All Glossary

Glaze Compression

In ceramics, glazes are under compression when they have a lower thermal expansion than the body. A little compression strengthens ware, too much can weaken and even fracture it.

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Details

Every solid has a coefficient of thermal expansion (COE), that is, an amount by which it expands and contracts on heating and cooling. COE is measured in thousandths of an inch but differences in body and glaze are significant because both are rigid and inseparable. If the thermal expansion of a glaze is excessive more than the body it is affixed to, to relieve the tension of being 'stretched on' it crazes (tiny cracks in the glaze). However, when the COE is significantly lower than the body the glaze can find itself under expressive compression. This condition is most commonly evident by shivering (flaking off on convex contours). But when a glaze is applied thick enough on convex surfaces it can fracture a piece.

This condition can be mistaken as dunting, but the two are completely different. This is caused by a thermal expansion mismatch between body and glaze, dunting occurs when thermal gradients are introduced as pieces cool too quickly in the firing (through quartz inversion they are unable to the differing thermal expansions of each).

A worst-case scenario for glaze compression is ware having a thickly-applied low-expansion glaze on the inside with a crazing glaze on the outside. Powerful internal stresses seek an opportunity for relief and the craze line cracks on the outside provide initiation points to almost assure failure - in vitreous or non-vitreous bodies. In severe cases, pieces will not survive cooling in the kiln without fracturing (individual shards of such pieces can explode into dozens of tinier pieces upon an impact). This type of failure is also very common in ware that is not glazed on the outside. In less severe cases, pieces can fail during use (e.g. when a hot liquid is poured in). In mild cases, ware is simply weaker or more brittle than it otherwise would be.

Although glaze fit can happen by accident it is better by design. The thermal expansion of glazes is a product of their chemistry and that is the ideal level on which to approach this challenge. At a minimum, looking at the oxide chemistry can give direction on how to adjust recipes for better fit. That being said, in industry it is common for technicians to take a material-level approach to achieving glaze fit, simply trading a percentage of lower expansion frit for one having a higher expansion. However, this approach often introduces unintended side effects (because frits having complex chemistries are being treated as just expansion-adjusters). For potters there is also a viable non-chemistry approach: Where two base glazes are known to work, a blend of one having a higher-than-needed COE with one having a lower-than-needed COE could achieve the desired fit. When opacifiers and stain additions affect COE the blend can also be adjusted to compensate. Potters are also the most flexible to delve into the chemistry, the prime origin of the fired properties of glazes. Using an account at Insight-live.com one can see the calculated thermal expansion glaze recipes. By placing recipes side-by-side (e.g. an original and an adjustment) one can observe changes in the expansion as ingredients are adjusted.

Related Information

This crack is "spring loaded" out of the kiln. Why?

It is possible for glazes to be under a condition called "glaze compression". This is L4410P, a low-temperature dolomite body, formulated to have the highest thermal expansion possible without the use of talc. This inside transparent glaze is G3879C (we made this as a brushing glaze, four coats are needed on this body). That glaze was formulated to have the lowest expansion possible (given frits we have). As the kiln cooled the body was unable to withstand the increasing compressive forces of the solidifying glaze inside, resulting in what you see here. In the days following the firing, it kept widening, branching and travelling until the mug spontaneously split in two. Another identical mug with Spectrum 700 clear inside (same outside glaze) bounced like a ball when dropped onto concrete from two feet (a subsequent ricochet right onto the handle took off a chip). But when I dropped this one the pent-up stresses within exploded it into dozens of pieces. Neither the body or glaze are at fault, it is their mismatch that causes this. The thicker the glaze the worse it is. If the outside glaze is also "pulling" (because of a higher thermal expansion than the body) the situation is even worse.

Why is this vitreous low fire ware cracking out of the kiln like this?

Simply put: Glaze misfit. The glaze is under compression and it is pushing outward. That compression was created as the pieces cooled in the kiln. After the glaze solidified, somewhere above red heat, it became a glass and began to contract. The body, to which that glaze is attached by a glass bond, had its own rate of contraction and that rate was more than the glaze. So, it is a battle! But the glaze has some advantages here. It is on thick, giving it extra power to assert its thermal expansion. The body is over fired and has become brittle. The unglazed outsides, incised designs and varying thickness provide points of weakness where a failure can start. The body is trying to resist the relentless force but the odds of stacked against it and the pieces do not even make it out of the kiln. What if we make the glaze thinner, fire lower, make a more even cross section, glaze the outsides. All will help but if the glaze still does not fit, eventually pieces will fail anyway.

Why are these vessels cracking when hot water is poured in?

They are time-bombs. Why? Yes, they have even wall thickness, the glaze is not crazing and the clay is vitrified. But, they are glazed only on the inside and that is a problem here. The glaze is under some compression, that is why it does not craze. That means that it is pushing outward from the inside. Ware can be very strong under glaze compression, but only to a point. When a hot liquid is poured into containers like these the inside glaze is the first to thermally expand, creating more even compression. The porcelain is under tension already, being stretched by pressure from within. And ceramics do poorly under tension. And third: The outer surfaces have incised lines that provide irregularities for internal forces to exploit and start cracks at. It is a perfect storm waiting for hot water to trigger it. Making the pieces thicker would help. Increasing the glaze thermal expansion would help. But unglazed outsides and incised lines will always be a weakening factor.

A low temperature vase has exploded while just sitting there! Why?

This is a slip-cast vase. The body is a typical 50:50 talc:ball clay blend. Only the outside of the vessel is glazed. The talc increases the thermal expansion so that during cooling in the kiln the body contracts more than the glaze, putting it under compression (and thereby preventing crazing). But, these bodies have no flux, they typically have 10%+ porosity and often are not strong enough to resist for long the tensive forces the glaze can put them under. This is especially so when walls are thin. Or when only the inside is glazed. Or when the inner glaze is under compression and the outer under tension. How could such a thing happen? When potters use glazes from one manufacturer and trust they fit bodies made by another (which is almost always).

A glaze is pushing outward from the inside of this cone 6 cup

Pushing enough to break the walls off the base. The body is a high-manganese black-burning stoneware. It is not just the higher thermal expansion of the glaze that is putting it under compression (as evidenced by the vertical crack that is being pushed apart), but also the fact that the glaze is applied thickly.

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.

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 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?

The glaze broke the bottom off the pot!

An example of a highly fluid cone 6 glaze that has pooled in the bottom of a mug (and crystallized). Glazes normally need to be under some compression to avoid crazing (by having a lower-than-the-body thermal expansion), but if they are thick like this the body does not have the strength to resist the extra outward pressure the glaze can be exerting at the base from the inside. The result here is a separated base. Conversely, if the glaze is under tension (having too high an expansion), the cracks that develop within it to relieve the tension are deep and wider and thus more likely to propagate into the body. The ultimate result: Poor ware strength and durability. A better solution is to use a liner glaze.

To gauge strength: Break pieces often. Pay attention to what happens.

This is M340 casting, L3798H. The handle was just glued on with slip. I had questions about this new body. Are my glazes under compression? How well is the handle stuck on? Because I have broken so many pieces I know how they break. It is a good sign when there are no fractures along the handle joins. And when the handle is the last to go. And when the item breaks into large pieces rather than shattering into small ones. Try doing this on some of your pieces and you might be surprised. Either by how strong they are. Or by how easily they break.

Yellow stain has decreased the thermal expansion of the base glaze

That has put it under more compression on the inside, it is pushing outwards. All it took was some hot coffee. That put it under even more compression and the mug "popped" to relieve the tension.

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.

My clear glaze outside. Commercial white inside. But a big problem!

I know my outside glaze recipe fits this terra cotta. It does not shiver on sudden heating or craze on sudden cooling. And I have a gallon so I can dip-glaze the outside and it dries perfectly in seconds. But that inside glaze? It is under too much compression, so much so that it is literally forcing the piece apart (that crack exploded onto the scene with a loud ping a day after firing). But I do not know the recipe. And I had to paint it on in three coats. The painting was difficult and it took ten minutes to dry each coat. A better way to do a cover glaze would have been to simply add 10-15% Zircon to my clear recipe (I can even adjust if the added zircon lowers its expansion too much). To apply that would have been a simple pour-in and pour-out. Or I could make my own pint-jar of brush-on by using a mix of gum solution and water (instead of pure water).

A novel way to test glaze compression and glaze fit

These are made from L4005D red cone 6 stoneware. Both are cast and thin-walled (half of what a thrown piece would be). They were glazed only on the inside to encourage cracking/splitting if the glaze is under excessive compression (that is, the thermal expansion of the glaze is significantly less than that of the body). And that is what happened here. The piece on the left cracked after a couple of taps with a hammer. Notice how the crack has opened. The piece is "spring-loaded" (press it together and it reopens on release). The glaze is GA6-B. The piece on the right is glazed with G1214Z1. It spontaneously blew in half, with a loud crack, a few 5 hours after exit from the kiln. On further taps with a hammer these pieces shattered into dozens of smaller ones! The white glaze is certainly under too much compression. Obviously, neither is under any danger of crazing. Is the compression too great on the dark glaze? It did not shatter the way the white one did on further taps. And, another thicker-walled piece exiting the same kiln was glazed inside and out with that glaze. It was very strong. The lesson: Glaze compression, if not too much, is good for ware strength - but pieces must be glazed both outside and inside. And, thin ware like this must have better-fitted glazes.

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.

Inbound Photo Links

An extremely runny glaze at cone 6. The runniness is manageable, it has other issues!

Shivering on terra cotta is a red light not to ignore

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