Designing Pottery: Choosing the Clay

Generally speaking, there are three kinds of clay: earthenware, stoneware, and porcelain. Earthenware is the most fragile because when it's fired to maturity (the highest temperature before things start to go wrong, like blistering, slumping, etc.) it remains porous, or unvitrified. Stoneware and porcelain, fired at higher temperatures, vitrify when mature and, for the most part, are no longer porous. The clay body is tighter and this makes it more durable. Given the time I spend on each piece, I want my work to last as long as possible. I use stoneware.

The clays we use at Mudville are cone 6 clays, mostly from Laguna Clay. I use grolleg porcelain and a number of different stonewares, a speckled tan, an unspeckled tan, a cream, a white, two different reds and a black. The clay you choose makes a substantial difference in the end result.

Glazes can change color depending on the clay you're using. In the two pots above, the blue water glaze is the same. The pot on the left is porcelain and the glaze is a purer color; the pot on the right, a cream-colored stoneware, the glaze is more muted. The difference is analogous to coloring with crayons on a white piece of paper as opposed to coloring on brown kraft paper.

On these two pots, both glazed with the same cream-colored glaze, you can see the difference between using a speckled or unspeckled clay. The speckled clay gives the finished pot a more rustic appearance and the eye something else to focus on besides the design. This is good when the design is fairly open without lots of detail but can be counterproductive when you want a crisp, more refined look.

Working with more deeply colored clay allows you to use the clay as part of the design. White or very light clay gets dirty with use so it is usually covered with a glaze to protect the clay. With a darker clay, it's viable to leave it unglazed, giving a direct connection to the clay. Also, there is an interesting contrast between the smoothness of the glaze and the rougher surface of the clay. The pots above are made with two different red stonewares and one black stoneware. The black looks grey due to the studio lights. The challenge of using colored clay is that the glazes are strongly influenced by iron and other colorants in the clay. The glaze on the wild sheep pot (red clay) is the same as the glaze on the rabbit pot (black clay). You can see how the white glaze becomes tan on the dark clay.

Porcelain is a manufactured clay, never found in nature. It is extremely smooth, more difficult to throw, and can have a tendency to slump. Porcelain dries differently than other clays, going from wet to dry in a moment in time rather than gradually. The surface can be suddenly dry when the interior is still damp. This can lead to cracking as well as problems when attaching handles. I use white stoneware when I want a white background and am adding a handle, otherwise I prefer porcelain. There are some good white stonewares but the surface is inevitably rougher. Despite the problems, porcelain has no equal in providing a white background and a paper-smooth surface. Colors on porcelain can be intensely vivid, and the smooth surface allows me to add maximum detail, providing a tightness and sharpness, not attainable on stoneware clay surfaces.

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