CLAY WORKING INDUSTRIES. 175 



The Burning of Paving Material. The delicacy and importa ce of 

 this part of the process have been previously alluded to in various con- 

 nections in this article. Much of the trouble heretofore has lain in the 

 fact that the pioneers "in this business have had to build their own roads 

 and find their own way, for while there is very much in common in the 

 burning of sewer pipe and paving brick and in iact any vitrified clay 

 ware, still the difficulties involved have not only been those of vitrify- 

 ing larger masses of clay than was ever before attempted, but in also 

 satisfying a market that did not yet know what it wanted. 



What is really desired or needed to produce a perfect paving mate- 

 rial is now beginning to be better understood, and as it is in the burning 

 process, where these qualities are largely made or lost, it is best to dis- 

 cuss the subject in this connection. 



The qualities which a city street must have, to make it a good in- 

 vestment for the tax payers who build it are: 



1st. Durability under heavy traffic. 



2d. Sufficient smoothness to make traction easy and comparatively 

 noiseless, and yet not such smoothness as to make it slippery for horses' 

 feet. 



3d. Reasonable cost. 



4th. Good sanitary qualities; that is, that the street as a whole or the. 

 material of which it is composed shall not- be absorbent of the filth 

 which always finds its way there, and thus becomes a receptacle for 

 disease. 



Without entering into a general discussion, it is safe to say that vit- 

 rified clay is undoubtedly the most satisfactory material now in use in 

 regard to the 2d, 3d, and 4th specifications and that the position of 

 brick pavements yet remain to be proved in regard to the first point. 

 Sufficient time has not yet elapsed since the laying of any street with 

 what we now call good brick paving material, to enable us to make a de- 

 duction as to its relative wear and cost compared with granite or 

 other natural stone. 



The qualities then, which must be possessed by bricks to fill these 

 various requirements are : 



1st. Vitrification — by which the material becomes indestructible to 

 weather or any natural influence it will have to endure. 



2d. Toughness, which enables it to stand friction and blows with- 

 out undue wear. 



3d. Uniformity — which prevents the early failure of the material 

 in spots, which very soon extend to the ruin of the whole. 



Vitrification, as we have already shown is wholly a matter of burn- 

 ing. Toughness is a quality which depends on three conditions ; 



1st. The natural quality of the clays, some of which produce tough 

 ware, and some which do not. 



