Digitalfire Ceramic Glossary



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Dunting


Dunting is cracking associated with too rapid a cool-down of the kiln. Dunting often exhibits itself as simple hairline cracks, but ware can fracture into pieces.

More specifically, a body recipe may create a ceramic that is unable to withstand reasonable cooling shock (e.g. have too much silica and too little feldspar to produce a vitreous body with reasonable mullite development). Or ware may have a poorly fitting glaze that creates stresses and places for cracks to propagate. Ware of uneven cross section or larger pieces cannot withstand quick cooling nearly as well. Quartz and cristobalite inversions are the most sensitive periods of cooling, especially for ware of uneven cross section or where the kiln is not heating ware evenly. A worst case scenario is a large flat plate made from a high silica non-vitreous porcelain, covered with a crazing glaze on the outside and excessively compressed one on the inside, having a thin lip and a thick base, fit tightly into an electric kiln with the lip close to the elements and cooled quickly through quartz inversion.

Pictures

A dunting crack

Example of a dunting crack in a flat deep cone 6 porcelain bowl. The bowl has a wide bottom that heat-sinks to the shelf, so during firing there is a temperature gradient between the walls and the base. That difference in temperature translates to stress because it means that different parts of the piece are experiencing different thermal contractions as it cools in the kiln.

Can a glaze crack a plate? Yes.

This is a thin slip-cast plate made from a high-silica (therefore high thermal expansion) clay and a thick layer of low thermal expansion glaze. During the cooling in the kiln the clay shrinks and the glaze shrinks less. This puts the latter under compression, and the body under tension. A ceramic does not do well under tension. A week after firing the piece spontaneously cracked to relieve the tension.

Example of dunting, where a crack has released the stresses produced by uneven thermal contraction during cool-down in the kiln. This usually happens by cooling too quickly through quartz inversion.

An unevenly cooled tile has cracked

Example of a severely dunted cone 6 stoneware tile. This problem was deliberately created by stacking several tiles on top of this one. This set up a temperature gradient across it so that different parts passed through quartz inversion at different times.

Out Bound Links

  • (Glossary) Thermal shock

    Thermal shock refers to stresses imposed on a cera...

  • (Glossary) Quartz Inversion

    The term "quartz inversion" is used in two ways. O...

In Bound Links


By Tony Hansen




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