|Monthly Tech-Tip |
In ceramic glaze calculation, a 'system' refers to a collection of glaze recipes that share a common set of oxides and material types (e.g. cone 10 dolomite mattes, cone 06 fritted boron glossies, cone 6 alumina matte, cone 8-10 crystalline) and preparation, application and firing methods. Also, a 'system' implies or states limits for each oxide beyond which unpredictable results are likely to occur. Ways of relating the oxide formulas of glazes to their physical fired results also imply confinement to a system.
For example, expansion calculations are relative within systems. For example, if you have a dolomite, whiting, feldspar, kaolin, silica glaze and you try a bunch of variations, the calculated expansions will give you an indication of which variations have higher and lower expansions. But if you introduce lithium carbonate, or boron frit, or zinc, for example, now you have a different system. Also, some oxides, like Li2O do not impose their expansions in a linear fashion, thus they compromise a system because they do not calculate as well. Another critical factor is melting: If a glaze is not completely melted the expansion calculation is invalid, the glaze is not within the system. A third factor: Crystalization: When a glass crystallizes its physical properties are different. A fourth factor: Non melting particles, like zircon, impose their expansions in a manner different than if they melt and participate in the glass chemistry.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
The melting temperature of ceramic glazes is a product of many complex factors. The manner of melting can be a slow softening or a sudden liquifying.
Desktop Insight 1A - Compare Theoretical and Real-World Feldspars
While comparing a real-world and theoretical feldspar learn to enter, edit, save, normalize recipes and the materials dialog. Glaze chemistry concepts.
|By Tony Hansen|
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