Type of Clay for Ceramics
Clay is similar to bonsai potting mix in that there is so much variety in the ingredients and their proportions depending on the result you want, and your personal preference.
Most clays contain several different types of clay minerals with different amounts of metal oxides and organic matter.
Clay differs from the inelastic soil and fine sand because of its ability, when wet with the proper amount of water, to form a cohesive mass and to retain a shape. This quality is known as clay’s plasticity. When heated to high temperatures, clay also partially melts, resulting in the tight, hard rock-like substance known as pottery or ceramics. (See the Ping Test post.)
There are different types of clay used in ceramics, and they can be characterised by the temperature the clay must be fired to in order for it to become mature, or reach its optimum hardness and durability.
The four most commonly used clay types are earthenware clay bodies ( fired at 950 to 1100 C), mid-fire stoneware clay bodies( fired at 1160 C to 1225 C), high-fire stoneware clay bodies( fired at 1200 C to 1300 C), and porcelain clays which fire to maturity at about 1800 C.
Various additives can be used to change the properties of clay, for example additions of ball clay will make the clay more plastic, additions of iron impurities will make the clay speckled and addition of grog ( fired clay that is then ball milled into fine pieces) will make the clay textured and more resilient and some can add crushed volcanic rock such as trachyte which tends to melt and add strong specks and texture to a finished pot.
The type of clay body makes a big difference to the expression of a glaze. A white clay with few impurities will give a stronger, cleaner colour than the same glaze on a darker clay body containing iron and grog where the glaze expression will be more earthy and variegated.