|Monthly Tech-Tip |
Alternate Names: Volclay HPM-20
Description: Microfine Sodium Bentonite
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).
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.
Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times as much!
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.
Volclay SPV 200 Bentonite
National Standard 325 Bentonite
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
National Premium WT Bentonite
Big Horn CE 200 Bentonite
Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
Volclay HPM-20 Data Sheet
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
|% Passing 325 Mesh Wet||99%|
|Apparent Viscosity (cps)||12 cps minimum @ 6.25% solids|
|Surface Area (m2/gm)||750|
|Trace Minerals||feldspar, quartz, calcite, gypsum|
|pH for dry powder||8.5-10.5 @ 2% solids|
|% Passing 200 Mesh Wet||99.75%|
|By Tony Hansen|
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