Porcelain is made by blending clay with other substances and firing it at high temperatures so that it achieves vitreous, or glasslike, qualities. True porcelain is slightly translucent, nonporous, and very hard, making it suitable for a range of applications, from plates to electrical insulators. There are three main categories of the material: hard paste or true porcelain, soft paste or china, and bone china. Each is made with different ingredients, resulting in products with varying qualities.
The concept emerged in China around the seventh century CE. The Chinese discovered that a mixture of kaolin clay and a substance called petuntse yielded extremely strong, sturdy pottery. Kaolin clay is a form of feldspar that is a very clear white, and petuntse is another form of feldspar that will turn into glass when fired at high temperatures. The kaolin clay is elastic and strong so that it holds the shape of the object during firing, while the petuntse fuses with the kaolin to finish the piece.
When porcelain was first brought to Europe, it attracted widespread interest. European potters began experimenting with a range of clays in an attempt to replicate the pottery, and after much experimentation, soft paste porcelain was produced. Soft paste was made by mixing a pale clay with glass, although many modern producers use kaolin and silica. When fired, it is a creamy color, and it is much softer than true porcelain.
Bone china is hard paste that has been mixed with bones and ash. It is hard and translucent like true porcelain, and it is closely associated with England and Germany, two major producers of the material. The addition of animal bones is believed to have arisen from confusion about how hard paste was made in China.
In all cases, the first step in making this pottery is grinding the ingredients down to a uniform small size. They are sieved to remove pieces that are too small or too large, and then a large magnet is run over the ground clay and rock to remove any traces or iron. Next, water is added to create a slurry that can be molded, cast on a wheel, or shaped in other ways prior to firing. Porcelain is bisqued first in a low heat kiln to set its shape and prevent shrinkage, and then it is glazed. After glazing, it is fired at extremely high temperatures to fuse the components, and then it is allowed to slowly cool.