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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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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. However, the opposite can be the case - when COE is too much lower the glaze is under compression.
The compression occurs while the piece is cooling in the kiln. When the glaze solidifies it acquires the characteristics of a solid. This might happen at 1500F, for example. As the piece continues to cool body and glaze are contracting. If the body is contracting more than the glaze then the glaze becomes more and more compressed by the body. Some compression is actually desirable and strengthens the piece (like prestressed concrete). However, too much compression puts the piece under internal stresses seeking an opportunity for relief. In severe cases, a piece will not survive cooling in the kiln without fracturing. When individual shards of such pieces are dropped onto a cement floor, for example, they can explode into dozens of tiny pieces. Ware loss is most likely where the inside glaze on a vessel is under compression and the outside is not (or worse, is covered with a high-thermal-expansion-glaze that is under tension). If the inside glaze is thick enough it can literally force the piece apart. In less severe mismatches, glazes will flake off areas where they wrap around contours (e.g. the lips of mugs), this is known as shivering (this can be serious if it occurs while a piece is being used and someone ingests a flake having razor-sharp edges).
The thermal expansion of glazes is a product of their chemistry. Although glaze fit can happen by accident it is better by design. In industry, technicians often take a nontechnical approach to matching glaze fit with the body, simply adjusting recipes by trading a percentage of lower expansion frit for one having a higher one (however this often introduces unintended side-effects). For a potter, a best-case scenario is where two base glazes are available (having the desired surface characteristics), one of higher-than-needed COE and one of lower-than-needed. These can be blended to achieve the desired fit for the body being used. When opacifiers and stain additions affect COE the blend can be adjusted to compensate. Potters are 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. Placing recipes side-by-side (e.g. an original and an adjustment) one can observe changes in the expansion as ingredients are adjusted.
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.
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.
Why? Glaze fit. These are available on Aliexpress (as Drip Pottery or Drippy Pottery) 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. It has to fit better than normal because of the absence of an outside glaze. Too low an expansion and the compression (outward pressure) will fracture body (these are thin-walled pieces making them vulnerable). 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. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same (they are often called Gloop Glazes).
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.
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.
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.
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.
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, is good for testing that compression.
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. 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.
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.
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 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 people do something very unfortunate: they use glazes from one manufacturer and trust they fit bodies made by another without doing any fit-testing.
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.
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.
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).
An extremely runny glaze at cone 6. The runniness is manageable, it has other issues!
Ceramic Glaze Defects
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
|By Tony Hansen|
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