FIRE GLAT8. 133 



the fusible impurities of a fire clay should not exceed 

 3i or 4 per cent., but these limits may be extended 

 somewhat in either direction depending upon the 

 nature of the flux and whether the clay is fine or 

 coarse grained. 



The shrinkage of a fire clay in burning may often be 

 counteracted by the addition of grog, i. e. sand, 

 ground fire brick, or similar substances. Fire clays 

 which are too fat and plastic are likely to crack in 

 burning, but at the same time they give a dense body. 

 It is desirable that any burned clay or grog which is 

 mixed with the raw material should have previously 

 been burned as dense as possible. Fine grains of pow- 

 dered grog permits the brick to shrink more in burning 

 than the course and bricks with the latter generally 

 stand changes of temperature better. Next to burn- 

 ed clay, quartz is perhaps the most important grog, 

 and flint clay serves a simila^" purpose. 



If a fire brick made only of clay and clay grogs 

 still shrinks when placed in the furnace, sharp quartz 

 grains should be added, as they have a tendency to 

 expand on repeated heating. Fine grained quartz 

 sand should in no case be added if the brick is to be 

 exposed to high temperaturesi, for in such cases it 

 tends to flux the clay in burning, furthermore the 

 addition of coarse quartz must also be within limits 

 for if in too large quantity the quartz grains loosen the 

 brick -by their expansion. A good fire brick is some- 

 times made by mixing a non-plasitic refractory clay 

 with a very plastic dense burning, semi-refractory 

 one. 



No fixed rules can be laid down to govern the 

 selection and valuation of a fire clay for the reason 

 that the use to which it is to be put determines its 

 qualities to a large extent. All fire clays should 



