CLAYS OF NEW YORK Y45 



water was required to work it up. The air shrinkage was 4^. Up 

 to cone .03 it was lOi^. It vitrified at 2 and became viscous at 5. 

 The tensile strength was from 60 to YO pounds a square inch. 



Manufacture of paving hrich 



Shale is used more than claj in the manufacture of paving brick. 

 It has to be prepared first by crushing in a dry pan, then screened. 

 This screened clay is mixed with water and tempered either in a 

 pug mill or in a wet pan. (For description see " Manufacture of 

 .common brick ", p. 653) 



Paving brick are commonly molded in an auger machine; they 

 • are either end-cut or side-cut. At a few factories the soft mud 

 method is used, but in this state it is only employed at Syracuse. 

 Repressing the green brick is commonly practised, but there is a 

 difference of opinion as to whether it improves the quality of the 

 brick, though the experiments given on the following pages tend to 

 indicate that the end-cut repressed brick are the strongest, while 

 still more recent tests somewhat disfavor this view. 



The green brick are usually piled on cars and dried in tunnels. 



Paving brick should be burned in down-draft kilns, as they give 

 better results than the up-draft kilns, and there is no loss from 

 crushed, overbumed brick. 



The type of kiln used in this state is either the rectangular down- 

 draft kiln or a continuous one. The latter is extending in favor, 

 as it is the more economical and yields good results. Circular kilns 

 are but little used in this state for burning paving brick. 



Tests of paving hrich 



Por a long period there has been some difference of opinion as to 

 what constitutes the qualities requisite for a paving brick. Engi- 

 neers have frequently laid considerable stress on the crushing test 

 and the color. The latter is of no value as a guide; the former be- 

 yond certain limits is to be looked on in the same way. With a view, 

 therefore, to determine what the requisite qualifications of a paving 



