Vitrification is the change clay goes through at high temperatures as part of it melts into glass that binds the remaining particles and fills the pores. A vitrified body is dense, strong and absorbs very little water.
What Is Vitrification?
As a clay body is fired hotter, fluxes such as feldspar begin to melt and form glass. The glass flows into the spaces between particles, pulling them together, which is why fired clay shrinks and becomes harder. The more vitrified a body is, the lower its absorption and the stronger it gets.
Stoneware and porcelain are designed to approach full vitrification at their rated cone, which is what makes them suitable for tough, watertight tableware. Earthenware is fired well short of it, so it stays porous.
There is a limit. Fired past its rating, a body can become overfired: too much glass makes it soften, slump, bloat or blister. Clay makers choose a rated cone that balances density against the risk of warping, and Digitalfire describes this as a balancing act for every vitreous body.
Measuring absorption on fired test bars is the usual way to see how vitrified a clay is at a given cone.
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.