250 CLAYS AND CLAY INDUSTRY. 



may be due to two. causes, viz., (i) difference in the three linear 

 shrinkages, and 2) differences in the degree of burning. 



The air shrinkage of the soft-mud bricks varies between slightly- 

 wider limits than the stiff-mud, but the average air shrinkage of 

 the stiff-mud bricks is slightly lower than that of the soft-mud 

 ones. In both the stiff-mud and soft-mud the linear fire shrinkage 

 is rarely high, while in the S0'ft-mud ones the fire shrinkage is 

 lower than the air shrinkage in most of those measured. Again, 

 in the stiff-mud bricks, the number showing a fire shrinkage 

 greater than the air shrinkage is about equal to those showing the 

 reverse. 



The low fire shrinkage of the soft-mud brick is due partly to 

 the sandy character of the clay mixture, and partly because of the 

 low temperature of burning. A difference of two- or three cone 

 numbers in burning may make considerable difference in the 

 shrinkage of a plastic clay, as can be seen by reference to the 

 tables of physical tests given in Chap. XVIII. 



TESTS OF NEW JERSEY BRICKS. 



In order to test the qualities of the bricks, made in New Jersey, 

 a number of samples were collected from different localities. 

 Where several yards were in operation at the same locality, and 

 using the same clay, samples were taken from only one, the 

 object being to show the character of the product obtainable from 

 different clays, or by different methods from the same clay. 

 A number of samples were in every case collected at each locality, 

 and always by members of the New Jersey Geological Survey. 

 They were taken at random either from the stock pile or kiln, 

 and, unless otherwise stated, were normally burned brick. 



The samples thus obtained were subjected to several different 

 tests in order to determine, 1) their crushing strength; 2) their 

 transverse strength, and 3) their absorptive capacity. The first 

 two tests were carried out by Prof. I. H. Woolson, of the De- 

 partment of Mechanical Engineering, Columbia University, 

 assisted by Mr. R. H. Waters. The absorption tests were made 

 by the writer. In nearly every test from 5 to 7 bricks were 



