle sizes range between 3-5 nm and 7-9 nm in low firing temperature and 5-8 nm and 9-11 nm in high firing temperature. In addition, using different lustre paste compositions on same glaze effected the nanoparticle sizes even the same lustre firing regime. In case of using kaolinite in the lustre paste resulted forming smaller silver nanoparticles in both firing regimes. Applying different lustre pastes over a same high lead content glaze resulted in the formation of metallic silver nanoparticles with different crystallites. Investigations after lustre firing process revealed silver chloride in the lustre paste which caused higher crystallinity than silver carbonate. Although metallic silver was observed in both samples, copper was in ionic and oxide form and dispersed in the glaze. In addition, the chemical composition of the lustre pastes effected the final colour of the thin lustre layer. Silver chloride in lustre paste caused golden colour. On the other hand, silver carbonate caused purple colour with golden spots. Increase in nanoparticle size was observed in purple coloured sample over same glaze composition due to using different lustre paste composition. Silver nanoparticle radius changed with the silver source of the lustre paste. Silver carbonate in the lustre paste caused formation of the bigger nanoparticles than silver chloride. Silver nanoparticle sizes were found approximately 6 nm and 12 nm. Raman investigations of the lustred samples were carried out to compare the reproduced tiles and traditional Iznik ceramics and to observe the silicate network modification. Peaks obtained in the reproduced tiles with lustre decoration were similar to the traditional tiles. However, the peak intensities of the reproduced samples with lustre decoration were lower than traditional tiles. Lustre layer with metallic nanoparticles in its structure on the surface of the glazes caused decline of the Raman peak intensities of the investigated samples.