• Materials for Ceramics

HPM-20 Volclay Bentonite

Alternate Names: Volclay HPM-20

Description: Microfine Sodium Bentonite

Notes

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).

Related Information

Two bentonites that should not look this similar

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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.

This bentonite fires almost black.

So why put it in a porcelain?

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Only a small addition of this material, often 1–5%, can dramatically increase the plasticity and dry strength of a porcelain or other clay body. Why not use one of the very white specialty plasticizers instead? Cost: they can be ten or even twenty times more expensive.

This is a typical Wyoming bentonite. It is so plastic that, to make these SHAB test bars without excessive drying cracks and warping, we mixed it 90:10 raw and calcined powders. These fired bars, cone 1 through 7 oxidation (bottom to top), reveal the baggage that comes with that plasticity. The material is iron-bearing and fires progressively from reddish brown to very dark brown as vitrification advances. At the higher temperatures, the surface becomes strongly glassy (because of the soluble salts it also contains). Yet, at an addition of only a few percent, its contribution of iron and other impurities to the total body is small enough that the loss of whiteness can be surprisingly modest. That is why an inexpensive, decidedly non-white bentonite can still be a very practical porcelain plasticizer.

Two practical ways to test incoming bentonite shipments:

Both dilute its extreme plasticity with a non-plastic material

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If you manufacture clay bodies, bentonite deserves more incoming-material testing than its small percentage in recipes might suggest. Because of its highly concentrated nature, bodies are disproportionately affected by variations in its plasticity, soluble salts, particulate contamination, fired color (and another property we will see in a moment). Consider why it is wise to accumulate test data to establish a reliable baseline against which to compare every shipment.

The top two bars are a Wyoming bentonite fired to cone 8 and 2 oxidation. But they are not made from the raw bentonite alone. The test mix is 15% raw bentonite and 85% of the same material after calcining. The calcined portion acts as a non-plastic filler, reducing the extreme plasticity enough that SHAB test bars can be formed and dried reliably. They provide a revealing picture of the fired properties as a pure material.

The bottom two bars use a different approach: A standardized dilution of 15% bentonite mixed with 85% silica. These make evident its effect on the fired characteristics of a pure white, refractory and non-plastic material. The loss of whiteness is obvious, and the lack of fired specks notable. But another thing still reinforces viewing this bentonite as a “controlled impurity” addition: These bars should be much more refractory, but they are porcelain-like, albeit with high porosity, the bentonite itself is acting as a body flux.

Bentonite powders compared in two cast crucibles

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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.

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