Digitalfire Ceramic Materials Database

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Alternate Names: Montmorillonite, Bentonite USA

Oxide Weight707.76
Formula Weight786.40

Bentonites are classified according to several types (e.g. calcium, sodium, potassium) but in the ceramic industry sodium bentonite is the material of commerce (this is the same material used by the drilling industry). Sodium bentonites have incredibly small particle sizes (and thus surface areas). Combine this with the active particle surface chemistry (which makes them hold onto water) and 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.



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 it 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 much different effects on the plasticities.










White firing bentonites: There are a number of white firing and highly refined (and highly expensive) bentonites produced for the ceramic industry (and others). However they can have less plasticity. Testing is needed to determine if the plasticity and plastic character is sufficient and if the extra cost is worth it. 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 can have up to 0.5% iron. 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. If you used only 2.5% bentonite the extra iron may not be an issue at all.



Out Bound Links

In Bound Links

Example of the oversize particles from a 325 mesh screen (left) and 200 mesh (right) fired to cone 8. This bentonite (Volclay 325) has 2% residue on 325 and 1% on 200.

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Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and dry shrinkage vary widely. Materials normally acting as fluxes are refractory.

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Plainsman P580 (35:17:1.5 ball clay:kaolin:bentonite), H570 (10:46:2.5), P700 (50:5 Grolleg:bentonite) and Crysanthos Porcelain (China) fired in oxidation at cone 10.

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A comparison of the plasticity of Volclay 325 Bentonite:Silica 25:75 (top) and Hectalite 200:Silica 50:50. Both are mixed with silica powder. The latter (a highly refined bentonite) is much less plastic even though it is double the percentage in the recipe.

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An example of a DFAC drying test of a bentonitic clay. This disk has dried under heat (with the center part protected) for many hours. It is very reluctant to give up its water and shrinks alot during drying.

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A few drops of water on top of a tiny pile of bentonite powder. Notice the water just sits there, it does not soak in because the bentonite gels in contact with the water and that gel acts as a barrier.

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An example of how a dried piece of clay, left (having lower bentonite content) absorbs a drop of water faster. After 10 seconds (middle picture) it is beginning to disappear while the other is still wet. By 30 seconds (bottom) it is gone.

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By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="Bentonite" descrip="" searchkey="Montmorillonite, Bentonite USA" loi="0.00" casnumber="52623-66-2"> <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="LOI" name="Loss on Ignition" percent="10.000" tolerance=""/> </volatiles> </material>

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