Bentonite Alternate Names: Montmorillonite
Bentonites are classified according to several types (e.g. calcium, sodium, potassium) but sodium bentonite is used by the ceramic industry and the drilling industry. it is the most plastic and impermeable common clay material used in ceramicsIt is typically 10 times finer than ball clay. It can have a surface area of almost 1000 square meters per gram i.e. 50 times that of kaolin, 5000 times that of silica flour.
Plasticity: Because of their active electrolytic behavior and fine particle size, bentonites exhibit extremely high plasticity Drying performance: Bentonite makes bodies more plastic and dry harder but this comes at a cost, they shrink more during drying and thus potentially crack more.
Welling: Sodium bentonites expand (as much as 15 times) when added to water. This characteristic is valuable in thickening liquids and slurries and is another contributing factor to maintaining suspensions.
Bentonite is used in large quantities in the gas and oil drilling industries to suspend high specific gravity slurries which are used as a medium to float out the chunks of rock cut by the drill bit.
Binder: Bentonite binds particles together in ceramic bodies to make them stronger in the green or dry state. Its minute particles fill voids between others to produce a more dense mass with more points of contact. Adding bentonite to glazes also imparts better dry strength and a harder and more durable surface.
Sorptive properties: Granules of this material can be heat-treated to nullify gelling properties but greatly improve liquid absorption, creating hard micro-sponges (e.g. for cat litter and other absorptive products of many types).
Ball clay is the most common type of secondary clay. They are much more plastic than kaolin because the particle size is much smaller. Ball clays are available with a wide range of plasticities (a product of varying particle sizes). They have higher iron content than kaolins so bodies made from them are not as white. Raw ball clays tend to be grey in color since they commonly contain some lignite. Ball clays are fairly refractory.