If the potter so desires, he can apply a glaze once the vessel is sun-dried. Sometimes the vessel undergoes a first ("biscuit") firing, usually at temperatures around 900-1000°C, before the glaze is applied, as this makes the vessel stronger for dipping into the glaze and the body is still porous enough for the glaze to adhere. This also means that all the shrinkage of the vessel has taken place, and helps the "fit" of the glaze to the body: if the glaze contracts slightly after cooling, this compresses and strengthens the body.
The glaze may "puddle" in the well of the vessel, indicating that the vessel was upright during firing; alternatively, drips standing up around the rim of a pot show that in was face-down during firing. "Crawling" is another effect which shows on the vessel, where the glaze pulls away from certain areas on the surface of the pot for some reason, leaving spots where the surface is unglazed.
A glaze is a glass which melts and fuses to the vessel during firing at high temperatures; as it cools it solidifies into a glassy coating which may be opaque or transparent depending on the ingredients. If the glaze and body material share ingredients (such as quartz) then the fusion during firing is stronger, as the shared properties (quartz) fuse together.
A "flux" is employed to lower the temperature at which the glaze melts, thereby necessitating less fuel in the firing process. The flux is usually a metallic oxide, and glazes are usually called after the flux employed: eg. lead-glaze, alkaline-glaze, or salt-glaze. Some glazes are more appropriate for firing at high or low temperatures: alkaline and lead glazes are low-fired glazes, whereas porcelains are fired at very high temperatures until the clay body vitrifies and itself becomes a glass. Certain ingredients also give off toxic gases so the craftsman also needs to know when to keep away.
|D.5.b.||Preparation of glaze-frits:|
The glaze-frit is the pre-prepared raw material for a glaze, a mixture of two or more materials which have been fused together by heating and then cooled rapidly into solid granules. These can be stored for as long as necessary. To make the glaze the granules are ground into a powder and dissolved in water or a weak acid such as vinegar, and applied to the vessel through dipping or painting. Abul-Qasim describes the process in Kashan for making glaze-frits for tin glazes.
Glazes did not have to be transparent, and Islamic potters made efforts to make their glazes opaque (or opacified) by adding quantities of tin oxide. Opacity in a glaze means that particles are dispersed through the glaze, absorbing light and/or scattering it back towards the surface before it reaches the underlying body. Tin oxide makes an excellent opacifier as it is highly refractive (ie. deflects light), has small particle size and low solubility. Thus the potters hoped to imitate the opaque white bodies of Chinese porcelain vessels, which were formed through no such additions, but the pure white kaolin clay used by the Chinese was fired at such high temperatures that it vitrified (turned into glass) and this gave the fine porcelain its characteristics of translucency and resonance (a porcelain bowl will ring when you tap it).