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Alkaline Earths

Glaze chemistry models fired glazes as constructed of oxides decomposed from the materials in the recipe. Fired properties of glazes (like melting temperature, thermal expansion, surface character, even color) are a product of the oxide makeup (the chemistry). Oxides are grouped in various ways to simplify deriving relationships between the glaze chemistry and what happens when it is fired. This group is a good example.

The common alkaline earths are the fluxing oxides at medium and high kiln temperatures: MgO, CaO, SrO, BaO, ZnO. These are not as active as the alkalies but have much lower thermal expansions. They also tend to create matte glazes when adequate amounts are present and the host glaze chemistry has the right balance. Thus, it makes sense to substitute these one-for-another to maintain the general glaze character but fine tune a specific property that one favors over another.

While in a general sense, each of these oxides can produce matteness in glazes, each has their own mechanism that creates the visual phenomenon which can vary with the temperature range (eg. crystallization, surface ripples, multiple melt phases). Matte glazes are quite difficult to formulate, the oxide balance to create the effect can be fragile and fall to a gloss easily. Of course, each of these oxides can exist in a gloss glaze also and thus the stability of that matte effect varies.

Out Bound Links

  • (Oxides) SrO - Strontium Oxide, Strontia
  • (Oxides) MgO - Magnesium Oxide, Magnesia
  • (Oxides) CaO - Calcium Oxide, Calcia
  • (Glossary) Alkali

    Alkalies are the strong base fluxing oxides of Na2O and K2O. Feldspars are by far the most common alkali sourcing materials, thus the term 'alkali feldspars'. However frits are also an excellent source and sometimes the only alternative to meet the chemistry requirements (e.g. low alumina and high N...

  • (Oxides) BaO - Barium Oxide, Baria
  • (Glossary) Oxide

    An oxide is a combination of oxygen and another element. There are only about ten common oxides that we need to learn about (most glazes have half that number). CaO (a flux), SiO2 (a glass former) and Al2O3 (an intermediate) are examples of oxides. CaO (calcium oxide or calcia), for example, is cont...

  • (Glossary) Matte Glaze

    A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. I...

By Tony Hansen

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