Bentonite

Source: http://ctmpotterssupplies.co.uk/SMLCTM%2044%20Bentonite%2017022017.pdf

Al2O3.2H2O.SiO2 Most plastic and impermeable common clay materials. It binds particles together making bodies stronger in the green and dry state. Its' minute particles fill voids between others to produce a more dense mass. Adding bentonite to glazes imparts better dry strength and a harder more durable surface. 1 part bentonite can plasticize 10 parts of kaolin. Bentonite bodies are stronger but dry slower, crack more and fire darker with potential iron specks. 1-2% bentonite added to the dry powder can greatly improve glaze suspension. Bentonite containing clay bodies may look dry but may not be - they must be very slowly dried to prevent steam being produced and creating cracks. Make into a paste before adding to glaze batches if not added dry to the powder.