Tin-glazing - The Nature of Tin Glaze

The Nature of Tin Glaze

For glaze use only one tin compound, tin (IV) oxide Tin dioxide (SnO₂), and also called stannic acid, is commercially exploited. Opacity is produced in glazes by the addition of some substance to scatter and reflect some of the incident light.

The opacity of glaze could be determined by the particles which spread through the glaze, therefore the light is absorbed by the particles, being scattered back before reaching the ceramic body, leading to the opaque glaze. As a result, the concentration of the absorbing or scattering particles in the glaze could determine the degree of opacification. Generally speaking, the more different the refractive index between the particles and the glaze matrix, the larger the opacity. Similarly, the closer the particle size to the light wavelength (100-1000 nm for visible light) and the more irregular the surface, the larger the degree of opacification.

Tin oxide remains in suspension in vitreous matrix of the fired glazes, and, with its high refractive index being sufficiently different from the matrix, light is scattered, and hence increases the opacity of the glaze. The degree of dissolution increases with the firing temperature, and hence the extent of opacity diminishes. Although dependant on the other constituents the solubility of tin oxide in glaze melts is generally low. Its solubility is increased by Na₂O, K₂O and B₂O₃, and reduced by CaO, BaO, ZnO, Al₂O₃, and to a limited extent PbO.

Some research on medieval tin glaze has shown that the particle size of tin oxide which appears as cassiterite is around several hundred nanometer, which corresponds to the range of wavelength of visible light. In some cases, the tin oxide is presented not only as small crystals but also as aggregates of particles. These factors – the high refractive index, the low solubility in glazes and the particle size make tin oxide an excellent opacifier.

In the beginning of the use of tin oxide, it is mainly viewed as a slip layer between the glaze and ceramic body. This could be seen from the SEM photomicrographs of some earlier Islamic glazed ceramics, of which the particles of tin oxide are concentrated at the interface, together with the existence of wollastonite, diopside and air bubble as other opacifiers. The microanalysis of later tin glazes reveals the distribution of tin oxide through the glazes rather than just at the interface, which indicates that tin oxide is really acting as an opacifier instead of only a surface coating layer.

Lead is usually brought into the glazes with tin oxide. The reaction between lead and tin oxide results in the recrystallisation of tin oxide, and thus enhances the degree of opacification in tin-opacified glazes than in tin-opacified glass. A high PbO/SnO₂ ratio is often found in ancient glazes. During the firing process, lead oxide react with quartz at approximately 550℃ to form PbSiO₃, which then reacts with tin oxide to produce lead-tin oxide (PbSnO₃) at a temperature higher than 600℃. After the formation of lead-tin oxide, the melting of PbSiO₃, PbO and PbSnO₃ occurs at the temperature in the range of 700℃ to 750℃, resulting in the dissolution of PbSnO₃ to SnO₂. The degree of the crystallisation of SnO₂ increases with the increasing of temperature. During either heating or cooling, the recrystallisation is taken place until the supply of tin is exhausted. In the second heating, lead in the form of lead oxide no longer reacts with tin oxide to form lead silicate, thus the recrystallised cassiterite (SnO2) remain undissolved and precipitate in the glazes. The nucleation and growth rates of the precipitation depend upon temperature and time. The particle size of the cassiterite developed is also dependent on the temperature, and smaller than that used in the very beginning. It is the smaller particle size of the recrystallised SnO₂ in glazes that increases the opacity in tin-opacified glazes. Besides the increasing the opacity, the high lead oxide to tin oxide ratio also reduces the melting point of glazes, lead to a lower firing temperature during production.