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

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 outside-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, if you make thick-walled ware and the glaze thickness is not really crazy, then you can probably get away with doing everything that I have advised against above.

$Glaze compression fractures porcelain mug$

Related Pictures

A mug cracks before your eyes because of glaze compression

The glaze on this highly vitreous, thin-walled mug is normally perfect, it is under enough thermal compression to really increase ware strength. But, since this mug is glazed inside only, the compression is too great. While it looks OK, the glaze is constantly pressing outward, looking for relief. Watch as a tap with a spoon is enough to trigger a sudden crack. And it opens under the pressure, clearly revealing the piece was spring loaded. This is a simple test. A typical mug of this clay would survive hundreds impacts of this nature. Further, this did not happen just because it was not glazed on the outside. A mug with glaze under compression on the inside and under tension on the outside would fail this test even more dramatically.

Here is how far some potters are taking drip glazing

Glaze compression is fracturing these pieces. The maker of these had been doing it for some time with success but a change in the clay body recipe started to cause problems. These are his experiments to formulate a body/glaze combination that could survive in spite of the glaze being almost as thick as the clay in some sections.

The handle is strong, the mug is weak. Learn why.

A broken mug, lying in pieces, but the handle is intact

This body vitrifies to incredible strength on its own. But applying a glaze complicates that. The inside glaze, G2926B, is under COE glaze compression. The outside glaze (which I neglected to sieve) only covers about half of the surface. Another mug just like this, glazed inside and out with the same glaze, required a dozen hammer whacks to break (it sounded like a piece of steel on impact)! But the lower section of this broke off with the first impact and the other on the second. But notice that the entirely glazed handle has not broken. Notice wall inside the handle has separated latterly - the the handle is holding fast! This is a testament to how much the glaze strengthens the handle (by its full coverage) and how much it weakens the body (by partial coverage).

Drip glazing and bare outsides: Deceptively difficult.

Why? Glaze fit. Do these yourself and they might end up being glaze compression demonstration pieces. These are available on Aliexpress (as Drip Pottery, Drippy Pottery or Goopy Glazes) and they are made by a manufacturer that has close control of body maturity (and thus strength) and the capability to tune the thermal expansion fit of glaze-on-body. Glaze fit has to be better than normal because of the absence of an outside glaze. Too low an expansion and the compression (outward pressure) will fracture body (especially for thin-walled pieces). Too high and it will craze. And the glaze is thick, it will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They likely deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. And applied the glaze to preheated ware. If done right these pieces are a visual and technical achievement. However hobbyists, for example, often just brush multiple layers of commercial glaze that only by accident fits the body they are using. No wonder their pieces often end up as time bombs or crazed bacteria farms.

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.

Links

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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