512 ' NEW YORK STATE MUSEUM 



I 



compounds or mineral species, which either already exist in the 

 clay or may be added to it artificially. The influence of fluxes in- 

 creases not only with the amoimt present but also with the state 

 of division, they being the more active the more finely they are 

 divided. If the flux is present in the form of large grains, 

 these grains will only exert a fluxing action on their surface, 

 whereas the single grains alone will act more like quartz grains, 

 that is, as diluents of the shrinkage. The minerals which may 

 be present as fluxes or may sometimes be added are mica, feld- 

 spar, and similar silicates, slags, lime carbonate, magnesia car- 

 bonate, and various compounds of iron and manganese. In addi- 

 tion they may be present as soluble salts. It is* usually the oxids 

 of iron, manganese, and complex silicates containing these as well 

 as lime, magnesium, potash, and soda that determine the degree of 

 fusibility of the clay. 



The amount of fluxes which a clay contains has an important 

 bearing on its applicability. For some purposes it is desirable as 

 well as necessary that the percentage of fluxes 'should be low 

 (producing refractory wares), not only with a view to^ refractoriness, 

 but also, as in porcelain or white earthenware manufacture, to 

 prevent discoloration of the ware. Again, the combination of 

 fluxes in large amount may be desirable for the production of a 

 vitrified body, such as is required for paving brick or sewer pipe* 



In kaolins the amount of fluxes may be as high as Ifo provided 

 they do not exert a coloring action. Some of the best kaolins known 

 contain about 35^ of feldspar, which means about 5.5^ of potash. 

 In fire clay 4-5^ is the permissible limit, depending on the physical 

 properties of the clay, while in a paving brick clay the total of 

 fluxes may run as high as 16^. 



Alkalis 

 These are never present in a clay in the form in which they axe 

 determined in the ordinary quantitative analysis, but generally as 

 a constituent element of one or more minerals. Clay may contain 

 two classes of alkalis, flxed and volatile. The former are soda, 

 potash and lithia, the latter ammonia. 



