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 containing even small amounts are disproportionately affected by variations in its plasticity, soluble salts, particulate contamination, fired color and the degree to which it promotes vitrification.
The top two bars are a Wyoming bentonite fired to cone 8 and cone 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. The bottom two bars use a different approach: 15% bentonite mixed with 85% silica, fired here to cone 10 and cone 6. This standardized dilution makes it easier to see what the bentonite contributes to an otherwise non-plastic matrix.
That may sound like excessive testing for a material used at only a few percent. From the standpoint of fired whiteness imparted to porcelains, it behaves somewhat like a controlled impurity addition. We need to understand how impure it is. This becomes more apparent on the bottom two tests bars. They are 85% silica and bentonite is a clay, not a flux. They should be much more refractory. So, it appears the bentonite itself is, in fact, a body flux. This is an added reason to accumulate testing data to establish a reliable baseline and compare every shipment against it.

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#1: I got it in a foot-thick layer in a gravel pit in Leader, Saskatchewan (half way up the slope). I code numbered it L3822.
#2: The two lower bars are L3822, fired at cone 1 and 2 (at cone 2 the center Aero-chocolate textured material bursts out). The upper bars, 3822A, are a 50:50 mix with Pioneer kaolin (also cone 1 and 2).
#3: L3822A fired at cones 04,03,02,01,3 (top to bottom).
#4: 90:10 mix of M2, the mid-temperature red burning material Plainsman Clays uses, and L3822 (Cone 06,04,03,02,01 bottom to top). The 10% addition supercharges the M2 plasticity beyond what is practical to dry.
#5: The pure material, leather hard mugs (with exceptionally thin walls (because it is so plastic).
#6: The pure material cracks, only one of the 50:50 mix survived drying (even though they were dried over a period of weeks).
#7: Bisque fired 50:50 and pure material mugs.
#8: Cone 6 mug of the 50:50 mix. Notice it gases and clouds the clear glaze.
What is this clay? It is not a balanced material, but highly bentonitic. With lots of fluxes (like KNaO, CaO, MgO), it matures below what is possible with mixes of feldspar and kaolin. Its super fine particle size (and thus high surface area) enable imposing its maturity (even when mixed with a refractory material like kaolin). Treating this as a bentonite seems best, adding no more than 5% to improve body plasticity. Notice that it fires to a much nicer surface than the commercial raw bentonites shown below.

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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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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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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. |
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