Calcia Matte

Calcium oxide acts as a flux and, in excess, tends to crystallize as the glaze cools. These crystallites scatter light and create a matte finish. Ca mattes form from phase separation and crystal growth (anorthite, wollastonite) during cooling, rather than surface microtexture alone.

Matte but sometimes with a softer, less velvety surface than Mg mattes.
• Often shows subtle crystal formations or a “dusty” matte look depending on cooling and silica/alumina ratios.
• Smooth and matte — but not as silky or buttery as Mg mattes.
• Can be more finely textured matte than gloss but usually not velvety.
• Relatively neutral on color — doesn’t push or pull color hues strongly. Can bleach or soften colors, especially with cobalt (pastel tones)

The characteristic soft satin surface can be explained in simple physical terms: A micro-wrinkle surface forms from a high viscosity, elastic melt that is too viscous and elastic to fully level on freezing. But there are additional mechanisms at play. A microstructure also develops while the melt is stiffening. That process needs time in the glass transition zone (~900–700 °C). Slow cooling stretches that zone out. Submicroscopic phase separation occurs as the glaze cools, the melt becomes less chemically comfortable holding MgO evenly dissolved in the silica-rich glass and it begins to separate into Mg-rich and an Si-rich glassy phases. These have different refractive indices and viscosities, light hits the boundaries and scatters. Fast cooling, by contrast, freezes the melt before this separation develops..

The minimum MgO level in the unity formula is typically 0.3 accompanied by high Al₂O₃ and a low Si:Al ratio (assuming slow cooling). For matte surfaces with faster cooling, MgO may need to be as high as 0.4 (and SiO₂ lower).

At lower temperatures there is also recipe level matte mechanism with MgO. Talc, dolomite and magnesium carbonate are all refractory, their resistance to dissolving in the glaze melt can stiffen it and produce a matte surface (although not normally silky). This type of glaze falls outside of this discussion.

At stoneware temperatures, it can be tricky to produce a functional magnesia matte that resists cutlery marking, staining and leaching (one reason why my G2571A recipe is popular). The first challenge is that a viscous melt is a requirement so flux levels must be lower, this introduces the possibility of inadequate melting. Second, lower MgO and higher SiO₂ favours better functionality; the more firings can be control-cooled the more these are enabled. Getting a specific surface becomes a question of whether to adapt the firing to the recipe, or the recipe to the firing.

Related Information

Calcia vs Magnesia matte - Different mechanisms

This picture has its own page with more detail, click here to see it.

This melt flow test was done at cone 6 to demonstrate the difference in melt viscosity between a calcia matte (left) and a magnesia matte (right). In simplest terms, the former depends on a fluid melt to provide the needed mobility for tiny crystals to form during cooling, those crystals scatter the light and soften the surface to give the matte effect. The latter requires a stiffer melt to help prevent leveling during cooling and host phase separation to produce a surface that scatters light.

By Tony Hansen
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