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.

Related Pictures

Is inside-only glazing a good idea? We say it is not.

$Glaze compression fractures porcelain mug$

This mug is made from 325 mesh MNP, the strongest porcelain I have. Since the walls are of even thickness with no abrupt corners or contour changes and the glaze is thinly and evenly applied I thought I could follow a social-media-driven trend and glaze only on the inside. But I got glaze compression time-bombs waiting for hot coffee triggering! Three other mugs failed this same way! But four with this same glaze inside and out were fine. Why? The outside glaze counters the inside one pushing outward. And it closes crack initiation points.

I got lots of pushback on social media saying glaze compression problems are overblown. But I also got stories and pictures much worse than this (especially with thick and drippy glazes). But, some still feel that inside-only glazing can work by carefully tuning the thermal expansion fit between body and glaze. Or even by accident. Either way, there is still an elephant in the room: Glaze fit has to be just right - too much and pieces break, too little and the glaze crazes. That is a problem because it brings intolerance of even slight changes in body, glaze or firing.

Of course, by making thick-walled ware having a glaze thickness is not excessive, you may still get away with doing everything just advised against! But think about this: This coffee could have ended up in my lap rather than on my paper?

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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