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Glaze Compression

Every solid has a thermal expansion, that is, an amount by which is expands and contracts on heating. If the thermal expansion of a glaze does not match the body it is on, then the glaze either cracks (when it is under contraction) or chips off when under compression.
The compression occurs while the piece is cooling in the kiln. When the glaze solidifies it acquires its solid characteristics. This might happen at 1500F, for example. As the piece continues to cool in the kiln it contracts. If the body is contracting more than the glaze then the glaze is being put under compression. Some compression is actually desirable and strengthens the glaze-body combination. However too much compression puts the piece under internal stresses seeking an opportunity to relieve themselves. When the difference is severe the 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 will literally explode into hundreds of tiny pieces. 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 micro-flake of glaze (having razor sharp edges).

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

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 even 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 the 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. And its surface is not as good as mine. A better way would have been to simply add 10-15% Zircon to my clear recipe. That would have been simple to pour in and pour out to apply. Or I could make my own pint-jar of paint-on from 450g glaze, 50g zircopax, 75g gum solution and 280g water.

Drip glazing and bare outsides: Deceptively difficult.

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 a dilatometer to precisely match the thermal expansion of the glaze with the body. The inside glaze has to fit better than normal because of the absence of an outside glaze. Too low of a thermal expansion and it's compression (outward pressure) will fracture body (especially if the latter is thin). Too high a thermal expansion and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And there is one more problem: How to get the glaze on thick enough to flow like this. The answer is two-fold: Deflocculate it, take the specific gravity up to 1.7 or more. Glaze the inside, let it dry, then glaze the outside.

Glaze under excessive compression can flake off the engobe

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?

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

They are even thickness. The glaze is not crazing. The clay is vitrified. Glazing closed containers only on the inside to be very challenging. Three factors I see here produce the very cracking you are getting: 1. The glaze is designed to be under compression to avoid crazing. That means that it is pushing outward from inside. Ceramics are very strong under compression. When a hot liquid is poured into the container the glaze inside is the first to thermally expand, creating even more pressure. 2. Ceramics do poorly under tension. The porcelain is under tension, being stretched by pressure from within. It is seeking opportunity to relieve that pressure. 3. Your outer surfaces have carving, incised lines that provide irregularities for cracks to start at. Glazing the outside of the piece would mitigate all three of these factors. It might seem that making the pieces heavier would give the porcelain enough strength to resist, but that would also exacerbate the gradient created when hot liquids are poured inside.

An extreme example of what happens when glaze is under excessive compression

An extreme example of what happens when glaze is under excessive compression

This is a low temperature slip-cast vase. Only the outside of the vessel is glazed. It burst apart like this on its own after firing.

Out Bound Links

  • (Glossary) Crazing

    Crazing refers to small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. Even though the amount of change is very s...

  • (Glossary) Co-efficient of Thermal Expansion

    A measure of the reversible volume or length change of a ceramic material with temperature. The more the expansion during heating the more contraction must occur while cooling it back down. Expansion values are very small and recorded in scientific notation (e.g. 6.5 x 10-7 which is 0.00000065). Typ...

  • (Glossary) Calculated Thermal Expansion

    Digitalfire Insight-live and desktop Insight calculate the thermal expansion of a glaze from its oxide chemistry (based on the contributing expansion factors and amounts of each oxide in the formula). These numbers are very small and recorded in scientific notation (e.g. 6.5 x 10-7 which is 0.000000...

  • (Glossary) Shivering

    A defect in glazed ware where the glaze is compressed too much by the body, the glaze actually peels off the ware on edges to relieve the stress. Shivering is thus the opposite of crazing and is also less common. This problem is potentially dangerous, since the tiny flakes of glaze having razor-shar...

By Tony Hansen

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