When kneading bread one is trying to get air into the dough. When wedging clay, you're trying to get the air out. They look similar, but the goal is opposite.
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Wedging is a process which is used to prepare clay for working. Without being wedged, clay will have an uneven texture which makes it hard to work with, and it could potentially develop problems as it is worked and fired. In extreme cases, clay which has not been wedged will explode in the kiln, ruining many of the pieces which are fired with the clay. In the case of shared kilns, this can attract ire from people with work which was ruined as a result of the explosion.
In the wedging process, clay is repeatedly kneaded on a porous surface to draw some of the water out while distributing the moisture evenly, eliminating hard spots in the clay. In the process, air bubbles are forced out of the clay, ensuring that these bubbles will not heat and cause the clay to explode in the kiln. Properly wedged clay is very smooth, with an even texture which is easy and pleasant to work with. People can also set clay up for specific tasks while they wedge, as for instance in the case of a conical wedge of clay which is ideally suited to working on the wheel.
Wedging can also be used to blend multiple types of clay, ensuring that the clay is thoroughly and evenly worked so that there are no patches and pockets of a particular type of clay. Potters who work with recycled and reclaimed clay often take special care over their wedging to confirm that the clay has been properly prepared.
Setting up a good space for wedging is critical. A counter or block is needed to work the clay on, and it should be positioned at a comfortable height for kneading the clay. Tables often work very well for this, although unusually tall or short people may need to find a more adjustable surface so that they can wedge without straining their arms, shoulders, and backs. In a pottery studio, a wedging table may have a slab of basalt, concrete, or similarly porous material so that clay can be worked directly on the surface, but a canvas sheet can also be used for wedging.
Everyone wedges slightly differently, but the goal is to knead and work the clay into a smooth mass. Some people slam the clay during the wedging process, while others prefer to create a churning motion with their hands which keeps the clay in constant rotation to press the air bubbles out. People who bake bread often wedge clay just like they knead. Some people utilize a technique known as wire wedging, in which the clay is cut, stacked, and slammed against the table several times, with this process being repeated several times to thoroughly blend the clay.
People who have just started to wedge clay may want to begin with two different colored pieces of clay and wedge them together. This will allow for frequent checks on the wedging process, as the mixture of colors will be a visual indicator of their progress. Once people get familiar with the texture and feel of wedged clay, they can work with a single color of clay.
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