Microstructure and light
Under a microscope, fired bone china is a composite of fine crystals in glass. That structure explains both its glow and its strength.
The finished body of bone china is generally described as two crystalline phases — anorthite, a calcium aluminosilicate, and beta-tricalcium phosphate — embedded in a substantial amount of glass. Hard-paste porcelain, by contrast, typically contains needle-like mullite crystals and undissolved quartz in its glass.
How structure makes light
Light is scattered whenever it meets a boundary between materials of different refractive index — and most strongly at pores, because air bends light very differently from glass. Bone china’s near-zero porosity removes the worst scatterers. Its abundant glass carries light, and its fine crystals scatter it softly rather than blocking it. The result is translucency with a diffuse, warm glow.
How structure makes strength
Cracks start at flaws, and pores are flaws. A dense body has fewer of them. The interlocking fine crystals then help to deflect or arrest a crack that does begin. This is why a thin bone china wall can carry loads that would break a more porous body of the same thickness.
How the properties are measured
- Water absorption — a proxy for porosity; well-fired bone china is close to zero (EN 1217).
- Translucency — light transmitted through a sample of known thickness (EN 1184).
- Impact — EN 12980 and ASTM C368 for edge and centre impact.
- Crazing — resistance of the glaze to fine cracking (EN 13258).