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Bone china is a type of porcelain that is composed of bone
ash, feldspathic material and kaolin. It has been defined as 'ware with a
translucent body containing a minimum of 30% of phosphate derived from animal
bone and calculated calcium phosphate. Developed by English potter Josiah
Spode, bone china is known for its high levels of whiteness and translucency,
and very high mechanical strength and chip resistance.
History
The first development of what would become known as bone
china was made by Thomas Frye at his Bow porcelain factory
near Bow in East London in 1748. His factory was located very close to
the cattle markets and slaughterhouses of Essex,
and hence easy access to animal
bones. Frye used up to 45% bone ash
in his formulation to create what he
called ‘fine porcelain.’ Although in quality it rivaled porcelain imported from
Europe and China
the factory was not a commercial
success.
Later, Josiah Spode in Stoke
further developed the concept, and finalized his formulation sometime between
1789 and 1793. Amongst his developments was to
abandon Frye’s procedure of calcining the bone together
with some of the other body raw materials, instead calcining just the bone.
Bone china quickly proved to be
highly popular leading to it being
introduced by other English pottery manufacturers. Both Spode's formulation and
business were successful: his formulation of 6 parts bone ash, 4 parts china stone and 3.5 parts china clay remains the base for
all bone china still, and it was only in 2009 that his company, Spode, went into receivership before eventually being purchased by
Portmeirion.
From its initial development up to
the twentieth century, bone china remained almost exclusively an English
product, with production being effectively localized to
Stoke-on-Trent.
Production
The production of bone china is similar to porcelain, except more care is needed because of
its lower plasticity and a narrower vitrification range. The traditional
formulation for bone china is about 25% kaolin, 25% Cornish stone and 50% bone ash. The bone ash that is used in
bone china is made from cattle bones that have a lower iron content. These
bones are crushed before being degelatinised and then calcined at up to 1250°C to
produce bone ash. The ash is milled to
a fine particle size. The kaolin component of the body is needed to give the unfired body plasticity which allows
articles to be shaped. This mixture
is then fired at around 1200°C. The raw materials for bone china are
comparatively expensive, and the production is labour-intensive, which is why
bone china maintains a luxury status and high pricing.
Bone china consists of two crystalline phases, anorthite
(CaO.Al2O3.2SiO2) and ß-tricalcium phosphate (3CaO.P2O5) embedded in a
substantial amount of glass.
Characteristics
Compared to most
porcelain bodies, bone china has superior whiteness and translucency. Also, its
high strength allows it to be
produced in thinner cross-sections than other types of porcelain.
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Berta Hedstrom 