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These glaze cones are fired at cone 6 (without slow cooling) and have the same recipe: 20 Frit 3134, 21 EP Kaolin, 27 calcium carbonate, 32 silica. The difference: The one on the left uses dolomite instead of calcium carbonate. Notice how the MgO from the dolomite mattes the surface (without slow cooling) whereas the CaO from the calcium carbonate produces a brilliant gloss (because it needs slow cooling).

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This melt flow test was done at cone 6+ (without slow cooling) 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 produce 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.
| Materials |
Calcium Carbonate
In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes. |
| Materials |
Dolomite
An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes. |
| Oxides | MgO - Magnesium Oxide, Magnesia |
| Oxides | CaO - Calcium Oxide, Calcia |
| Glossary |
Magnesia Matte
Magnesia matte ceramic glazes are “microstructure mattes” while calcia mattes are “crystal mattes”. They have a micro-wrinkle surface that forms from a high viscosity melt and microscopic phase separation, both of which prevent levelling on freezi |
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