Monday, 17 November 2014

Opacification of Glazes

Glaze opacifiers are materials which when added to a glaze change the level of transparency of the glaze. Glazes are often described as clear/transparent or opaque. In order to achieve color tone and hide the body color it is often necessary to add opacifier particles which stay discrete in the glaze after firing. Materials which have a significant difference in refractive index from the parent glaze produce the most effective opacification. To the right you can see the effect of adding an opacifier to a transparent glaze. It becomes whiter and milky (opacified ) in nature and flows less.

Below you can see the effect of adding opacifier to a transparent glaze containing different colored pigments

Top = transparent glaze            Bottom = opaque glaze

Typical glaze opacifiers are zircon (zirconium silicate), tin oxide, and titania (titanium dioxide). 

Zircon opacifier powder


Zircon is the preferred opacifier for glaze due to the low cost, inertness and stability in glaze. To achieve full opacification, the opacifier content and the particle size are important factors. For addition of zircon to the glaze mill during grinding a particle size of 95% less than 3 microns and a content of 5-10% ensures sufficient opacifier is dispersed to create optimum opacity.  

However an exceptional level of opacity can be achieved by pre-melting the zircon into a glass (called frit) and then using the frit as part of the glaze recipe. These frits are transparent before firing but crystallize zircon during the glaze firing process to give a high level of opacity. 

Tin Oxide

Tin oxide has historically been used as a glaze opacifier but its high cost has limited its recent use to low temperature majolica or special effect glazes. At a level of 5% in a transparent glaze a high level of opacity can be achieved. Its lower solubility in glaze compared to zircon means that lower levels of tin oxide can be used to create the same level of opacity as zircon.. Tin gives a slightly blue white tinge and also has a lesser effect on the glaze appearance than zircon which increases glaze viscosity during dissolution.One major drawback of tin is it reactivity with some oxide pigments. For example, with chrome oxide a pink discoloration may result from the formation of a chrome-tin spinel crystal.
Titania reactive glazes


Titania is also a very costly opacifier and is used primarily where reactive special effect glazes are required. Like zircon it has a higher solubility than tin in the glaze and even at levels of 4%  tends to give a yellow tinge to the glaze after firing. It readily reacts with other materials in the glaze to create many unusual crystalline phases on cooling.This is ideal for special effect glazes but undesirable for standard opaque glazes.

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