Raw materials of ceramics
Raw materials are the natural minerals we start with. Each has a job.
- Clay (kaolin and other clays): gives plasticity. Wet clay can be shaped, and it holds the shape when dry.
- Quartz (silica sand): hard and stable. It forms a skeleton and cuts shrinkage and cracking.
- Feldspar: a flux. A flux melts at a lower temperature and glues the other grains together in the kiln.
Advanced ceramics use purer powders such as alumina (Al2O3) or zirconia. For them, purity and grain size matter even more.
Processing steps: crush, grind, sieve, clean
1. Crushing. A jaw or roll crusher breaks big lumps into small pieces.
2. Grinding. A ball mill is a turning drum with hard balls. The balls tumble and smash the pieces into powder. It can be done dry or with water (wet milling).
3. Sieving. A sieve with a known mesh size lets only fine grains pass. The coarse part goes back to the mill. This is called size classification.
4. Removing iron. A magnetic separator pulls out iron bits. Iron can turn the ware dark or spotty.
5. Mixing and preparing. The powders are mixed with water to make slip, or dried into free-flowing granules for pressing.
Batch calculation
A batch is one recipe weighed for one run. The recipe is given in percent by mass.
Mass of a material = (percent ÷ 100) × batch mass.
Raw clay often holds water. If you need a dry mass, the wet mass to weigh is wet = dry ÷ (1 − w), where w is the moisture fraction (20% = 0.20).
When a material is heated, some mass is lost as water and gas. This is the loss on ignition (LOI): LOI % = (mass lost ÷ starting mass) × 100.
Key formulas and definitions
- Mass of material = (% ÷ 100) × batch mass
- Wet mass = dry mass ÷ (1 − w) (w = moisture fraction)
- Percent passing = (mass passed ÷ sample mass) × 100
- Loss on ignition % = (mass lost ÷ starting mass) × 100
- All percents of a batch add up to 100%
Worked examples
1. A 200 kg batch is 50% clay, 30% quartz and 20% feldspar. Find the mass of each.
Clay = 0.50 × 200 = 100 kg. Quartz = 0.30 × 200 = 60 kg. Feldspar = 0.20 × 200 = 40 kg. Check: 100 + 60 + 40 = 200 kg.
2. A 400 g powder sample is sieved. 372 g passes through. What percent passed?
Percent passing = 372 ÷ 400 × 100 = 93%. So 7% stayed on the sieve and goes back to the mill.
3. You need 80 kg of dry clay. The clay holds 20% water. How much wet clay do you weigh?
w = 0.20. Wet = 80 ÷ (1 − 0.20) = 80 ÷ 0.80 = 100 kg.
4. 100 g of clay is heated and weighs 88 g afterwards. Find the loss on ignition.
Mass lost = 100 − 88 = 12 g. LOI = 12 ÷ 100 × 100 = 12%.
5. A 500 kg batch needs 40% dry clay. The clay has 10% moisture. How much wet clay is weighed?
Dry clay = 0.40 × 500 = 200 kg. Wet = 200 ÷ 0.90 = 222.2 kg.
6. A batch is 48% clay, 32% quartz, 20% feldspar. A plant makes 3 batches of 250 kg. How much feldspar is needed in total?
Total mass = 3 × 250 = 750 kg. Feldspar = 0.20 × 750 = 150 kg.
Common mistakes
- Adding percents that do not make 100%. A batch must total 100%.
- Forgetting moisture. Weighing 80 kg of wet clay gives less than 80 kg of dry clay.
- Thinking finer is always better. Very fine powder needs more water and can crack while drying.
- Skipping the magnet step. Iron specks stain the ware and cannot be removed after firing.