The silica-alumina ratio of a glaze or clay body is the ratio of the number of silica (SiO2) molecules to the number of alumina (Al2O3) molecules. This ratio is important as it determines the behavior and appearance of clays and glazes.
The glossiness of a glaze is determined by its silica-alumina ratio. The highest gloss is achieved by a ratio between 7 and 9. Both high silica and high alumina increase matteness, but the results have different textures. However, glazes can be glossy or matte for other reasons.
Glazes may phase separate and form microscopic droplets which alter the color. From my research in Oribe (Oribe Development) and Jun (Jun Development) glazes, phosphorus-based phase separation requires a silica-alumina ratio of at least 10.
In Soda Firing, high alumina ceramics resist interaction with the vaporized sodium. For clay bodies and slips, Gail Nicols recommends a silica alumina ratio of 3 in her book Soda, Clay and Fire. These High Alumina Clay slips and bodies absorb enough sodium to seal the surface and develop Alkali Flashing, yet resist the soda enough to avoid heavy build up of glass.
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