the alumina-silica system

If a ceramist could memorize only one binary diagram, it would be this one. Alumina (Al2O3) and silica (SiO2) are the two most abundant oxides in clay, and their diagram is the backbone of porcelain, stoneware, fireclay bricks, and most aluminosilicate refractories. Read left to right it runs from pure silica to pure alumina, and the whole story of how a fired clay body behaves at temperature is written along it.

The diagram has one crucial landmark: a single stable crystalline compound sitting in the middle, mullite (about 3Al2O3 to 2SiO2). On the silica-rich side there is a eutectic between cristobalite and mullite at about 1587 degrees C — a low, dangerous temperature, because a body that is even slightly silica-rich will start to form liquid there and can slump far below alumina's own melting point. On the alumina-rich side mullite meets corundum; mullite itself is usually shown melting incongruently at a peritectic near 1828 degrees C, decomposing to corundum plus liquid. The exact melting behaviour of mullite has been debated for a century (congruent versus incongruent, and the precise temperatures), a good reminder that even canonical diagrams have honest uncertainty.

The practical lessons fall straight out of the map. Because the silica-mullite eutectic is so low, high-temperature refractories are made alumina-rich: push the composition past mullite toward corundum and the first-liquid temperature climbs, so more alumina buys more refractoriness. Porcelain works because a controlled amount of eutectic liquid forms on firing to bond and densify the body, while needle-like mullite crystals grow to give strength; too much liquid and the ware deforms. From this one diagram you can read firing temperature, the liquid-versus-solid balance (with the lever rule), and the service ceiling of a whole industry's refractories.