porcelain

Porcelain is the fine white ceramic of good teacups, bathroom sinks, and dentures, the aristocrat of traditional clay wares. It began in China (hence the everyday name china) and is prized for being white, hard, ringing when tapped, and, when thin, faintly see-through. What makes it special is that it is fired hot enough to partly turn to glass inside, sealing itself so that, unlike ordinary earthenware pottery, it holds water without a glaze.

It starts as a wet mix of three traditional ingredients: clay (kaolin, which makes it plastic and workable when wet, and white), feldspar (which melts during firing to form glass), and quartz (which gives a rigid skeleton). You shape it, dry it, then fire it very hot, around 1200 to 1400 degrees C. During firing the feldspar melts into a glassy liquid that flows around and fuses the other particles, while long needle-like mullite crystals grow through it and knit everything together. The result is a dense body that is part crystal, part glass, a natural composite. This partial melting and sealing is called vitrification.

So porcelain sits neatly between two worlds: it is a traditional clay-based ceramic, yet its strength and translucency come from the glass phase locked inside it. That glass is also its weakness: like all ceramics it is hard and stiff but brittle, and it chips and cracks from tiny surface flaws with little warning. Compared with modern engineering ceramics like alumina it is far weaker, but it is cheap, beautiful, chemically inert, and easy to clean, which is why it still tiles our bathrooms and sets our tables.