Clay is the material that makes pottery and ceramics possible. Clay is made mostly of alumina, silica and water, along with smaller amounts of other materials. It was formed during the weathering of feldspathic rocks over the ages, and organic acids acting on the clay particles as vegetation decayed.
Properties of clay
Clay has two key properties that give its utility for ceramics:
Possibly the most important quality that distinguishes clay from other minerals and soil is plasticity.
Plasticity refers to the ability of the clay to take and hold the form that the potter gives it.
Clays that are easily molded without cracking are considered more plastic than those that will not bend easily.
2. The ceramic change at high temperatures
Equally important for being able to make functional pottery wares is the ‘ceramic change’. When fired at high temperatures, clays used for pottery become hard and permanently change their shape.
Some clays also vitrify, meaning they become glass-like, and will be waterproof. Other clays ‘mature’ which means they are permanently changed, but remain porous.
3 basic types of clay body
Clay dug straight from the ground may not be have the correct properties for making ceramics, potters usually will combine different kinds of clays or add different materials to our clay. These man made clays are called clay bodies.
Clay bodies are divided into three basic categories based on the fired density of the finished pottery. The terms earthenware, stoneware and porcelain are used with other properties of the clay (such as color or workability) and each of these terms refer to the amount of density or porosity of the fired works.
The most commonly used clay bodies fall into one of the following categories:
Earthenware is available in a wide range of colors from white to dark brown. It is usually plastic, matures at low temperatures, but remains porous.
The term earthenware can be used to describe any type of clay which has a ten-to-fifteen percent absorption rate after it has been fired to maturity.
Earthenware is the most commonly found clay in nature and is the raw material usually used to make tiles, bricks, and most of the pottery in the world. Of the three types of clay it is the most porous and soft because it has the lowest firing temperature.
It has a high proportion of iron and mineral impurities which means it matures between 1300°F/ 705°C and (2120°F/ 1160°C (Cone 018 to Cone 3). These impurities also make this clay look brown, red, gray or green. When it is fired it can be from red or tan to brown or black.
Stoneware is available in various colors. It is usually plastic, and vitrifies at medium to high temperatures.
Stoneware clays are named this because when fired they have the characteristics of stone, which is a hard, dense surface sometimes with a variegated grayish brown color.
Because they are fired to temperatures from 2100°F/ 1150 to 2327°F/ 1275°C (Cone 3 to Cone 11), they have an absorption rate of only two to five percent. It is usually completely leak proof after firing to maturity.
It is tough and forgiving during both the throwing and firing stages, and is mainly used for industrial ceramics.
The higher the firing temperature of the stoneware, the more durable the product is. The fired pottery ranges in colors from tan or light gray to brown or dark gray. Because of it’s durability it is an excellent choice for dinnerware or other everyday wares.
Porcelain is white, relatively non-plastic, and vitrifies at high temperatures.
Porcelain is the product of different techniques of many early Chinese pottery workers. Its main ingredient is kaolin, also know as china clay.
Kaolin has a melting point of 3275°F/ 1800°C and is difficult to form because of low plasticity. However, the addition of other clays makes it a dense, hard, white and translucent clay known as porcelain.
This allows it to be fired to maturity at a lower firing temperature. Usually porcelain is fired at 2300°F/ 1260°C (Cone 9), or as low as 1900°F/ 1040°C (Cone 04) with the addition of other ingredients.
Porcelain clay is composed of extremely tiny particles, so it has the highest level of smoothness and can be worked even when extremely thin to create delicate translucent pieces.