Every solid has a coefficient of thermal expansion (COE), that is, an amount by which it expands and contracts on heating and cooling. If the thermal expansion of a glaze is too much more than the body it is affixed to, then it is basically "stretched on" and to relieve the tension it develops a crack pattern called crazing. If its COE is too much lower, however, it is under compression and bad things can happen!
The glaze broke the bottom off the pot!
A 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 below. The ultimate result: Poor ware strength.
Drip glazing and bare outsides: Deceptively difficult.
Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.
BEWARE of leaving outsides of functional ware unglazed
This mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? Glazes need to have a lower thermal expansion than the body so they do not craze over time. When ware is glazed inside and the compressive forces the glaze finds itself under keep it crack free and also significantly strengthens the piece (like pre-stressed concrete). But here there is no outside glaze to be counteract the inside one pushing outward. When suddenly heated it pushes even harder. Structural weak points, outside surface imperfections or pronounced contour or thickness changes provide crack-initiation-points to relieve the stress. The only way to make this inside-only-glazed technique work is carefully tuning the thermal expansion of the inside glaze. That means a lot of testing and a lot of broken pieces.
A mug cracks before your eyes because of glaze compressionShow on Post Page
A highly vitreous, thin walled mug is glazed inside-only. The glaze has a thermal expansion that is too low and it is under compression, pressing outward. A tap with a spoon is enough to trigger a sudden crack. It opens under the pressure. Had it been glazed on the outside also it would likely survive, but this test still indicates that it would be better to raise the expansion a little.
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).
Glaze under excessive compression can flake off the engobe
The amber glaze on the outside of the left mug contains 20% super-low thermal expansion Ferro Frit 3249 as the melter. With no underlying engobe it can form enough of a bond with the body that it does not flake off at the rim (even though it is under excessive compression because its low thermal expansion). This flaking is called "shivering". The engobe, which does not melt like a glaze, has a more fragile bond with the body (and the glaze is pushing enough to make that bond fail). The mug on the right employs 20% Frit 3195 melter instead, producing a glaze that fits better. I hammered both of these rims repeatedly with a metal object to stress them, that one on the right definitely fits better.
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.
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.
A fired, low temperature vase has exploded while just sitting there! Why?
This is a slip-cast vase. The body is a typical talc:ball clay blend. Only the outside of the vessel is glazed. Why did it just burst apart like this on its own after firing. Without the almost 50% talc these bodies have, all but very specially formulated glazes will craze severely. 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% or more 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 and only the inside or outside is glazed (or when a glaze is under compression on one side of the wall and tension on the other). This issue is more important at low fire but potters do something very unfortunate: they use glazes from one manufacturer and trust they fit bodies made by another without do any fit-testing.
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.
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