Wollastonite is considered the preferred raw material source for calcium oxide in glazes or in glaze frits and usage of wollastonite in ceramic bodies can amount to as much as 40% or more. Rheological clay additives Additives for Glazes and frits E-SPHERES are an extremely consistent refractory product with a wide variety of uses in heat and flame resistant products and applications. They are extensively used in lightweight insulating refractory castables, for both casting and gunning and are also used in bricks and blocks for various kiln insulating applications and in chimney stack and flue linings.