Wedging clay is similar to kneading bread dough. Clay tends to set up over time and the growth of mold discolors the surface and cross section and create non-homogeneous stiffness across the matrix. The process of wedging it loosens the overall stiffer clay and evens out the stiffness, returning it to the stiffness at the time of production. It is not uncommon for the clay to soften quite dramatically on wedging, this is thought to occur because of the mobilization of water between the flat particles of clay and the disruption of a stable electrical charge pattern between water and clay that develops over time. Wedging also performs the function of lining up the flat clay particles concentric to the center of the mass allowing them to slip over each other more easily in that direction. When clay is wedged well before throwing, it is much easier to center the clay and make a non-wobbly piece during throwing. Likewise, hand-built pieces will warp less and dry more evenly.
Wedging is a very effective mixing method
16,000 layers of clay, but still not mixed! Soft and stiff slabs were interlayered, then the piece repeatedly cut in half and slammed downward to re-flatten (doubling the number of layers each time). Yet it is still not mixed! 30 seconds of wedging is all it takes to finish the job, wedging is a very effective mixing technique.
Another reason why clay should be wedged or kneaded
Left: A high-contrast photo of a cut across the cross section of an eight-month-old slug of Plainsman M370 pugged clay. Right: A cut of a just-produced material (which will exhibit the same pattern in eight more months). You can feel different stiffnesses as you drag your finger across this clay, these are a product of the aging process combined with the natural lamination that a pugmill produces. Clearly, the older material needs to be wedged before use in hand building or on the wheel.
Aged clay really needs to be wedged before use
This is a cut through an eight-month-old slug of pugged clay. The cut was done near the surface. The patchy coloration is a by-product of the aging process. If a slice of this was fired in a kiln, an even and homogeneous white surface would emerge, with no hint of what you see here. A few moments of wedging will mix the matrix and ready it for wheel throwing or hand forming.
Mold has appeared on the surface of an eight-month-old slug of potters clay (Plainsman M370). It is part of the aging process and can appear on any clay, depending on the conditions of storage. On the right a thin slice of the material has been fired to cone 6, showing that these marks are not iron. Mixing these specks back into the interior of the slug can be done quickly by wedging, greatly diminishing worries about any health issues involved with them.
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