18 GENERAL DISCUSSION OF CLAYS. 



cotta, they are frequently utilized either because no 

 others are available or to obtain a buff colored ware. 



Some soft body porcelains have an appreciable 

 amount of lime, much of the Hungarian containing 

 from five to fifteen per cent, of CaO.* The bone china 

 made in England at the present day also contains 

 lime and some white earthen ware manufacturers use 

 lime instead of feldspar. 



Much buff ware is now made from semirefractory 

 clays, which, on account of their low percentage of 

 iron, burn to a creamy color. 



The one objection to highly calcaeous clays is that 

 the points of incipient fusion and vitrification (see 

 Fusibility of Clays) lie so close together that it 

 is not safe to burn them hard without running 

 the risk of fusing them. Experiments 'have shown 

 however, that it is possible to separate these two 

 points, by the addition of quartz and feldspar to the 

 clay, of sand containing a large percentage of these 

 two minerals. 



In addtion to lowering the fusibility of clay, lime 

 also affects the fusion and absorptive power, thus 

 Segar found that limy or marly clays required us- 

 ually only twenty to twenty-four of water to convert 

 them from a dry condition to a workable mass, where- 

 as other clays needed twenty -eight to thirty per cent, 

 of water to accomplish the same result. In burning 

 the calcareous clays have not only their combined 

 water to lose, but also the carbonic acid gas, and con- 

 sequently the bricks are more apt to be light and po- 

 rous unless they can be burned to vitrification. The 

 shrinkage of calcareous clays is also less than that of 

 others, and it sometimes happens that this shrinkage 

 i*> not only zero, but that the brick even swells. 



Many clays contain lime in the form of gypsum, the 

 hydrated sulphate of lime. It generally results from 

 the action, on carbonaie of lime, of sulphuric acid set 

 free by the oxidation and leaching of pyrite in the clay. 



*Sprechsaal, 1896, p. 2. 

 gHecht, Thonindustrie Zietung. 

 % Thonindustrie Zietung, 1877, p. 131. 



