Classification of clay into these three categories is based on the fired density of the finished wares. While the terms "earthenware," "stoneware," and "porcelain" are also used in conjunction with other characteristics of the clay, such as color or workability, each of these terms refers to the degree of density or porosity of the fired works. For example, the term "earthenware" can be applied to any type of clay which has a 10 to 15 percent absorption rate after it's been fired to maturity. To determine the absorption rate of a clay, you can perform the following test: Weigh the fired piece, boil it in water for two hours, dry with a towel, re-weigh it, and figure the percentage of weight gain (absorption) by using the formula which follows:
(Saturated weight - Dry weight)
DIVIDED BY Dry weight
Earthenware, the type of clay with the lowest firing temperature of the three, is relatively porous and soft. It is the most commonly found clay in nature, and is the raw material used to make tiles, bricks, and most pottery products found across the globe. Earthenware clay contains a percentage of iron and mineral impurities high enough for it to mature at firing temperatures from "bonfire" heat at around 1300 degrees F, up to about 2120 degrees F (cone 018-cone 3*). In its raw state, the presence of iron oxide makes this clay appear brown, red, gray, or greenish. When fired, it's anything from red or tan to brown or black.
Earthenware has a low shrink factor and is porous, leaky, and stains easily. Because of its porosity, earthenware is a good clay for making planters and oven steamers, yet would not work well for pitchers or vases.
Stoneware clays have an absorption factor of about 2 to 5 percent, and are usually fired to temperatures ranging from 2100 degrees to 2372 degrees F (cone 3-cone 11*). As one might guess, stoneware clays are so named because the fired wares exhibit stone-like characteristics: a hard, dense surface with often variegated color. Stoneware usually bonds well with its glazes, and should be completely leak-proof after firing to maturity. It is tough and forgiving during throwing and firing stages, and is often used for industrial ceramics.
Fired stoneware can yield pleasing colors ranging from buff or light gray to brown or dark gray. Because of its hard, tight qualities, this clay is an excellent choice for functional applications, especially dinnerware. The higher the maturing temperature of the stoneware, the more durable the product.
Porcelain, characterized by an absorption rate of 0 to 1 percent, is the product of many hundreds of years of development by early Chinese potters. Its main ingredient is kaolin, also known as china clay. By itself, kaolin has a melting point of around 3275 degrees F, and is difficult to form because of low plasticity. To achieve the dense, hard, white, translucent clay body known as porcelain, kaolin is mixed with other clays to achieve a high degree of workability and to lower the firing temperature. Typically, porcelain is fired at temperatures above 2300 degrees F (cone 9*), yet can be formulated for firing at temperatures as low as 1900 degrees F (cone 04*) by adding unusual ingredients.
Because it's composed of such tiny particles, porcelain offers optimum qualities of smoothness, and can be manipulated even when extremely thin to achieve translucent, delicate forms. However, it is difficult to work with during the throwing stage because of its low plasticity. It cracks and deforms easily during the firing stage, because its optimum density isn't achieved until nearly the melting point of the clay.
*Cone equivalents are based on use of large Orton cones during a firing with heat rise of 108 degrees F/hr.
Earthenware or stoneware clay can be found just about anywhere, but your best bet is to investigate nearby creeks or river beds. To determine whether or not the clay can be used for throwing, start by picking up a handful of moist dirt, then squeeze it in the palm of your hand. If it crumbles, it's probably not plastic enough for throwing. If it seems to hold together, it could be just what you're looking for. Dig up a bucketful and take it back to the studio for testing.
Your clay probably contains such unwanted ingredients as stones, grit, and roots. To clean, spread out on wooden boards and let dry in the sun, then break up with a wooden block or a mallet. Now mix the clay powder with twice as much water, letting slake for a few hours. Strain the mix through a 40-mesh sieve. Let it settle for a few hours, then pour off excess water from the top. Next, dry out the thick clay slip to working consistency by spreading it out on plaster bats, then wedging.
Now it's time for testing. Throw a few pieces on the wheel, and also cut some test strips. Measure and fire them at various temperatures, logging their characteristics. Measure again to check shrinkage. Note the workability of the clay, its fired color, its porosity (see article above), and the highest firing temperature before slumping. (To check for slumping, span a test strip across two previously fired test tiles.) Once you've determined the maximum firing temperature, try a glaze firing in that range to check glaze fit.
Test various clays until you've isolated at least two with the qualities you want. Because each clay varies in particle size, the smaller particles of one type of clay will fill in the gaps where larger particles bond together. This mixture of clays can then be used as 50 to 80 percent of the ingredients to make your final mixture, or clay body. Other ingredients should include 10 percent silica and up to 20 percent each of a feldspar and a filler.
To mix the clay body, again spread out your strained, raw clay on plaster bats, dry out completely, then pulverize to a fine powder (always wear a face mask to protect from the dust). Weigh out and dry-mix the clay ingredients with test proportions of silica, feldspar, and filler. Grog is an excellent filler because it reduces shrinkage, provides throwing strength, and minimizes warping and cracking. You can make your own grog by pulverizing some dried clay, then firing it to maturity in an unglazed, bisqued dish. The fired particles may be crushed again to the point where they will pass through a 30-mesh screen, yet rest on an 80-mesh screen. Add up to 10% grog to the dry clay body ingredients, mixing thoroughly.
Now add water to the clay mix and blend into a thick, smooth slip. Once again, dry on plaster or wood and wedge until it's ready for throwing. Store in tightly covered plastic bags in a cool, dark place. Allow to age as much as possible.
After trying out your clay, you may need to modify the mixture. If your clay body is too porous, alter the recipe by adding more feldspar. If it shrinks too much, add grog. Plasticity can be increased by adding 1 to 2% bentonite. If the clay body is too plastic, more filler can be added. To increase either density or porosity, try adding 5-20% more silica.
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