Library: Mineralogy vs. Chemistry
Materials of the same chemistry can have very different firing behavior. This can be attributed to differences in mineralogy and physical properties. Understanding that materials have a chemical, physical and mineralogical presence is a key to controlling all aspects of glazes.
Talc is an interesting example. At low temperatures it is refractory, it is often added to clay bodies to increase their thermal expansion (its ability to do this relates to the fact that it does not melt, but retains its mineral character). However talc is composed of SiO2 and MgO, both of which are low expansion oxides, yet together in the mineral talc the expansion is high. Talc will matte low temperature glazes, simply because it is stiffening the melt. Yet, at middle temperatures (e.g. cone 6) a small amount of talc (e.g. 2-4%) can greatly improve body maturity, several cones can be gained. However, if more talc is added, the effect is lost. Explaining such behavior involves interactions, mineralogy, chemistry and the physical presence of the material. At high temperatures, the talc does dissolve in the glaze melt liberating the MgO as a powerful flux that behaves as chemistry calculations suggest it should. At cone 8, plenty of boron is needed to coax talc to decompose. However, talc can also be fired to form cordierite crystals (thus a man-made mineral), cordierite is very refractory and used to make kiln furniture. Out Bound Links
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