Quartz inversion is the sudden change in the crystal structure of quartz at about 573 °C, which makes it expand on heating and shrink on cooling by a small but abrupt amount. Passing through it too quickly can crack pottery.
What Is Quartz Inversion?
Quartz, a form of silica, is present in nearly all clay bodies. At around 573 °C it changes between two crystal forms, alpha and beta, and its volume changes abruptly by a small amount, often quoted at around 1%. This happens on the way up and again, in reverse, on the way down, in every firing.
The danger comes when different parts of a piece pass through the inversion at different times, because one part expands or shrinks while another hasn’t yet. Digitalfire describes this as a leading cause of dunting, especially in thick pieces, wide plates, and work sitting near elements or on heavy shelves.
Stoneware fired high can also form cristobalite, another form of silica with its own sudden change at a lower temperature, roughly 220 °C, which adds stress on cooling.
The practical answer is to heat and cool steadily through the inversion ranges, keep walls even, and resist opening the kiln before it has cooled. Many firing schedules include slower segments around these temperatures for large or thick work.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.