Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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11.12 Bone China

Bone china is a particularly British body developed in the 18th century in the search for hard-paste porcelain. Various reasons are cited for the paucity of hard-paste porcelain in Britain, but the lack of refractory materials with a low enough iron content for kiln furniture, short-flamed British coal compared with continental brown coal and high fluorine content in Cornish stone discolouring wares all may have contributed. Bone china is more difficult to manufacture as its low clay content makes it less plastic, with a poor “green” (unfired) strength compared with hard-paste porcelain and the firing has to be more precise (within 20 degrees C). It is remarkable that they were able to achieve this accuracy in bottle kilns.

The exact formula for Thomas Frye’s first bone china in 1748 is not known, but the wide range of constituent proportions that has been used was quoted by chemical analyst P. P. Budnikow as: bone ash 20-60%, kaolin 20-45%, feldspar 8-22% and silica 9-20%. The British potter Josiah Spode carried out significant development of the bone china body, starting production in the late 18th century. Today bone china is almost exclusively a British product, and apart from that made by the Worcester factory, European hard-paste porcelain has been predominantly from the Continent.

The bones used in bone china are calcined at 900 to 1000 degrees C and then ground to produce particles less than 1 micron. The bone ash is mainly made up of calcium hydroxide and calcium phosphate. Cattle bones are used as they have less iron contamination. The role of calcined bone as a ceramic constituent is not fully understood, but it contributes to the high mechanical strength of bone china, the strongest of all ceramic tableware, and to its translucency, as well as being responsible for the particularly white colour of its body. An example of its strength was used as a marketing idea when a racing car was supported on four Wedgwood bone china teacups.

One formula used for Royal Worcester bone china was 50% calcined bone, 25% clay, 12.5% feldspar and 12.5% silica. To maintain a pure white body, crushed flint (especially Norwegian) is preferred instead of sand to provide the silica. The present Worcester recipe for bone china is 50% calcined ox bone, 25% China stone (feldspar) and 25% clay. The clay used is invariably kaolin. Today ball mills containing very hard high-alumina ceramic balls are used to grind the raw materials.

After forming, the walls of bone china vessels could be further reduced on a lathe, as it is capable of achieving a very thin strong body. Bone china was biscuit fired in the kiln for some 60 hours, with a final firing temperature of 1260 degrees C, when it shrank by about 17% and became hard, non-absorbent, white and translucent. It was necessary to have a soaking time of about 2 hours at the maximum temperature, but the body becomes very soft at this temperature and would slump if it did not have much more support than usual in the kiln. Moulds were used for support during firing and ground quartz was dusted in them to help to prevent the item from collapsing.

The vessels were then glazed and refired in another kiln at 900 - 1100 degrees C for some 24 hours – the glost firing. As bone china glaze needs to melt at a lower temperature it needs more powerful fluxes, usually compounds of boron and lead, although it permits a greater glaze colour range. The glaze components have to be fritted and subsequently reground into a powder before applying by dipping or spraying. The low porosity of bone china (0.2%) makes glazing difficult and spraying is preferred. Interestingly whereas glaze strengthens earthenware, as mullite crystals are formed in the pores, it slightly weakens bone china.

As mentioned earlier bone china is harder than soft-paste porcelain and more resistant to chipping than hard-paste porcelain, as it has a high content (around 70%) of fine crystals (much smaller than those found in hard-paste porcelain) and 30% glass. Although bone china is not suitable for cooking, it became very successful and is ideal for figures and fine table services.

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