CLAYS OF KEW YORK 637 



age of the clay in the burning, and it will be tlie more injurious ilie 

 less finely divided its condition; consequently if a brick clay con- 

 tains lime it should be seen to that the substance is finely and 

 evenly disseminated throughout the material, for if in lumps it is 

 very apt to split the burned brick. Many brick clays which con- 

 tain lime pebbles are often dried and screened before using. 



Sand decreases both the plasticity and the tensile strength of the 

 clay as well as the shrinkage. This fact is frequently known and 

 utilized by the manufacturer to diminish the shrinkage of his clay 

 in both drying and burning, consequently reducing the danger of 

 obtaining a warped or cracked product. Some clays will stand as 

 much as 25^ of sand. The coarser the sand, the more marked will 

 be its effect on the shrinkage. On the other hand, if the grains 

 of sand arQ angular and of too large size, they may of themselves 

 produce a cracking of the ware; and it should be borne in mind that 

 there is danger of adding too much sand to a clay, for the tendency 

 will be to produce a weak, porous brick, specially if the latter is 

 hand-molded. 



Tine-gTained clays and very plastic ones generally need to be 

 dried very slowly, the reason being that on account of the smallness 

 of the pores the moisture can not escape readily, and the outer por- 

 tion of the brick dries and shrinks more quickly than the interior, 

 with the resultant cracking. Rapid drying may be prevented 

 somewhat by adding salt water to the clay, and this is a common 

 practice in parts of Missouri. (Mo. geol. sur. 11: 481) 



Fine-grained clays often have to be heated slowly in the early 

 stages of burning, though if such a clay contains an abundance of 

 fine sand particles, the contrary is possible. 



