660 KLEIN: CONSTITUTION OF PORCELAIN 



formation of amorphous sillimanite mainly and but little crys- 

 tallized sillimanite, while the quartz is undissolved and the 

 grains may still be of considerable size, up to 0.2 mm. or more, 

 depending upon the fineness of grinding. 



By burning these bodies at 1380° to 1400° the feldspar glass 

 dissolves considerable quartz, there being only a comparatively 

 small amount of residual quartz remaining. The quartz grains 

 are much rounded and etched and they seldom show a length 

 over 0.06 mm. The clay is dissociated with the formation of 

 crystallized sillimanite, although an extremely small amount of 

 amorphous sillimanite may be present. 



The changes involved by burning commercial bodies are 

 identical with those of laboratory prepared bodies. The quartz 

 grains observed in whiteware and in low-fired vitreous ware 

 are large and angular, showing a size of 0.2 mm., or more, 

 whereas in the hard porcelains, due to solution, the quartz 

 grains are rounded and etched, and seldom exceed 0.05 mm. 

 in length. 



The constitution and the microstructure of porcelain depend 

 upon the temperature of burning, and change as this temperature 

 changes. This has served as a basis for the estimation of the 

 probable burning temperatures of the commercial bodies, a 

 fact which was accomplished with success, the error involved 

 being within 25°. It appears that the time-of-burning factor is 

 by no means as important as that of the burning temperature 

 in determining the constitution and microstructure of the ware. 



No cristobalite or tridymite has been definitely observed in 

 any of the laboratory or commercial bodies examined. It ap- 

 pears that the quartz dissolves in the feldspar glass more readily 

 than it inverts to the oMier modifications of silica. 



In conclusion, it may be stated that the petrographic micro- 

 scopic study of porcelain has led to interesting and, it is to be 

 hoped, important technical results. It has placed the chemical 

 and physical processes involved in the formation of porcelain 

 on a more quantitative thermal basis. Furthermore it has 

 offered a means of estimating the burning temperature of a ware 

 by an examination of a fragment much too small in size to be 

 satisfactory for even a chemical analysis. 



