THE PHYSICAL PROPERTIES OF CLAY. 93 



black clay., which cannot be used alone for bricks, as it shrinks 

 too much, especially in burning, and consequently a more sandy 

 clav (Lab. No. 603) is added to it. 



Table showing air shrinkage and tensile strength of a fat and of a sandy clay. 



Per cent, of water Per cent, of air Tensile strength 



required. shrinkage. lbs. per sq. in. 



Lab. No. 606, Loc. 222, . . 30 6. 91 



" 603, " " . . 23 5.3 106 



While coarse or sandy clays shrink less than fine-grained ones, 

 they may sometimes absorb considerable water, especially if they 

 are silty in their character, but the fact that their pores are much 

 coarser allows the water to escape rapidly, and thus often per- 

 mits more rapid drying. The cracking of some fine-grained 

 clays in drying is due partly to the surface shrinking more rapidly 

 than the interior, because the evaporation there is greatest. As 

 the outer portion of the product cannot stretch, it must pull 

 apart and crack. 



Fire shrinkage. — All clays shrink during some stage of the 

 burning operation, even though they may expand slightly at 

 certain temperatures. The fire shrinkage varies within wide 

 limits, the amount depending partly on the quantity of volatile 

 elements, such as combined water, organic matter and carbon 

 dioxide present in the clay, and partly on the texture. It reaches 

 a maximum when the clay vitrifies, but does not increase uni- 

 formly up to that point, and, in fact, is very irregular. Thus 

 a certain amount of shrinkage takes place when the combined 

 water begins to pass off, namely, at -400° C. (752 F.), and an 

 additional amount occurs at higher temperatures, but not appar- 

 ently the result of contraction following volatilization of some 

 of the elements. 



Wherever the fire shrinkage is given in this report, it refers 

 to the linear shrinkage occurring during burning, and is ex- 

 pressed in terms of the length of the bricklet when molded. 

 Thus, if the fire shrinkage at cone 1 is given as 4 per cent., it 

 means that the amount of fire shrinkage at that cone is 4 per 

 cent, of the length of the bricklet when freshly molded. 



