640 SOILS: PROPERTIES AND MANAGEMENT 



cient as well as a very permanent means of hind drainage 

 at relatively small cost. If they are well installed and of 

 good material, they should continue to operate for cen- 

 turies with very little attention. As noted above, tile 

 drains have been in continuous operation in America for 

 seventy-five years and are still firm and efficient. 



540. Quality of tile. — There may be a considerable 

 range in the quality of tile made of either clay or concrete. 

 Clay tile is made of several grades of clay and sand mixed 

 and burned at a high temperature. Material that is 

 fused slightly is thereby vitrified, and forms a tile having 

 a very dense, impervious wall. This is vitrified tile. 

 Burned at a lower temperature the walls are more porous 

 and less resistant. Some material does not fuse at any 

 temperature to which it may be raised, and produces a 

 tile having soft, porous walls. This makes soft, or brick, 

 tile. Still another grade of tile is made of clay — usually 

 fire clay — dipped into a salt solution before firing. This 

 gives a smooth glaze, commonly seen in sewer tile. This 

 is glazed tile. 



Of the three grades mentioned, the vitrified tile is 

 normally the best because of its strength and resistance 

 to the destructive agencies in the soil. The most notice- 

 able of these agencies is frost. Even burned clay cannot 

 resist the destructive action of freezing water. Any tile 

 that has walls porous enough to absorb an appreciable 

 amount of water — and the larger the amount, the greater 

 is the danger — will, if frozen and thawed a few times, be 

 shattered into flakes. The walls of soft tile will absorb 

 capillarily from 8 to 20 per cent of moisture, and under 

 the action of frost will go to pieces rapidly. Glazed tile 

 is less injured, especially when the glaze is intact; but 

 once a crack has formed the tile is rapidly destroyed. 



