Bentonite
Chemistry
|
Volatiles
|
Mineralogy
- Montmorillonite
Links to Other Materials
- Ball Clay - Related
- Smectite - Related
- Veegum - Related
- Hectorite - Related
- Attapulgite - Related
- Acti-Gel 208 - Related
- Cache
Hazards
- Bentonite
- Quartz, Crystalline Silica
Miscellaneous
- Family: Clay Other
- Region: North America
- Mined At: Unspecified
- Raw Mineral: No
- Generic: Yes
Notes
Bentonite is valuable because of its incredibly small particle size. 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.
Raw bentonite is generally a pale green, buff, cream, or grey material composed of the clay mineral montmorillonite. Its origin can be traced to ancient volcanic eruptions where fine volcanic ash particles were carried by winds and deposited in discrete layers which altered over time from the glassy state to claystone. 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.
In North America, bentonites are mined in Montana, Wyoming, South Dakota and Saskatchewan and used in things like porcelain, toothpaste, tablets, cosmetics, oil drilling mud, oven cleaners, insecticides, putty, paint, ink, paper, polishes, cleansing agents, explosives, detergents, plastics and rubber.
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. 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.
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 10-20% virgin material with 80-90% calcined can be extruded and formed (test specimens will still shrink to a very small size).
Permeability, Swelling: 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 attractionb (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.
Firing: Standard grades typically vitrify (around Orton cone 6-10) to 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. For good reason, bentonite is considered a very dirty material. However commercial micro-fine grades (much finer than the standard 200 mesh of other materials) are available and thus particulate problems are not an issue. 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 few percent bentonite without firing significantly darker.
White firing bentonites: There are a number of white firing and highly refined bentonites produced for the ceramic industry. However they tend to have much less plasticity and are many times more expensive. Consideration and testing are thus needed to determine if a white burning material twice as expensive (or more) of which you need to use twice to
four times as much is worth it (in a worst case scenario that may be ten times more expensive!). Normal microfine bentonite 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 is not, several percent water tightly-bound between bentonite particles 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.
(Richard Willis)
Mineral earth of decomposed volcanic ash origin, principally of the hydrous alumino-silicate mineral montmorillonite (Al2O3, 4SiO2, 9H2O) + nH2O, nFe2O3, nTiO2, nCaO, nMgO, nNa2O, nK2O, nCO
2) with a typical empirical analysis of K2O 1.85%, Na2O 2.10%, CaO 3.05%, MgO 5.33%, Al2O3 18.40%, SiO2 59.30%, Fe2O3 2.17%, TiO2 0.23%, CO2 1.18%, H
2>2O 6.79%. Molecular weight is estimated at 504 .
Bentonite is a smectite clay which bloats while wetting and shrinks dramatically while drying — owing to its principal component, montmorillonite (Al2O3, 4SiO2, H2O + nH2O), a uniquely water-retaining alumina-silicate. Even though, it is quite useful as a recipe ingredient for clays and glazes precisely for these characteristics (to thicken a slurry,
improve its suspension, and toughen it as well) and particularly for its ash origins and their potential.
There are three types of “bloatable” bentonite earths: high in sodium and low in calcium, very bloatable; high in calcium and low in sodium, slightly bloatable; sodium and calcium content almost equal, moderately bloatable. All three are usually fossil and shell-rich soils, albeit minuscule, so that if the earths are pulverized as is, without dissolving out the fossils and shells (and bones), the calcium content will be higher (and will have significant traces of phosphorous) than is normally a “refined” industrial powder (a commercial powder can go well beyond a #300 mesh). see fuller's earth
Apart from montmorillonite and imported impurities added through the years by wind-blown dust, erosions, etc., bentonite is somewhat similar in compositional percentage-profile to pumicite (a caution: not to equate the two since pumicite does not contain montmorillonite nor crude fossil-shell-bone remains). Common to ocean floors and (old, very old) lake bottoms. see pumice
Properties
Body Plasticity - Porcelain
1 part bentonite can plasticize a body as much as 10 parts kaolin. Bentonitic bodies are stronger in the dry form but dry slower, crack more and fire darker with potential iron specks (get a super fine ground grade).Glaze Suspender - Fritted glazes
1-3% bentonite can greatly improve glaze suspension by geling it. In addition it will harden the dry layer. Coarser varieties can impart some glaze speck. If a glaze already contains more than 15% clay (kaolin, ball clay) you should not need more than 1% bentonite.
Suppliers
- Generic
Authors
- Tony Hansen (Owner)
XML
<?xml version="1.0" encoding="UTF-8"?>
<material name="Bentonite" descrip="" generic="1" rawmineral="0" searchkey="Montmorillonite" loi="0.00">
<families>
<family name="Clay Other"/>
</families>
<regions>
<region name="North America"/>
</regions>
<oxides>
<oxide symbol="CaO" name="Calcium Oxide, Calcia" status="" percent="1.000" tolerance=""/>
<oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="" percent="2.000" tolerance=""/>
<oxide symbol="K2O" name="Potassium Oxide" status="" percent="1.000" tolerance=""/>
<oxide symbol="Na2O" name="Sodium Oxide, Soda" status="" percent="3.000" tolerance=""/>
<oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="20.000" tolerance=""/>
<oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="59.000" tolerance=""/>
<oxide symbol="Fe2O3" name="Iron Oxide, Ferric Oxide" status="" percent="3.500" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="" name="" percent="10.000" tolerance=""/>
</volatiles>
<references>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
</references>
<minerals>
<mineral name="Montmorillonite" detail=""/>
</minerals>
<hazards>
<hazard name="Bentonite"/>
<hazard name="Quartz, Crystalline Silica"/>
</hazards>
<suppliers>
<supplier name="Generic" country="" url="" label=""/>
</suppliers>
<notes>
<note>Bentonite is valuable because of its incredibly small particle size. 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.
Raw bentonite is generally a pale green, buff, cream, or grey material composed of the clay mineral montmorillonite. Its origin can be traced to ancient volcanic eruptions where fine volcanic ash particles were carried by winds and deposited in discrete layers which altered over time from the glassy state to claystone. 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.
In North America, bentonites are mined in Montana, Wyoming, South Dakota and Saskatchewan and used in things like porcelain, toothpaste, tablets, cosmetics, oil drilling mud, oven cleaners, insecticides, putty, paint, ink, paper, polishes, cleansing agents, explosives, detergents, plastics and rubber.
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. 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.
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 10-20% virgin material with 80-90% calcined can be extruded and formed (test specimens will still shrink to a very small size).
Permeability, Swelling: 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 attractionb (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.
Firing: Standard grades typically vitrify (around Orton cone 6-10) to 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. For good reason, bentonite is considered a very dirty material. However commercial micro-fine grades (much finer than the standard 200 mesh of other materials) are available and thus particulate problems are not an issue. 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 few percent bentonite without firing significantly darker.
White firing bentonites: There are a number of white firing and highly refined bentonites produced for the ceramic industry. However they tend to have much less plasticity and are many times more expensive. Consideration and testing are thus needed to determine if a white burning material twice as expensive (or more) of which you need to use twice to
four times as much is worth it (in a worst case scenario that may be ten times more expensive!). Normal microfine bentonite 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 is not, several percent water tightly-bound between bentonite particles 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.
</note>
<note>Mineral earth of decomposed volcanic ash origin, principally of the hydrous alumino-silicate mineral montmorillonite (Al<FONT SIZE=2><SUB>2</SUB></FONT>O<FONT SIZE=2><SUB>3</SUB></FONT>, 4SiO<FONT SIZE=2><SUB>2</SUB></FONT>, 9H<FONT SIZE=2><SUB>2</SUB></FONT>O) + nH<FONT SIZE=2><SUB>2</SUB></FONT>O, nFe<FONT SIZE=2><SUB>2</SUB></FONT>O<FONT SIZE=2><SUB>3</SUB></FONT>, nTiO<FONT SIZE=2><SUB>2</SUB></FONT>, nCaO, nMgO, nNa<FONT SIZE=2><SUB>2</SUB></FONT>O, nK<FONT SIZE=2><SUB>2</SUB></FONT>O, nCO<FONT SIZE=2>
<SUB>2</SUB></FONT>) with a typical empirical analysis of K<FONT SIZE=2><SUB>2</SUB></FONT>O 1.85%, Na<FONT SIZE=2><SUB>2</SUB></FONT>O 2.10%, CaO 3.05%, MgO 5.33%, Al<FONT SIZE=2><SUB>2</SUB></FONT>O<FONT SIZE=2><SUB>3</SUB></FONT><SUB> </SUB>18.40%, SiO<FONT SIZE=2><SUB>2</SUB></FONT><SUB> </SUB>59.30%, Fe<FONT SIZE=2><SUB>2</SUB></FONT>O<FONT SIZE=2><SUB>3</SUB></FONT><SUB> </SUB>2.17%, TiO<FONT SIZE=2><SUB>2</SUB></FONT><SUB> </SUB>0.23%, CO<FONT SIZE=2><SUB>2</SUB></FONT><SUB> </SUB>1.18%, H<FONT SIZE=
2><SUB>2</SUB></FONT>O 6.79%. Molecular weight is estimated at 504 .
Bentonite is a smectite clay which bloats while wetting and shrinks dramatically while drying &#151; owing to its principal component, montmorillonite (Al<FONT SIZE=2><SUB>2</SUB></FONT>O<FONT SIZE=2><SUB>3</SUB></FONT>, 4SiO<FONT SIZE=2><SUB>2</SUB></FONT>, H<FONT SIZE=2><SUB>2</SUB></FONT>O + nH<FONT SIZE=2><SUB>2</SUB></FONT>O), a uniquely water-retaining alumina-silicate. Even though, it is quite useful as a recipe ingredient for clays and glazes precisely for these characteristics (to thicken a slurry,
improve its suspension, and toughen it as well) and particularly for its <B><I>ash</I></B> origins and their potential.
There are three types of &#147;bloatable&#148; bentonite earths: high in sodium and low in calcium, <B><I>very bloatable</I></B>; high in calcium and low in sodium, <B><I>slightly bloatable</I></B>; sodium and calcium content almost equal, <B><I>moderately bloatable</I></B>. All three are usually fossil and shell-rich soils, albeit minuscule, so that if the earths are pulverized as is, without dissolving out the fossils and shells (and bones), the calcium content will be higher (and will have significant traces of phosphorous) than is normally a &#147;refined&#148; industrial powder (a commercial powder can go well beyond a #300 mesh). <FONT SIZE=2>see <B><I>fuller\'s earth</I></B></FONT>
Apart from montmorillonite and imported impurities added through the years by wind-blown dust, erosions, etc., bentonite is somewhat similar in compositional percentage-profile to pumicite (a caution: not to equate the two since pumicite does not contain montmorillonite nor crude fossil-shell-bone remains). Common to ocean floors and (old, very old) lake bottoms. <FONT SIZE=2>see <B><I>pumice</I></B></FONT>
</note>
</notes>
</material>
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