2 6o CLAYS AND CLAY INDUSTRY. 



strength, and also low in porosity, contrary to what we might 

 expect. 



6. Re-pressing increases the strength of the brick so far as the 

 New Jersey experiments go ; it also increases the density and, 

 therefore, decreases the absorption. As an example of this, we 

 may take samples 26 and 27, representing the red brick made 

 around Trenton. In this case the re-pressing has increased the 

 crushing strength about 40 per cent, and the transverse strength 

 nearly 30 per cent., while the absorption has decreased 2.24 per 

 cent. A similar difference is observable in specimens 30 and 31, 

 where the difference is still greater, but in this case it has been in- 

 creased somewhat by harder burning. 



7. The material added to decrease the shrinkage may seriously 

 affect the strength of the brick. Thus Nos. 20 and 21 are com- 

 mon bricks made from' the same clay bed, at two different, but not 

 widely, separated points. No., 20 contains clean, sharp sand as 

 an anti-shrinkage ingredient, while No. 21 has a sandy loam 

 added to it in tempering. The latter has evidently made the brick 

 more porous, softer, and less ringing. It also lowers its crushing 

 strength more than 66 per cent, and its transverse strength over a 

 half. 



8. In one instance the same yard was found to be using both 

 the stiff-mud and the soft-mud process ; the clays used were ob- 

 tained from the same bed, and the mixtures used differed but 

 little. No. 4 represents the stiff-mud, and No. 22 the soft-mud 

 brick. The greater crushing strength of the latter is probably due 

 to its more homogeneous character, for the former contains many 

 small cracks. 



9. The effect of hard burning is seen in Nos. 11 and 12. Both 

 lots came from the same down-draft kiln, but No. 12 was taken 

 from the top, where it had been subjected to greater heat. Its 

 strength is more than double that of No. 11. 



For the sake of completeness of data, and also to indicate the 

 character of the clays used, the conditions surrounding the manu- 

 facture of each brick are given here in detail, the numbers corre- 

 sponding to those given in the first column of the table of tests. 



