Objects made from clay and hardened by exposure to high temperatures are commonly called pottery. At temperatures of around 500°C chemical changes take place in the clay, causing it to harden and lose its plastic properties. At temperatures of over 1600°C clay vitrifies and is therefore no longer porous.
The three main types of pottery are earthenware, stoneware and porcelain, the differences between them being the firing temperature, clay composition and opacity.
Earthenware is the term used for nonvitreous pottery that is both coarse and porous; this type of ware is sealed with a glaze to make it waterproof. When such materials as silica are added to the clay and firing is at around 1200°C, the resulting pottery is called stoneware. If china-clay (kaolin) and feldspathic china-stone (petuntse) are used and fired at a higher temperature, the ware is called porcelain and is usually translucent. However, the dividing line between stoneware and porcelain is vague.
Forming and Hardening Clay
Until the invention of the potter's wheel in around 3000 BC pottery was made by hand.
The clay was pressed into shape either by hand or in a mold, being formed from slabs or built up by coiled layers that were later smoothed. These ancient techniques are still used today. More regular shapes can be achieved by the use of a potter's wheel, which is a circular table that is turned by a foot pedal or, nowadays, an electric motor. The clay used must be free of impurities and has to be kneaded to remove air bubbles.
A lump of wet clay is 'thrown' onto the wheel and 'centered'. While being rapidly rotated the clay is hollowed in the center, and the sides are 'raised' by the hands until it reaches the correct height. The potter's wet hands smooth the object while it is spinning.
The pottery is dried until hard and then placed in a kiln to be 'fired'. Early kilns were simply pits in which the objects were placed and a fire was built over them; later oven-style kilns were introduced. When pottery has been fired once it is called 'biscuit'.
Decoration is applied to the clay body by various means. The earliest and simplest decorations were pressed or incised designs worked into the raw clay and other techniques included molded decorations, which were applied separately, carving and piercing. In addition, the body of the pot could be covered by a slip, a cream-like mixture of clay and water, either white or colored, with adhesive as well as decorative properties. The most common decorative treatment for pottery is glazing. A finely ground glass-like powder, usually mixed with water, is applied to the 'biscuit' pot, which is then refired to fuse the vitreous particles into a layer that seals the pores of the clay.
Different oxides, such as lead and copper, are used to give pottery color. The colors vary, depending on heat and the flow of oxygen during firing. Pottery can also be painted, usually after glazing, when it must then be refired; douse firing prevents smudging, which often happens if the pottery is painted before firing. Much of the finest ancient and modern pottery (for example, that of classical Greece and seventeenth and eighteenth century Europe) was decorated in this way. Pottery is often marked with a symbol to represent the artist or factory. Marks on Chinese porcelain usually indicate the dynasty and emperor's name, whereas fine European porcelain usually bears factory marks, such as the crossed swords of Meissenware. Some Greek vases were actually signed, occasionally with the names of the potter and vase painter.
Development of Different Types of Pottery
The art of pottery-making dates from prehistoric times and has been used in most parts of the world. Blue and green glazes were in use in Egypt by about 1500 BC and glaze was in use in China by 200 BC. Porcelain was invented by the Chinese at the time of the T'ang dynasty (AD 618-907). A mixture of white china clay and feldspathic china-stone, it was fired at around 145°C, becoming vitreous and translucent. Occasionally Chinese porcelain found its way to Europe and was highly prized but until the end of the seventeenth century Europeans had not been able to imitate it.
A 'soft' porcelain in imitation of the Chinese porcelain was created; it was made of clay and ground glass and fired at around 1200°C. This technique was invented in around the twelfth century, probably in the Middle East, and found its way to Europe. The secret of true hard porcelain was finally rediscovered in Germany in 1708 by Grafvon Tschirnhaus and Johann Bottger and they began to manufacture their porcelain near Meissen, Saxony. When this formula reached England Josiah Spode the Second added bone ash to the mixture and created bone china, which is stronger, more resistant to chipping and whiter than European porcelain.