Firing and vitrification
Bone china is fired hot first and cooler second — the reverse of hard-paste porcelain. That order shapes everything.
Most of what makes bone china special happens in the kiln, in the first and hottest firing.
The first firing: to vitrification
Bone china is usually fired twice. The first, hotter firing — typically around 1,200–1,280 °C, depending on the body — is done before glazing. The china stone melts into glass, the clay and calcium phosphate react to form new crystals, and the body shrinks and becomes dense. Almost all its porosity disappears. This is vitrification: the ceramic becomes glass-bonded and essentially non-absorbent.
The second firing: the glaze
The vitrified piece is then glazed and fired again at a lower temperature, so the glaze melts into a smooth, glassy skin without disturbing the body. Decoration often needs a third firing, lower still.
Why pieces need support
At the top of the first firing, bone china softens appreciably. Left unsupported, a plate would sag or warp. Makers fire flatware on shaped supports called setters and keep the firing within a narrow range: too cool and the body is under-fired and weak; too hot and pieces distort. It is one of the main reasons bone china is demanding to make.
Look closer: shrinkage
A bone china body shrinks noticeably between the mould and the finished piece. Every mould has to be made oversize by exactly the right amount, and every shape behaves slightly differently. Getting a set of plates to stack neatly is a real achievement — one that makers such as Narumi single out in describing their own work.