A super-fine, super-plastic wild clay that comes with baggage

#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.

Related Pictures

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.

Fired bars of a bentonite used in porcelains! Is this possible?

Fired bentonite test bars are dark brown

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 more!

Bentonite powders compared in two cast 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.

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