|Monthly Tech-Tip |
Refers to a group of ceramic fluxing oxides that contribute similar properties to fired glazes. They contrast with the alkalis which are stronger fluxes.
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.
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
|Oxides||BaO - Barium Oxide, Baria|
|Oxides||CaO - Calcium Oxide, Calcia|
|Oxides||MgO - Magnesium Oxide, Magnesia|
|Oxides||SrO - Strontium Oxide, Strontia|