2 BULLETIN 23, U. S. DEPARTMENT OF AGRICULTURE. 
This bulletin purposes to furnish information relating to the con- 
struction of brick roads, and to supply suggestions for aiding engineers 
in preparing specifications under which such work may be satisfac- 
torily performed. One of the most essential features of the construc- 
tion of brick pavements is the selection of the brick, since the success 
or failure of such pavements depends to a large extent on the char- 
acter of the material used. In order that the significance of the 
varying physical characteristics observea 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, however, the deposits are com- 
paratively 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 essen- 
tially a difference 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 
physical and chemical composition. By carefully mixing the mate- 
rials from different strata or from different parts of the bank, there- 
fore, a resulting material of the desired character may usually be 
obtained. It not infrequently happens, however, that in order to 
secure the best results sand or surface clay must be added in an 
amount depending on the relative "leanness" or "fatness" 1 of 
the material used. In this connection it may be noted, also, that a 
chemical analysis of a given fire clay or shale does not necessarily 
1 " Leanness" and " fatness " refer respectively to the lesser or greater amount of silica present in the 
material. 
