4 BULLETIN 23, U. S. DEPARTMENT OF AGRICULTURE. 
duces the better brick, and each method has many advocates. 
Entirely satisfactory pavements have been made from both re- 
pressed and unre-pressed brick, however, and it is very doubtful if 
the failures which have been observed in connection with either type 
could rightfully be attributed to this particular feature in the process 
of manufacture. 
Special shapes, such as nose bricks for use next to car tracks, and 
hillside block, which have one side thicker than the other and which 
are used on steep grades in order to give the pavement a rough sur- 
face, may be made either by special die or special re-press molds. 
The next step in the process of manufacture consists in drying the 
brick. In a properly systematized plant the brick are stacked upon 
drier cars as they leave the presses in such manner as to permit a 
free circulation of air between them. The loaded cars are imme- 
diately run into a tunnel drier, the temperature of which is main- 
tained at about 100° F. at the entering end. As cars containing 
"green" brick enter one end of the tunnel, which is usually more 
than 100 feet long, other cars containing dry brick are being removed 
at the opposite end. Air circulation in the dryer is effected by means 
of fans or high stacks. During drying the brick lose an amount of 
moisture equivalent to from 15 to 20 per cent of their own weight. 
The brick leave the dryer ready for burning, which is the last and 
undoubtedly the most important step in the process of manufacture. 
Upon the burning depends largely the quality of the finished product, 
and it requires the greatest skill so to regulate the temperatures and 
firing periods as to obtain the best results from a given material. 
Experience alone can demonstrate the manner in which the burning 
must be modified in order to suit varying sets of conditions. The 
kilns in which the burning is done are made of brick and are provided 
with numerous furnaces. The brick are placed in the kilns so as to 
permit a free circulation of the gases of combustion and the heated air. 
PHYSICAL CHARACTERISTICS. 
GENERAL REQUIREMENTS. 
Paving brick should be uniform in size, reasonably perfect in shape, 
and free from ragging, due to friction in the die, or kiln marks, caused 
by impressions from overlying brick in burning. They should be 
tough in order to resist crushing, hard in order to resist abrasion, and 
uniformly graded in order that the pavement may wear evenly. Each 
brick should be homogeneous in texture and free from objectionable 
laminations or seams. Fire cracks, caused by too rapid firing, should 
be limited in number and extent, and the entire brick should be 
vitrified and should contain neither unfused nor glassy spots. 
