People dig up kaolin clay from under the ground. They mix it with water and use it to make porcelain. Porcelain is a kind of pottery that is very white and glassy. It is so glassy that light shines right through it. Even though there is plenty of kaolin clay in North America and South America, Europe, West Asia, and India, at first the only place where people knew how to make porcelain was China. Everywhere else in the world people had to buy their porcelain from China, and it was very expensive. That’s why potters in the Islamic Empire invented lead glazes – so they could make white cups that looked like porcelain but were much cheaper. Today we still use white glassy glazes on most of our dishes.
Bibliography and further reading about clay and pottery: