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Thermal Shock Failure - TSFL

This test procedure was employed in the Foresight Ceramic Database and now is available for those having an account at Insight-Live.com. Accumulating test data using the variables defined in these procedures enables us to create tools that enable you to compare the physical properties of materials and recipes.


This imposes a flame on the center of a tile and reports the amount of time until it cracks and the fired size. If rolled from plastic clay, the tile is made to be 3 inches square and 3/16 thick.


CONE - Cone to which tile was fired (V)

SECS - Seconds until failure (V)

MODE - Manner of failure (V)

Silent (crack appears), Ping (audible), fracture (tile flies apart).

MM - Thickness of tile - mm (V)

CM - Width of Square Tile - cm (V)

Flame thermal shock failure test

Flame thermal shock failure test

A 3 inch by 3/16 thick tile is flamed and timed until it cracks. The crack usually is audible and sometimes spectacular (the tile flying apart). However if the body is heavily grogged the crack can grow slowly (often from the edge inward). We log the result using the TSFL test.

A flameware body being tested for thermal shock. Is this a joke?

A flameware body being tested for thermal shock. Is this a joke?

A recommended flameware recipe from a respected website (equal parts of 35 mesh grog, talc and ball clay). Looks good on paper but mix it up for a surprise. The texture is ridiculously coarse. Recipes like this often employ fire clays and ball clays, but these have high quartz contents (in a test like this a ball clay vessel could easily fail in 5 seconds). But this one is surviving still at the 90-second mark. Or is it? While porcelain pieces fail with a spectacular pop of flying shards, these open-porous bodies fail quietly (note the crack coming up to the rim from the flame). There was an intention to create cordierite crystals (the reason for the talc), it is hard to say whether than happened or not. But the porosity of 12.5% would be difficult to deal with. On the positive side, you could likely continue using this vessel despite the crack.

The porcelain is harder, but the terra cotta has it beat for thermal shock!

The porcelain is harder, but the terra cotta has it beat for thermal shock!

This terra cotta cup (center) is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 30 seconds under direct flame against the sidewall and turns red-hot before a fracture occurs (the unglazed one also survived 30 seconds, it only cracked, it did not fracture). The porcelain mug (Plainsman M370) is glazed with G2926B clear, it survived 15 seconds (even though it is much thinner). The porcelain is much more dense and durable, but the porous nature of the earthenware clearly withstands thermal shock much better. It is actually surprisingly durable.

Flame thermal shock failure in two porcelains: surprising result

Flame thermal shock failure in two porcelains: surprising result

The porcelain on the left is highly vitreous, it cracked in 12 seconds, but the other lasted until 25! Why? Only theories, both of these should be failing sooner. The vitreous nature of the one on the left could be giving it the sheer strength to endure longer. The one on the right is Crystal Ice (from Plainsman Clays). It has very high quartz, which should have accelerated the failure. However it is not completely vitreous, the voids within the matrix could be giving it resilience. Another factor could be that the glaze is under more compression, adding strength. This whole thing is counter intuitive, porous bodies like earthenware normally endure best and vitreous porcelains the worst. Yet this same test on my earthenwares cracks them in less than ten seconds?

In Bound Links

  • (Glossary) Ovenware

    Ovenware clay bodies have a lower thermal expansion than typical bodies so they can withstand more sudden changes in temperature without cracking. Flameware bodies are not the same, they can withstand an open flame and demand much more compromise in working properties, strength, glaze fit, etc. O...

  • (Glossary) Flameware

    Flameware is ceramic that can withstand sudden temperature changes without cracking (i.e. stove top burners). Ovenware is another class of ceramics, it is not as resistant to thermal shock as flameware. There is some confusion among clay buyers and retailers about this. For example Japanese Donabe w...

  • (Glossary) Thermal shock

    Thermal shock refers to stresses imposed on a ceramic by the volume changes associated with sudden shifts in temperature. Pouring hot coffee into a cup is a classic example, it is a mild thermal shock common to every day use, almost any type of clay product can withstand this (unless internal stress...

  • (Glossary) Liner Glaze

    It is common to glaze food surfaces of utilitarian ware with a brilliant glossy white or transparent glaze called a 'liner glaze'. This is done to avoid releasing in-glaze or on-glaze metallic colorants to food or drink (which could leach them away and be a health hazard). Liner glazes can be applie...

By Tony Hansen

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