Translucency
Hold a fine bone china cup to a window and you will see the shadow of your fingers through it. Here is why.
Translucency is the property most people remember. It is also the easiest to misunderstand: it is not a sign of thinness alone, and it is not the same as transparency.
Why light gets through
Light passes through a ceramic when three things are true: the body is dense, with almost no air pockets to scatter light; it contains a lot of glass, which carries light; and its crystals are fine enough not to block it. A thin wall helps, because light has less material to cross.
Well-made bone china meets all three conditions, which is why a thin cup glows. Earthenware and stoneware, by contrast, are opaque at ordinary thicknesses.
Hold it to the light
Translucent, and warm
Bone china’s colour is a separate effect. It typically fires to a warm, creamy white, while many hard-paste porcelains have a cooler, slightly blue-grey tone. Held to the light, bone china tends to glow golden rather than grey. Whiteness explains why.
Look closer: translucency is not a strength test
A highly translucent piece is not necessarily weak, and an opaque piece is not necessarily strong. Translucency tells you about the body’s density and glass content, and the wall’s thickness. Strength depends on the material, the shape and the rim. See Strength.
Translucency can be measured formally by shining a standard light through a sample of known thickness; the European method is EN 1184.