|Monthly Tech-Tip |
The process of mixing a plastic clay by hand before forming it. Similar to kneading of bread dough, it is considered an essential step by most potters.
Wedging soft, plastic clay is similar to kneading bread dough. Clay tends to set up over time. And the growth of mold can discolor the surface and cross section and create non-homogeneous stiffness across the matrix. Pugmills produce laminations in a slug of clay. The process of wedging loosens it (if it has stiffened) and evens it out. It is not uncommon for a 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 patterns between water and clay that develop over time. Wedging tends to line 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 it and make a non-wobbly piece during throwing. Likewise, hand-built pieces will warp less and dry more evenly when the clay is thoroughly kneaded before use.
Wedging is remarkably efficient at mixing clays (of color, texture, stiffness). This is especially so if the mass is halved and layered first. Each cut doubles the layers. So, doing it 8 times produces 256 layers. 20 times will produce 1,000,000 layers! Clay of completely different colors, even black and white, can be quickly hand wedged together and the process is so effective that no visible remnant of either remains. Or, plastic clays of very different stiffnesses can be blended to an intermediate state so completely that all tactile traces of the originals are gone. Even agglomerates of certain materials will disintegrate during wedging (if the clay is stiff enough).
Wedging is also important before using soft extruded clay. Pugmills create particle patterns that build in "laminations", these are hostile to survival of the drying and firing processes and for the strength of the ware. Of course, if clay is stiff (e.g. for sculpture or architectural pieces), it will not be possible wedge. Grog is normally incorporated to minimize the problem.
Soft and stiff slabs were inter-layered (giving eight layers total). Then the piece repeatedly cut in half and slammed downward to re-flatten. Eleven times (doubling the number of layers each time to get 16,536). Yet it is still not mixed! 30 seconds of wedging is all it took to finish the job. Wedging is a very effective mixing technique. A pugmill would easily mix this also.
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.
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.
Clay is Plainsman M370. This is part of the aging process and can appear on any clay, depending on the conditions of storage (especially if stored in a very warm place or one exposed to sunlight). On the right this material has been fired to cone 6. The spots, although dark in the wet state, are burned away during firing. Mixing these specks back into the interior of the slug can be done quickly by wedging (diminishing worries about any allergy issues).
The grogs shown here are Molochite 16/30, Christie Minerals STKO 22S and F65 Silica Sand. I wedged 150g of the molochite into 2300g of pugged porcelain (Plainsman M370). This calculates to 7.5% grog (based on 22% water content of the pugged clay). This produced the texture shown. Wedging a grog (or silica sand) into a soft clay body is easy, just sprinkle it on the table and wedge the clay over it. With each push it picks up more, the process is amazingly effective at quickly producing a homogenous material. If the clay is stiff, just moisten the aggregate. Knowing that pugged clays have about 20% water it is easy to calculate a grog addition: A 5 kg slug of clay thus contains 4 kg of powder. To add 10% grog you would add 400g. To add 10% of the total it would be 4000*10/90=444g. How much did this 7.5% grog reduce the drying shrinkage of this body? About 0.5%.
The brick-halves on the left cracked in two during drying, the crack opened at the center. I dried six of them and all cracked in the same way. The one-inch-slices were cut laterally from an extruded slug of clay and sun-dried. The radial pattern of the laminations are clearly visible on the break. These laminations are "a weakness" formed-into this extruded and unwedged clay, they would, of course, extend to fired integrity, weakening the piece. The halves on the right are from a brick that I made by first wedging (kneading) the clay, then forming and cutting it to size. It was likewise sun-dried. But did not crack. I broke it (with difficulty), notice the break followed the stresses of the breaking process, not internal lines of weakness.