Clay, hard-porcelain

If the feldspar and quartz content is increased over that in hard porcelain, soft porcelain is obtained which is fired at 100 to 150°C lower temperatures. Highly plastic clay (ball clay) is added to obtain plastic and easy to work pastes, despite the lower kaolin content. Sanitary porcelains for the manufacture of bathroom articles are soft porcelain articles. 

We stated at the beginning that this chapter is the pottery chapter. We will now summarize areas in which many of the techniques described above have been used, in some cases for milleimia, in pottery then we can do the same for glass. Classical porcelain can be as thin as a sheet of paper (<0.2 mm). Bone china, so called because even today it is made by adding -50% bone ash to a conventional hard-porcelain clay mixture, can be so thin that it is translucent. This ingredient is so critical that the UK imports bone ash from Argentina. 

Industrial applications. Filler for paper and board, coating clays, ceramics, bone china, hard porcelain, fine earthenware, porous wall tiles, electrical porcelain, semivitreous china, glazes, porcelain, enamels, filler for plastics, rubbers and paints, cosmetics, insecticides, dusting and medicine, textiles, and white cement. 

Hard Porcelain. True feldspathic porcelain (continental porcelain). Hard-paste Porcelain being the older term still used by collectors. The combined nomenclature (q.v.) defines hard porcelain as made from kaolin or kaolinitic clays, quartz, feldspar and sometimes calcium carbonate. It is covered with a colourless transparent glaze fired with the body. See porcelain. 

Ceramic compositions forming the basis for the whiteware industry are the mixtures of clay, feldspar, and flint. These compositions include hard porcelain for tableware, sanitary ware, electrical porcelain, and so on. 

Clay minerals are formed when igneous rocks weather. These minerals are the main constituent of fine-grained (<63 rm) particles in mud. In general these minerals are less cation-rich than their igneous precursors. Kaolinite has the simplest clay mineral formula because it is pure aluminosilicate. It is the mineral that held the secret to making porcelain, which was greatly valued by the emperors of China before AD 1000, after they discovered how hard and clear kaolin becomes when heated to 1300-1400 °C. Other, more complicated clay minerals, e.g. iUite and montmorillonite, have various amounts of cations added to their structures. 

A ceramic is defined as a material manufactured from a clay or similar substance and covers a wide range of cements, stoneware, earthenware, glass, porcelain, and silica. Ceramics are hard materials which are resistant to wear and chemical attack, but they are brittle and susceptible to damage when subjected to sudden changes in temperature. 

Stoneware. The combined NOMENCLATURE (q.v.) is Stoneware, which, though dense, impermeable and hard enough to resist scratching by a steel point, differs from porcelain because it is more opaque, and normally only partially vitrified. It may be vitreous or semi-vitreous. It is usually coloured grey or brownish because of impurities in the clay used for its manufacture, and is normally glazed . A translucency (q.v.) test is specified. Water absorption is to be 3 wt %. The body consists either... 

Hard-paste porcelains are obtained from mixtures made up almost exclusively of kaolin, quartz and feldspars. A little chalk (= 2% of the mass) can be added to favor the formation of the viscous liquid. This mixture is very similar to the one used to prepare fine earthenware. It differs from it only because of the almost exclusive use of kaolin as clay and the proportions of the various components. 

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