Example of COE curves considered a good fit for body and glaze

These are from a sanitaryware plant in India. Long-term glaze fit is essential for their products. The glaze thus needs to be under some compression. That means the body must have a slightly higher coefficient of thermal expansion (COE) than the glaze. These two charts were created on the same dilatometer by the same person using well-defined procedures (the glaze and clay each have their own procedures). A history of measurements and associated knowledge of how the data relates to the quality of the fired products provides a context to interpret these reports. In other words, technicians have learned that the difference shown here is what is required to achieve optimal glaze fit for this specific body/glaze combination. Of course, some sort of database system (e.g. lab notebook, an account at insight-live.com) is needed to record the history of testing to be able to effectively compare the past with the present.

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Outside-only glazing success is just about the body being vitreous, right? Wrong.

$Glaze compression fractures porcelain mug$

This mug is made from the strongest porcelain I have, 325 mesh MNP. The walls are of even thickness, there are no abrupt corners or contour changes and the glaze is evenly applied at the right thickness. Since the clay surface does not coffee-stain it seemed like a natural to follow a social-media-driven trend of glazing only on the inside. But - I had created glaze compression time-bombs waiting for hot coffee! While three other mugs failed this same way, four that I fully glazed were fine. Why? Glazes need to have a lower thermal expansion than the body so they do not craze over time, so they are naturally under compression (this strengthens ware - like pre-stressed concrete). But here, there is no outside glaze to counteract the inside one pushing outward. When suddenly heated from inside the glaze comes under much more compression. Even though there are no structural weak points the stress is enough to blow it in two. This technique can work, for vitreous or non-vitreous bodies, by carefully tuning the thermal expansion fit between body and glaze (with lots of testing and lots of broken pieces). Wait - that might be wrong. The glaze, GA6-B, fits all our bodies really well, under enough compression that pieces will never craze. But too much compression for inside-only glazing. So doesn't this mean that for the glaze to fit for this purpose it would eventually have to craze?

Links

Glossary	Co-efficient of Thermal Expansion The co-efficient of thermal expansion of ceramic bodies and glazes determines how well they fit each other and their ability to survive sudden heating and cooling without cracking.
Glossary	Sanitary ware A type of porcelain zircon-glazed ceramic that includes bathtubs, sinks, toilets, etc.
Glossary	Glaze Compression In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.

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