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Both of these bodies are 70:30 kaolin:feldspar. The kaolin on the left (Tile #6) contains finer particles and some natural bentonitic material, giving it higher plasticity and better ability to absorb drying stresses. The EPK on the right is coarser and has less ability to redistribute stress. As the mugs dry, circumferential stresses develop near the base. The body cannot stretch enough to accommodate them, so cracks form around the bottom (or, as happens here, leave residual stresses that reveal later in bisque firing). The Tile #6 body, having greater “stress-relief plasticity”, can tolerate the strain and dries without cracking.
Is it true that during forming, the larger kaolin particles line up concentrically to the center, producing more shrinkage along radius lines than along tangents? Not likely. Clay platelets do orient during forming, but the orientation tends to be parallel to the forming surface, not radially oriented toward the center. Further, shrinkage alone does not predict cracking; uneven shrinkage does. Despite efforts to prevent it, we could not eliminate it.
While these specific cracks appear during our bisque firing, as noted, the cause originates earlier during drying. Uneven shrinkage creates residual tensile stresses in the clay. While the dried ware may appear sound, the body weakens during early firing when organic materials burn out and particle bonds weaken, releasing the stored stresses as the circumferential crack.
| Materials |
Kaolin
The purest of all clays in nature. Kaolins are used in porcelains and stonewares to impart whiteness, in glazes to supply Al2O3 and to suspend slurries. |
| Materials |
EP Kaolin
A kaolin that gels slurries (thus handy to suspend ceramic glazes). It is plastic and fires white enough that it is also valuable in porcelain bodies. |
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