Bloating is a blister or swelling in a fired clay body, caused by gas trapped inside the clay after the surface has sealed. It is a firing fault rather than a glaze fault, and it almost always means the body was taken past what it could tolerate.
What is actually happening
Organic material and some minerals in a clay body give off gas as they burn out during firing. If the outside of the body vitrifies and seals before that gas has finished escaping, it is trapped, and at temperature the softened body swells around the pocket. The result is a bubble that stays after cooling, sometimes with a crater where it has burst.
Overfiring is the usual cause
A body fired above its stated range vitrifies too early and too far, sealing the surface while gas is still being produced. That is why bloating so often appears in a kiln that fires hotter than its controller reports, or in the hottest part of a chamber. The first thing to check is not the clay but the cone: a body bloating consistently is being taken too high.
The other causes
An underfired bisque, which leaves more organic material to burn out during the glaze firing. A schedule that climbs too fast through the burnout range without time for gas to leave. Reduction started too early, which produces carbon inside the body. And contamination such as plaster, which behaves differently but produces similar-looking craters.
What to change
Fire cooler, or bisque higher, or slow the climb through the burnout range and hold there. Put a cone in every firing so a drifting thermocouple shows itself before a batch of work does. And if bloating appears only in one part of the kiln, the problem is the packing or the element wear rather than the body.
Questions people ask about what is bloating
What is bloating in pottery?
A blister in the fired body, caused by gas trapped inside after the surface sealed. It is a firing fault rather than a glaze one.
What causes bloating?
Usually overfiring the body past its stated range, and sometimes an underfired bisque, too fast a climb, or early reduction.
How do I stop it?
Fire cooler or bisque higher, slow the climb through the burnout range, and use cones to check the kiln is where it says it is.