All Glossary

Dunting

Dunting generally refers to firing cracks that occur in ceramic ware as it is cooled in the kiln. The reason is generally uneven cross section or too rapid cooling.

Key phrases linking here: dunting, dunt - Learn more

Details

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.

Related Information

A dunting crack

This picture has its own page with more detail, click here to see it.

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 picture has its own page with more detail, click here to see it.

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.

Cracking from the rim during bisque:

Uneven heat-up or cool-down can do this

This picture has its own page with more detail, click here to see it.

This crack is characteristic of a firing dunt: stresses created when different parts of the piece are changing temperature, and therefore dimensions, at different rates. When those stresses exceed the strength of the body, a crack releases them. The quartz inversion around 570C is especially important - a temperature difference across the piece at this point means one area can be undergoing the quartz volume change while another is not.

Large, heavy pieces and those having uneven cross sections develop temperature gradients more readily. This piece has another disadvantage: its wide flat bottom is coupled thermally to a heavy kiln shelf while the vertical walls are much more directly exposed to radiant heat. That can produce substantial temperature differences during both heating and cooling, especially if one side faces the elements more directly. Slowing the firing through quartz inversion, on both the way up and the way down, and avoiding uneven radiant exposure can greatly reduce the risk.

An unevenly cooled tile has cracked

This picture has its own page with more detail, click here to see it.

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.

Low fire ware cracking during firing. Why?

This picture has its own page with more detail, click here to see it.

Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz particles present do the same. These are good for glaze fit but bad for ware like this. There are also sudden volume changes associated with cristobalite, but it forms (from quartz) at stoneware temperatures so should not be a concern in terra cotta or a white low fire body. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. The quartz in stonewares goes through a sudden volume change between 950-1150F on the way down. Quartz particles in low fire bodies will do the same. A simple fix is to slow down the entire cooling cycle like this potter did. Or, learn to program your kiln to approach this range more slowly, then ease down through it. No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).

Links

PayPal	No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me.

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk