Glazes did not have to be transparent, and Islamic potters made efforts to make their glazes opaque (or opacified) by adding quantities of tin. Opacity in a glaze means that particles are dispersed through the glaze which absorb light and/or scatter 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).
Previously the technique of opacification was thought to derive from pre-Islamic Sassanian glazing techniques but, again, Masons analyses have shown that the Islamic potters of the C9th developed new techniques according to their own ingenuity and spirit of innovation. As with the stonepaste bodies described above, there is a large element of experimentation in the development of the new opacifiers.
Mason findings show that Sassanian ceramics with opaque glazes generally contain no tin in the glazes; instead they make use of other opacifiers such as gas or air bubbles, undissolved quartz and feldspar, or crystals which have formed during firing. These opacifiers are suspended in an alkaline-lime glaze, ie. a glaze containing quantities of soda, potash, lime, and magnesium.
Islamic opacifiers were first developed in Iraq and did continue this earlier tradition, but with the significant addition of particles of tin oxide. At first (Masons speculated dates are c.700-750 AD) these were concentrated in a slip layer at the interface between the body and the glaze, with very few particles of tine oxide actually dispersed through the glaze. Furthermore, the glaze remained of the alkaline-lime type, with very small amounts of lead added (c.1%) to complement the presence of the tin. The development of the technique sees gradually more particles of tin dispersed through the main glaze, and a gradually increasing amount of lead in the alkaline glaze (c.750-800). With the emergence and rapid dominance of lustre (c.800), tin is usually the only opacifier and the lead content is as much as 6-8%. Mason also shows that this development migrates with the potters and the experimental stonepaste body to Egypt c. 975, where the technique reaches maturity.
This experimentation shows that Islamic potters had a considerable amount of skill when it came to chemistry and ingenuity at achieving what they required to match the market for Chinese porcelains that were reaching the Islamic world in increased numbers with the development of long-haul trade (Go to section on Abbasid Ceramics: C8th-C10th). Abbasid Iraq was an entrepôt for science and the arts as craftsmen of all kinds were attracted to the patronage of the court, and thus the best minds were working together on the development of these ceramic techniques.