Absorption is the percentage of water a piece of fired clay soaks up, measured by weighing it dry, soaking it and weighing it again. Low absorption means a dense, strong, watertight body; high absorption means an open, porous one.
What Is Absorption?
Absorption is the simplest way to tell how mature a fired clay is. As clay is fired hotter, its particles fuse and the pores close up, so the percentage of water it can take in drops. A stoneware fired to its rated cone often absorbs only a few percent or less, while bisqueware and low-fired earthenware can absorb 10% or more.
Potters test it with a small unglazed tile or bar of fired clay. You weigh it bone dry, boil or soak it in water, pat off the surface water and weigh it again. The gain in weight, divided by the dry weight and multiplied by 100, is the absorption percentage.
It matters most for functional ware. A porous body under a crazed glaze can take on water, weaken and harbour bacteria, and very porous ware can crack in a microwave. Clay makers publish absorption figures at different cones, which is one of the best ways to check whether a clay suits the temperature your kiln reaches.
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.