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.

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Bentonite is a super-plastic clay. This block of it took months to dry, the material really holds on to its water! It shrunk to about half the size and, of course, broke up into many pieces in the process (because bentonite has such a high drying shrinkage). That white powder is calcium sulphate, it is soluble and comes to the surface with the water as the clay dries. The finer the manufacturer grinds the material, the more salts are liberated. In most ceramic applications for commercial raw bentonites, these soluble salts are not an issue (but the iron content certainly can be). The reason these salts can be tolerated is that bentonite is normally employed in bodies and glazes in the 1-5% range.

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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.

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It took weeks, in our dry climate, for this small sample to dry. It is pretty much impossible to slurry up and dewater this clay in the normal way. So to make the thick gel on the left I blender-mixed a slurry as thick as I could make it and then let it sit to evaporate the water. By the time it was non-sticky enough to handle it was too stiff to shape. To test bentonites, it is thus necessary to mix them with a non-plastic powder (e.g. silica), usually at 90% or more. However, we have a better way: Roast or calcine some of the betonite powder to destroy the plasticity and mix that with the raw (usually 85% calcine and 15% raw).
| Materials |
HPM-20 Volclay Bentonite
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| Materials |
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. |
| Glossary |
Efflorescence
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface. |
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