Bentonite Lumps

Bentonite is an absorbent swelling clay consisting mostly of montmorillonite which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

Bentonite usually forms from the weathering of volcanic ash in seawater, or by hydrothermal circulation through the porosity of volcanic ash beds, which converts  the volcanic glass present in the ash into clay minerals.

Plasticity is the property of bentonite clay that allows it to be manipulated and retain its shape without cracking after the shaping force has been removed; clays with low plasticity are known as short or non-plastic. A small amount of bentonite added to clay can increase its plasticity, and hence ease forming of articles by some shaping techniques.

Ceramic glazes often contain bentonite. The bentonite is added to slow or prevent the settling of the glazes. It can also improve the consistency of application of glazes on porous biscuit-fired ware. Once a certain amount of glaze water has been absorbed by the biscuit the bentonite effectively clogs the pores and resists the absorption of further water resulting in a more evenly thick coat.