History of pottery
Pottery is one of the oldest crafts. People in East Asia were making fired clay pots more than 15,000 years ago. Farming people needed pots to store grain and water. The potter's wheel came about 5,000 years ago, and the cities of the Indus Valley made fine wheel-turned pots. Porcelain was perfected in China about 1,000 to 1,400 years ago and later spread to Japan, India and Europe. Glazes and kilns slowly improved so pots could be hotter, harder and prettier.
Raw materials
A pottery body has three parts:
- Clay (like kaolin): makes the wet mix sticky and easy to shape.
- Silica (sand or quartz): gives strength and stops too much shrinking.
- Feldspar (a flux): melts a little and glues the grains together at a lower heat.
Typical porcelain body: about 50% kaolin, 25% feldspar and 25% quartz.
Making a pot: the process
- Prepare: clean, crush and mix the materials with water.
- Shape: throw on a wheel, press in a mould, or pour liquid clay (slip) into a plaster mould.
- Dry: slowly, so it does not crack. It shrinks.
- Bisque fire: about 900 °C. The piece is hard but still porous.
- Glaze: dip or spray a glass-making powder in water.
- Glost fire: the glaze melts into a smooth glass coat and seals the surface.
Earthenware and porcelain
Earthenware: fired at about 950-1150 °C. The body stays porous (soaks about 10-20% water by weight), is orange, red or buff, and chips easily. Matkas, flower pots and many tiles.
Stoneware: fired at about 1200-1300 °C. Dense and strong, grey or brown. Used for pickle jars and plates.
Porcelain: fired at about 1250-1400 °C. White, very dense, soaks almost no water, and gives a ringing sound when tapped. Thin pieces let light through. Used for tea sets, dinner ware and electrical insulators.
Try it
Try it at home. Weigh a dry broken piece of a clay flower pot. Soak it in water for an hour, wipe the surface, and weigh again. Water soaked (%) = (wet - dry) / dry x 100. Now do the same with a piece of a china cup. Which soaks more? In the 3D, drag the Firing heat slider and watch the number.
Key formulas and definitions
- Water soaked (%) = (wet mass - dry mass) / dry mass x 100
- Porcelain body: about 50% kaolin + 25% feldspar + 25% quartz
- Earthenware about 1000 °C, stoneware about 1250 °C, porcelain about 1300 °C
- Key terms: bisque, glaze, slip, flux, kaolin, porosity
Worked examples
1. A dry clay piece weighs 200 g. After soaking it weighs 224 g. Find the water soaked in percent.
(224 - 200) / 200 x 100 = 24 / 200 x 100 = 12%. This is earthenware-like.
2. A factory makes 400 kg of porcelain body with 50% kaolin, 25% feldspar and 25% quartz. How much of each?
Kaolin: 0.50 x 400 = 200 kg. Feldspar: 0.25 x 400 = 100 kg. Quartz: 0.25 x 400 = 100 kg. Total 400 kg.
3. Why does a glazed earthenware pot hold water but an unglazed one seeps?
The glaze melts into a thin glass coat that covers the pores. Glass has no pores, so water cannot get through. The unglazed body has open pores that let water seep.
Common mistakes
- Thinking glaze is paint. It is a glass layer made in the kiln.
- Saying porcelain is just painted earthenware. It has a different recipe and a much higher firing heat.
- Putting a wet pot into the kiln. Steam can crack it.
- Mixing up the two firings: bisque is before glaze, glost is after.