Bentonite is a highly plastic clay originating in the decomposition of volcanic ash. Is extremely fine and can be dispersed widely through bodies and clays. In which it is used to impart plasticity and ability to keep a glaze slop in suspension. Up to 2% can be safely added to all bodies and will improve 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.
Plasticity: Because of their active electrolytic behaviour and fine particle size, bentonites exhibit extremely high plasticity (and associated high shrinkage). In pottery and porcelain clay bodies additions of only 2% can produce marked improvements in workability and dry strength without much effect on fired colour. The use of up to 5% is common, especially where high plasticity is needed it a white burning body. High amounts of bentonite will dramatically slow down the drying rate. In certain applications it is practical to use bentonite as the only plasticizer in a mix (e.g. 5-7.5% with Redart to make a terra cotta). The plasticity-producing effects of bentonite depend on the shapes, sizes, surfaces and electrolytics of the particles it is interacting with, equal additions of bentonite to two different host bodies may have much different effects on the plasticities. While bentonites can be called upon to assume the major burden of plasticizing a body, there is a need to assess the practicality of this (the body can become excessively plastic, take too long to dry and be too easily torn).
Suspension: Bentonite is used to keep particulates in suspension in all sorts of consumer and industrial products, and in glazes in ceramics. The mechanism is charge attraction, that is, opposite electrolytic charges develop on the surfaces and edges of dispersed particles and give rise to a stable 'house-of-cards" structure that can be disrupted by shear stress. However when the stress is removed, the structure re-establishes itself. The amazing thing is that large amounts of other types of particles can be tolerated within this structure, they are kept in suspension as well. Thus maximum suspending benefit can be achieved by blunging bentonite with the water before adding the other dry materials (to insure that every particle is whetted on all sides). However, this cannot be done without a powerful high-speed propeller mixer. Thus it is normal to blend dry ingredients including bentonite first and then add them to the water. However beware of too much bentonite in glazes, they will dry too slowly and will shrink too much during drying causing cracks that later turn into crawling during firing.
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. To fully appreciate how plastic, hard and strong bentonite can be, try mixing 25:75 with silica and preparing plastic test bars.
Firing: Standard grades typically vitrify (around Orton cone 6-10) from grey to deep red coloration. However soluble salts can be so high that they form a glaze on pure test specimens. Utility grades often contain granular iron material that causes specking in clay bodies, even materials rated at 325 mesh can contain significant speck-causing particles. For good reason, bentonite is considered a very dirty material. However commercial micro-fine grades (100% minus 325 mesh) are available (these are very expensive however). Barium carbonate can be added to bodies to precipitate the solubles bentonite brings. Thus the iron content is the only firing issue associated with visual character. Contrary to what many think, a white body can often tolerate a up to 5% bentonite without firing significantly darker.