What is a ceramic?
A ceramic is a hard solid that is not a metal and not a plastic. It is made from earth materials such as clay, sand and limestone. Ceramics are strong when pushed, they do not rust and they stay solid at high heat. But they are brittle: they crack when hit.
Ceramics are made of atoms joined by very strong bonds. That is why they are hard and why they need a lot of heat to make.
The five steps from powder to product
- Raw materials. Clay, sand (silica), limestone, feldspar and other minerals are dug and cleaned.
- Grind and mix. The rocks are crushed to a fine powder. The right amounts are mixed.
- Shape. Water is added. The soft mix is pressed, poured into a mould, squeezed through a nozzle or turned on a wheel.
- Dry. Water leaves slowly. The piece shrinks.
- Fire. A kiln or furnace heats it. The grains fuse at their touching points. This is called sintering. The piece becomes hard and strong.
Glass is the exception: its raw materials are fully melted, then shaped while hot.
Four big product families
1. Pottery and tiles (clay products): cups, plates, tiles, bricks, toilet basins. Fired at about 1000-1400 °C.
2. Glass and enamel: bottles, windows, glass-coated steel pots. Melted at about 1400-1600 °C.
3. Refractories: special bricks that line furnaces. They keep their strength above 1500 °C.
4. Cement: a grey powder made at about 1450 °C. Mixed with water it sets hard.
Old ceramics and new (fine) ceramics
Traditional ceramics use natural clay and sand. They are cheap and made in huge amounts.
Fine (advanced) ceramics use very pure, man-made powders such as alumina, zirconia, silicon carbide and barium titanate. They are made with great care. They go into spark plugs, cutting tools, mobile-phone parts, sensors and medical implants. These are called functional ceramics when they do an electrical, optical or mechanical job.
Try it
Try it at home. Mix a handful of clay soil with water. Press it into a small flat cake and mark its length with a ruler. Dry it in the shade for two days. Measure again. How much did it shrink? Do not fire it at home. In the 3D, press the Kiln heat slider and see how strength rises with heat.
Key formulas and definitions
- Key words: raw material, kiln, firing, sintering, brittle, refractory
- Steps: powder -> mix -> shape -> dry -> fire
- Shrinkage % = (size before - size after) / size before x 100
- Typical heats: pottery 1000-1400 °C, glass 1400-1600 °C, cement about 1450 °C, refractory use above 1500 °C
Worked examples
1. A tile is 30 cm long when shaped. After drying and firing it is 27.6 cm. Find the shrinkage in percent.
Shrinkage = (30 - 27.6) / 30 x 100 = 2.4 / 30 x 100 = 8%.
2. A factory wants 90 kg of fired clay product. It loses 10% of its mass as water and gases during drying and firing. How much shaped clay does it need?
Fired mass is 90% of the start. Start = 90 / 0.9 = 100 kg.
3. Name the step in which grains of the powder fuse at their touching points. Why does it make the piece strong?
This step is sintering (during firing). The fused contacts join all grains into one solid, so the piece no longer crumbles.
Common mistakes
- Thinking "ceramic" means only pots. Glass, cement, bricks and electronic chips are ceramics too.
- Saying a ceramic is soft like clay. Clay is soft only before firing.
- Forgetting that drying comes before firing. Wet pieces crack or burst in the kiln.
- Thinking sintering means melting. In sintering the grains stick without fully melting.