The manufacturer of china-ware and porcelain requires china clay or kaolin which, in addition to the foregoing characteristics, shall be highly refractory. It must, therefore, be free from more than about 2 per cent. of lime, magnesia, soda, potash, titanic acid and other fluxes. It is a mistake to suppose that all white clays of slight plasticity are china clays or kaolins. Some _pipe clays_ have these characteristics, but they contain so large a proportion of impurities as to be useless for the purposes for which china clay is employed and are consequently of small value. Users of china clays and kaolins generally find it necessary to carry out a lengthy series of tests before accepting material from a new source, as such a material may possess characteristics not readily shown by ordinary methods of analysis, but which are sufficiently active to make it useless for certain purposes (see p. 143). =Pottery clays= are, as their name implies, those used in the manufacture of pottery, and comprise the china clays already mentioned (p. 104), the ball clays and the less pure clays used in the manufacture of coarse red ware, flower pots, etc. The _china clays_ (p. 104) are not used alone in pottery manufacture as they lack plasticity and cohesion. In the production of china-ware or porcelain they are mixed with a fluxing material such as Cornish stone, pegmatite, or felspar, together with quartz or bone ash. Thus, English china ware is produced from a mixture of approximately equal parts of bone ash, china clay and Cornish stone, whilst felspathic or hard porcelain is made from a mixture of kaolin, felspar and quartz, a little chalk being sometimes added. The _ball clays_ (p. 64) form the basis of most ordinary pottery, though some china clay is usually added in order to produce a whiter ware. Flint is added to reduce the shrinkage--which would otherwise be inconveniently great--and the strength of the finished ware is increased, its texture is rendered closer and its capability of emitting a ringing sound when struck are produced by the inclusion of Cornish stone or felspar in the mixture. Small quantities of cobalt oxide are also added to improve the whiteness in the better classes of ware. [Illustration: Fig. 18. Mining best Potter's clay in Devonshire. (_Photo by Mr G. Bishop._)] The ball clays are characterised by their remarkably high plasticity, their fine texture and their freedom from grit. They are by no means so pure as the china clays, and unless carefully selected can only be used for common ware.