CLAYS OF NEW YOKK 563 



on the clay as a mixture of fused particles, with a skeleton of un- 

 fused ones. If tlie proportion of the former to the latter is very 

 small, there will be a strong hardening of the clay with little 

 shrinkage, and the burned clay will still be porous. With an in- 

 crease of temperature, and the fusion of more particles, the pores 

 fill up more and more, and the shrinkage goes on till at the point 

 of vitrification the spaces are completely filled. Above this point 

 there is no longer a sufficiently strong skeleton to hold the mass 

 together, and the clay begins to flow. 



The conditions which influence the difference in temperature 

 between vitrification and viscosity still remain to be satisfactorily 

 explained, but it probably depends on the relative amounts of 

 fluxes and nonfluxes, and the size of grain of the latter. 



The preservation of form in burning is primarily dependent on 

 the refractoriness of the mineralogic components which are pres- 

 ent in the greatest quantity, because these build a framework or 

 skeleton. In kaolins and some refractory clays this component is 

 the clay substance. 



A feldspar percentage aids the fusion above a certain tempera- 

 ture. At high temperatures the quartz tends to increase the fluid- 

 ity of the fused clay, but at lower temperatures the quartz is to be 

 classed with those components which aid in preserving the form, 

 and in low grade clays the quartz has an important office in this 

 connection. 



The recent experiments of Hofman lead him to believe that size 

 of grain does not influence the refractory qualities of a clay (Trans. 

 Amer. inst. min. eng. 0,ct. 1898), and in the case of fire clays 

 tested by him this seems to be true. 



