2 BULLETIN 373, IT. S. DEPARTMENT OF AGRICULTURE. 
roads possess may be stated briefly, as follows: (1) They are durable 
under practically all traffic conditions; (2) they afford easy traction 
and moderately good foothold for horses; and (3) they are easily 
maintained and kept clean. 
The principal disadvantage is the high first cost. The defects 
which frequently result from lack of uniformity in the quality of 
the brick or from poor construction are usually to be traced indi- 
rectly to an effort to reduce the first cost or to a popular feeling that 
local materials should be used, even when of inferior quality. 
This bulletin purposes to furnish information relating to the con- 
struction of brick roads and to supply suggestions for aiding engi- 
neers in preparing specifications under which such work may be satis- 
factorily performed. One of the most essential features of the con- 
struction of brick pavements is the selection of the brick, since the 
success or failure of such pavements depends to a large extent on the 
character of the material used. In order that the significance of the 
varying physical characteristics observed in brick manufactured 
under different conditions may be more readily understood, a brief 
discussion of the raw materials and processes used in the manufacture 
of brick will be given. 
THE RAW MATERIALS. 
Paving brick are made from shales and fire clays. The " lean or 
less refractory varieties of these materials, which are found in the 
carboniferous deposits broadly distributed throughout the United 
States, are best adapted for this purpose. 
Shales frequently occur in such quantity and are so located that 
they may be readily excavated by means of a steam shovel or other 
mechanical device. Occasionally the deposits are comparatively thin 
and underlie other material, making it necessary that they be mined. 
Fire clays are usually found interstratified with coal deposits which 
may or may not be workable, and must, therefore, generally be 
mined. The principal difference between fire clays and shales, in so 
far as the manufacture of brick is concerned, is essentially a differ- 
ence of color in the finished product. The shales always contain iron 
in some form, and brick made of shale are usually red. Fire clays 
are free from iron and should produce a light-colored brick. Some 
low-grade fire clays, however, may be darkened by certain firing 
conditions too complicated to be discussed in detail here. 
Shales and fire clays as they occur in nature are not always well 
suited for use in the manufacture of paving brick, but must fre- 
quently be subjected to some modifying treatment before being used. 
In general, deposits of these materials occur in layers or strata, and 
the different strata are almost always slightly dissimilar in both 
