Porcelain is made from about 50% kaolin, 25% feldspar and 25% silica and fired at 1,260-1,400 °C, hot enough for the feldspar to melt into a glass that fills the pores between the clay and quartz grains. That vitrified structure makes it waterproof, dense, chemically resistant and an excellent electrical insulator. The cost is a body that goes soft near its firing temperature and shrinks, so pieces must be supported and the kiln held steady.
Part of Advanced Ceramics
Porcelain is a high-fired vitrified ceramic body of kaolin, feldspar and silica, used for fine ware, laboratory vessels and electrical insulators.
Three Ingredients Do Three Different Jobs
The Advanced Materials article’s recipe is 50% kaolin (china clay), 25% feldspar and 25% silica by weight. Each does something specific.
- Kaolin is the plastic clay that holds the shape while wet and supplies alumina and silica.
- Feldspar is the flux. It melts first and, on cooling, becomes the glass that bonds everything.
- Silica is the skeleton, grains that stay solid and keep the piece from slumping.
The structure that results is crystal grains of quartz, mullite and clay residue held in a glassy matrix with few open pores. A less-fired earthenware is porous and absorbs water, while porcelain barely does. Flux Materials and Silica Sources cover the raw materials. Feldspar can be replaced in part by plant ash flux (see Plant Ash Flux) but then the body needs testing, because impurities, especially iron, change colour and melting.
Firing Is A Controlled Climb
Fire to the Advanced Materials article’s 1,260-1,400 °C, and judge temperature with test pieces and cones as in Pyrometric Cones and Firing Temperatures, which puts firebrick at 1200-1400 °C. Features to watch:
- Early heat. Raise temperature slowly while water and chemically bound water leave. Quick heating bursts the piece.
- Vitrification. As the flux melts the body shrinks and densifies. Test a small piece for water absorption: it should absorb almost nothing.
- Too hot. The glass fraction grows and the body softens, so thin parts sag or deform. Support them and do not overshoot.
- Cooling. Cool slowly through the range where the glass sets, to avoid built-in stress.
Glaze is a thin glass fused to the surface (see Glaze Materials) that makes the piece smooth, easy to clean and better at shedding water.
Worked Example: Batching A Test Body
For a 2 kg test batch at the Advanced Materials article ratio, kaolin is 0.50 × 2,000 g = 1,000 g, feldspar 0.25 × 2,000 = 500 g and silica 500 g. Make three variants (say 20%, 25% and 30% feldspar), fire them all together and compare absorption and sag. The best body for your local materials comes from the test, not from the first recipe.
Silica dust and kiln heat
Dry kaolin, feldspar and silica make fine dust that causes silicosis, a permanent lung disease. Mix wet or use a mask, and wipe surfaces damp rather than sweeping. Kilns at these temperatures burn skin and ignite wood at a distance, so keep the area clear and open the kiln only when cool enough to handle.
Common Mistakes
- Underfiring — the body stays porous and fails as an insulator or vessel.
- Overfiring — the piece slumps.
- Skipping the water test — the porosity problem only shows later in use.
What’s Next
- Learn next: Electrical Insulators — porcelain for voltage
- Combine with: Chemical Vessels — porcelain for acids