Bone china is a type of soft-paste 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.
From its initial development and up to the later part of the twentieth century, bone china was almost exclusively an English product, with production being effectively localised to Stoke-on-Trent. Most major English firms used it, including Mintons, Coalport, Davenport, Derby, Herculaneum Pottery, Wedgwood, and Worcester. In an English context, references to "china" or "porcelain" often actually mean bone china.
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 rivalled porcelain imported from Europe and China the factory was not a commercial success.
Later, Josiah Spode in Stoke further developed the concept, and finalised 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 its production 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, and it was only in 2009 that his company, Spode, went into receivership before eventually being purchased by Portmeirion.
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.
Its high strength allows it to be produced in thinner cross-sections than other types of porcelain.
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