96 CLAYS AND CLAY INDUSTRY. 



very high percentage of limonite, which would supply an addi- 

 tional quantity of chemically combined water. 1 



The amount of fire shrinkage shown by these samples is 

 equally interesting, for it is seen that although the loss in weight 

 between 500 C. (932 F.) and 900° C. (1652 F.) is con- 

 siderable, still there is little or even no shrinkage, so that after 

 the volatile elements have been driven off the clay must be very 

 porous, and remains so until the fire shrinkage begins again. 

 From the table it will be seen that with one exception, no shrink- 

 age occurred between 6oo° C. (1112 F.) and 900 C. (1652 

 F.), but between 900 C. (1652 F.) and 1000 C. (1832 

 F.), all except No. 663 decreased in size, and there was an 

 additional but greater shrinkage between iooo° C. (1832 F.) 

 and 1100 C. (2012 F.). None of the bricklets became steel- 

 hard, that is, sufficiently hard to resist scratching with a knife 

 until iooo C. (1832 F.), or even 1100 C. (2012 F.). In 

 the case of those burning red, a good red coloration began to 

 appear at 1000 C. (1832 F.). From this it can be seen, and 

 this is a fact already known, that up to 600 ° C. (1112 F.), a 

 clay should be heated slowly, but from that point up to 1000 

 C, the temperature can be raised quite rapidly, unless much 

 carbonaceous matter is present. The gradual burning off of 

 this carbon is well shown in PI. XVII, Fig. 1, which repre- 

 sents a series of bricks taken from a kiln, at regular intervals as 

 the burning proceeded. Further heating should be done slowly, 

 as the shrinkage recommences at the last mentioned temperature. 



Since many clays when used alone shrink to such an extent 

 as to cause much loss from warping and cracking, it is neces- 

 sary to add materials, which of themselves have no fire shrinkage, 

 and so decrease the shrinkage of the mixture in burning. Sand 

 or sandy clays are the materials most commonly used for this 

 purpose, but ground bricks (grog) and even coke or graphite 

 may be employed. These materials serve not only to decrease 

 the shrinkage in drying and burning, but also tend to prevent 

 blistering in an easily fusible ferruginous clay when hard fired. 

 They furthermore add to the porosity of the ware, and thus 



1 See Chapter III, p. 73. 



