Bone China
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Bone China |
Stronger than hard-paste porcelain and easier to manufacture.
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Bone China is a hybrid hard-paste porcelain containing bone
ash.
The initial development of bone china is attributed to Josiah Spode, who introduced it around 1800.
The original basic formula of six parts bone ash, four parts china stone, and three and a half parts china clay remains the standard English body.
Hard porcelain is strong but chips fairly easily and, unless specially treated, is usually tinged with blue or gray. Bone china is easier to manufacture, is strong, does not chip easily, and has an ivory-white appearance.
Very soon the bone china was copies by Minton, Coalport, Davenport, Derby, Worcester, and the Herculaneum factory at Liverpool.
Later on it was used by New Hall in 1810, Wedgwood in 1812, and Rockingham in 1820.
The quality, as much as form and decoration, varied from factory to factory; some tended, after about 1820, toward brilliant colour, lavish gilding, and overcrowded design; others produced tasteful, simply ornamented tableware. Since much early bone china was issued unmarked, it is often difficult to attribute the pieces.
Bone china is extremely hard, intensely white and will allow
light to pass through it.
Strength is provided by the fusion of body ingredients during
firing. This unique English pottery body is made from the following: 50% animal
bone, 25% china clay, 25% china
stone. First or biscuit firing 1200 C - 1300
C. Second or glost firing 1050 C - 1100 C.
Porcelaneous ware was first made in China,
hence its common name china. Chinese porcelain is less vitrified (and
therefore softer) than its modern European counterpart, which was developed in
Germany in the early 18th century.
Josiah Spode II (1754-1827) introduced in his new bone
china pottery in c1797. This was to prove the English solution to the quest for
porcelain. Technically bone china is a form of hard paste porcelain because it
is a mixture of clay and another non-glassy material. The standard formula is
25% china clay, 25% Cornish stone, 50% bone ash. Bone china became the
English porcelain because - It is less liable to loss in firing than soft paste
porcelains which contain glass. The firing temperature is much lower (1250º C)
than for hard paste porcelain (1400º C). The potters could use their existing
methods and ovens. The brilliance of enamel colours and gold was greater than on
other porcelains.
It very quickly became a popular body for several
reasons - The diminishing trade with China caused by very heavy import duties on
porcelain (108% in 1799). Less merchant shipping available because of the need
to sustain naval and military forces overseas. The patronage of the Prince of
Wales, leader of taste at the beginning of the nineteenth century. The growth of
the professional and merchant classes provided a market. It was easy for
existing factories to convert to making bone china because the sequence of
processes as well as the glost and enamel firing temperatures were the same as
earthenware. Bone china is more difficult to use in a plastic state that
earthenwares, but quicker to cast with than earthenwares.
Calcined bone ash:
Calcined bone ash is used in the production of bone china and makes up about 50% by weight of the final body recipe. It is produced from animal bone, which is first processed to remove any adhering meat which is generally sold as pet food. The bone is then treated to remove glue, which is processed and upgraded for use in normal applications where glue is used, and also for the sizing of expensive paper. The raw bone which is left after the meat and glue have been extracted is then heated to about 1000 C at which temperature any residual organic material is burned off and the structure of the bone is changed to form suitable for the manufacture of bone china. The high temperature used also sterilises the bone. Prior to use the bone is finely ground with water before inclusion in the bone china body and it is calcined bone which gives traditional English bone china its translucency and whiteness.
questions / comments? email Steve Birks steveb@netcentral.co.uk