Bentonite

Bentonite is a clay mineral with incredibly small particle sizes. This, in combination with the active chemistry on the surface of the particles (that makes them hold onto water), makes it the most plastic and impermeable common clay material used in ceramics. Its contribution to working properties in glazes and clay bodies is balanced by the undesirable properties that are also imparted. Thus anyone who uses this material should have their eyes open to its advantages and disadvantages.

There is a huge variation in the chemistries of bentonites, it is impossible to specify an average (bentonite is not employed in ceramics for its chemistry). Any generic chemical analysis is thus only an attempt to represent the amounts you might find in a common variety. Because of the high iron content, bentonite is considered a dirty material and thus the tug-of-war between the valuable working properties it imparts and the need for whiteness or pure color that it impedes.

Fine particle size: Bentonite is colloidal (particles are so small the action of water molecules is enough to keep them in suspension). It is typically 10 times finer than ball clay. It can have a surface area of almost 1000 square meters per gram (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 (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 color. The use of up to 5% is common, especially where high plasticity is needed in a white burning body. However the need for higher additions than this may indicate a lack of other clean or adequately clays in the recipe. Also, 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 (in larger percentages). 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 considerably different effects on the plasticities.

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.

Bentonite is far too plastic to prepare test specimens (e.g. for drying, strength and shrinkage evaluation). However, a mix of 20-30% virgin material with calcined material can be extruded and formed (test specimens will still shrink to a very small size).

Permeability: Most bentonites are impermeable to water. To demonstrate this fill a tall glass cylinder with bentonite to near the top and then carefully pour water on top. The water will penetrate down into the clay only a few millimeters and no matter how long you leave it it will not penetrate further. This occurs because the powder swells as the water penetrates and adjacent particles 'hang onto' the water between them. The water thus becomes a glue that holds the mass together and prevents more from entering or passing through. This phenomenon accounts for why glazes and bodies of high bentonite content dry slower. As an example, if you pour a slurry of silty clay onto a plaster surface the water is often pulled out in seconds. However a bentonite slurry may require days or weeks to pull the water out evenly.

Swelling: Most 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.

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.

Thixotropy: This is a tendency of a suspension to gel after sitting for a time and then re-liquify when it is agitated. Clay bodies also exhibit this behavior, stiffening on aging but then re-softening when worked. Thixotropy is valuable in clay slurries for this reason, they gel when not being used and thus do not settle out. While typical industrial thixotropic agents employ various mechanisms bentonite works by charge attraction (see above).

Chemically inert, Inorganic, Non-irritating: Formulations that are not fired are not altered chemically by bentonite additions. Bentonite does not support organic growth. Thus it is suitable as a carrier for personal care products like hand cream and cosmetics.

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.

Normal microfine bentonite or raw bentonite even at 5% does not darken the color of the porcelain as much as you might think. Examining recipes often shows that the kaolin and ball clay are contributing more iron than the bentonite. Even white plasticizers often have up to 0.5% iron also. 5% bentonite increases the iron content of the body by .25% without considering the factors above. Considering them it might cut it to half that. Use only 2.5% bentonite and it is not really an issue.

Firing cracks, explosions: Bentonite slows down water penetration. Not only does a bentonite-containing clay body dry slower but it does not dry as completely. Although ware might look dry it likely is not, several percent tightly-bound water remains. If ware is not temperature-dried before being fired there is a risk that water will not be able to escape fast enough during firing and ware will crack, fracture under steam pressure.