Brand : REFSAN
Price : 1,27€


(60-65 % Zr)

Zirconium silicate (or zircon) is extremely stable (refractory, hard, dense). Large quantities of zircon are used by the tile, sanitaryware and tableware industries.

Individual zircon particles are angular, very hard and refractory and amazingly, they do not readily dissolve into glaze melts even when ball milled to exceedingly small particle sizes. Zircon is the generic name for zirconium silicate, the trade names are different (for example, Zircopax, Zircosil). The refractive index of zircon is high (particularly with micronized zircon, size less than 5 microns). Notwithstanding this, some sources list Zircon as a source of SiO2 in glazes (meaning that it does decompose). This view is plausible since smaller amounts of zircon do not opacify glazes, in fact, they are purposely added to raise refractive index to encourage transparency and high gloss. That means it dissolves when percentages are low and precipitates to opacify when they are higher. Interestingly, it dissolves so well at low percentages that are is sometimes added to clear glazes to make them more transparent (because it has a high refractive index).

Zircon is normally used in glazes for opacification (converting a transparent glaze to an opaque). The silicate form of zirconium does not matte glazes (like pure zirconium oxide does). The exact amount needed varies between different glaze types. 10-12% is normal, but up to 20% may be required to opacify some transparent glazes. When the saturation point is achieved crystallization begins to occur. It is most effective at low temperatures. Tin oxide can be a more effective opacifier than zircon (it has various advantages and disadvantages).

High amounts of zircon opacifier can cause cutlery marking (because of abrading angular micro-particles projecting from the glaze surface).

As a glaze opacifier, the white color produced by zirconium silicate is often characterized as 'toilet bowl white'. Tin oxide, by contrast, can produce more of a blue-white, but tin is subject to alteration of the color (toward pink) if there is any chrome in the kiln atmosphere (zircon is unaffected by this). If the shade of white is too harsh, it can be toned by shifting part of the opacification burden to tin or by adding a tiny amount of stain (e.g. blue, brown, grey).