Calculated Thermal Expansion
INSIGHT software calculates the thermal expansion of a glaze and part of its chemistry. The number it reports is based on the contributing expansion factors and amounts of each oxide in the formula. Results are determined by the set of expansion numbers and method of additive calculation method chosen (based on formula or mole%). Thermal expansion values predicted by calculation are relative (not absolute) and apply within 'systems'. Thus, if a glaze calculates to a higher expansion than another, and is in the same system, then it is more likely to craze. Conversely, so solve a crazing problem, move the expansion number downward.
Another factor is the homogeneity of the material. Frits, for example, compared to raw materials, have glass particles of the same chemistry, thus every particle is going to do something predicable during melting. Raw materials, on the other hand, have particles of possibly a dozen different minerals, each having it's own complex melting behavior that is a product of it's mineralogy as well as it's chemistry and particle size and shape. In addition, these particles interact in complex ways. Thus, the calculated thermal expansion of some materials may not be accurate.
The thermal expansion of bodies cannot be calculated either because there are too many factors other than chemistry that impact the thermal expansion of a clay body. The most fundamental in that clay bodies do not melt like glazes, the oxides do not form a glass; thermal expansion calculations depend completely on this assumption. Expansion calculations also depend on the assumption that no crystallization is taking place (once a glass crystallizes is thermal expansion will change completely, clay bodies are loaded with crystallization). Particle size distribution, mineralogy of the particles, the degree to which the body is fired are other factors that affect expansion. For example, while the SiO2 content may be similar in two bodies of similar chemistry one may have most of the SiO2 in quartz grains and the other might have it in feldspar and kaolin. These will have vastly different thermal expansions.
Real-world expansion numbers are extremely small and refer to the amount by which an item expands per degree rise in temperature. INSIGHT removes the decimal to produce a simple number that generally falls between 5 and 8 (for the default expansion number set). This number is relative only. Thus if a glaze is crazing you need to adjust the formula to bring the expansion number down. If it is shivering you do the opposite. The amount by which you change it comes with the experience of seeing a fired result and comparing it with the degree of change in the chemistry. Different glazes systems behave differently in the degree to which they respond to chemistry changes. For example, if a glaze is crazing badly and the calculated thermal expansion is 8.0, then try effecting a chemistry change that will bring that number down to 7.5, test and see whether it needs to be reduced further or less. Out Bound Links
In Bound Links
- (Glossary)
Glaze Compression
Every solid has a thermal expansion, that is, an a... - (Glossary)
SiB:Al Ratio
This number is reported by INSIGHT software as par... - (Glossary)
Si:Al Ratio
This number is reported by INSIGHT software as par...
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