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
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.
Exploding clay. How do you do that?
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The two terra cotta test bars at the top were fired at cones 01 and 02. They expanded internally and split open, revealing an “Aero chocolate bar” interior. This is severe bloating: As the clay develops enough glass to close its pore network, gases become trapped. If the body is soft enough to expand, but still viscous enough to hold the bubbles, it swells into a ceramic foam. Its denser outer skin can split as the interior expands. Native clays can contain a mixture of gas-producing constituents, and oxygen-starved interiors can develop different chemistry and melting behavior from their surfaces. The precise reactions depend on the clay and firing schedule.
Guess what happened when I mixed this clay 50:50 with Redart: The bloating disappeared, and the blend fired to a strong red stoneware at cone 02. Although also a terra cotta, Redart changed the balance between gas release and softening—apparently allowing the body to mature without puffing up. But what is a defect in a test bar can become an artistic effect. Clayton Bailey explored this in his “exploding” pots, shown below. To pursue it, start with a clay that shows a tendency to bloat and use small, supported tests to explore its firing range. The key is trapping gas while the clay is soft enough to expand, yet stiff enough to retain its shape. Clay composition, thickness and firing schedule all influence that balance.
How far can a terra cotta clay go before it melts down?
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These bars of a locally collected wild clay were fired, from bottom to top, at cones 2, 6, 8 and 10. At cone 2, the clay has already vitrified to stoneware density while retaining a reddish colour—well above the usual cone 06–04 terra cotta firing range. Additional tests, not shown here, reveal that it turns brown by cone 3 and begins to soften and bloat by cone 4. By cone 6, the second bar from the bottom has an “Aero chocolate bar” interior! Gas-producing reactions within the softening body expand it into a ceramic foam.
At cones 8 and 10, the bars are severely swollen and glassy, yet still recognizably bar-shaped. Why haven’t they flowed into puddles? Even with extensive melting, the material surrounding the bubbles remains viscous enough to resist collapse. This clay demonstrates that vitrification, bloating and flowing freely are very different stages of firing.
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