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HPM-20 Volclay Bentonite

Microfine Sodium Bentonite

Alternate Names: Volclay HPM-20

Oxide Weight767.95
Formula Weight806.67
If this formula is not unified correctly please contact us.
SAMG - Surface Area (m2/gm) 750
P325 - % Passing 325 Mesh Wet 99%
TRMN - Trace Minerals feldspar, quartz, calcite, gypsum
AVSC - Apparent Viscosity (cps) 12 cps minimum @ 6.25% solids
pHPW - pH for dry powder 8.5-10.5 @ 2% solids
P200 - % Passing 200 Mesh Wet 99.75%

HPM-20 is a micro-fine ground version of Volclay 325 bentonite. The extra grinding triples the price, thus this material is not commonly employed in ceramics (it is used as a suspending agent, viscosifier, binder, plasticizer and emulsion stabilizer). However, typical ground bentonites, like Volcay 325, can have more than 2% residue on the 200 mesh screen, this poses the possibility of fired specks in porcelain. The exceedingly fine HPM-20 largely removes that possibility. As a testament of how hard bentonite can be to grind, their data sheet still shows 1% passing 325 mesh. Also, although this material fires brown, even chocolate brown at higher temperatures, because it is extremely plastic little is required in the recipe. For example, if 2% is used it will add 0.1% to the overall iron content of the body, not usually enough to notice a fired color impact.

The analysis we provide here has been adjusted from that on the Allied Colloids data sheet. Their analysis totals 105 (including a 5% LOI), so we have corrected it to total 95) to make room for the LOI).

Natural bentonites fire to a surprisingly dirty surface!

Natural bentonites fire to a surprisingly dirty surface!

Pure HPM-20 micro-fine bentonite fired to cone 8 (top) and cone 2 (bottom) oxidation (it is actually a mix of raw and calcined material to make it possible to make the bars). Below that is an 85% silica:15% HPM-20 bentonite mix; they are fired to cone 10 (top) and 6 (bottom); these lower bars tell us the degree of plasticity imparted but also how much the bentonite is staining a normally paper-white burning material. HPM is a very expensive micro-pulverized bentonite, but, like other common bentonites, it still has significant iron. However note that much of the color on the top bars is from the soluble salts on the surface. These salts do not appear to come to the surface in the same way when mixed with the silica. It is very common to put these relatively dirty materials into porcelains to plasticize them. Why? The alternative is a material like VeeGum, it is 10-15 times the price! Still, if only a few percent of this is added, the color is affected less than you might think.

This is an expensive, quality bentonite! It is used in porcelains!

This is an expensive, quality bentonite! It is used in porcelains!

HPM-20 micro-fine bentonite fired from cone 1 to 7 in oxidation. This bentonite is expensive compared to others and it used for the guarantee that there are no speck producing particles. However it is still high in soluble salts (that melt by cone 4) and is very dark burning in color. It is not unusual to put 3-5% of this (and other dirtier bentonites) into Grolleg porcelain bodies (where whiteness is supposedly important).

Two bentonites that should not look this similar

Two bentonites that should not look this similar

The powders of HPM-20 bentonite (left) and National Standard 325 bentonite (right) fired to cone 6. Both have sintered into a solid mass. The HPM-20 is much more expensive because of the extra grinding done to make it micro-fine (for non-ceramic uses). However, its data sheet shows an Fe2O3 content double that of the National Standard material. That means the latter should be firing to alot lighter color. But they seem very similar.

Bentonite powders compared in two crucibles

Bentonite powders compared in two crucibles

HPM-20 Bentonite powder (left) and National Standard 325 (right) powder samples fired to cone 9 oxidation. These have shrunk alot since cone 6, but are not yet melting.

Out Bound Links

In Bound Links

By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="HPM-20 Volclay Bentonite" descrip="Microfine Sodium Bentonite" searchkey="Volclay HPM-20" loi="0.00" casnumber="52623-66-2"> <oxides> <oxide symbol="CaO" name="Calcium Oxide, Calcia" status="" percent="1.200" tolerance=""/> <oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="" percent="2.500" tolerance=""/> <oxide symbol="K2O" name="Potassium Oxide" status="" percent="0.300" tolerance=""/> <oxide symbol="Na2O" name="Sodium Oxide, Soda" status="" percent="2.300" tolerance=""/> <oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="19.300" tolerance=""/> <oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="64.900" tolerance=""/> <oxide symbol="Fe2O3" name="Iron Oxide, Ferric Oxide" status="" percent="4.500" tolerance=""/> </oxides> <volatiles> <volatile symbol="LOI" name="Loss on Ignition" percent="4.800" tolerance=""/> </volatiles> </material>

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