HIGHWAY CONSTRUCTION 



land or overflow pitch, and iron pitch. The first and most valuable 

 kind is obtained from the so-called Pitch Lake. 



The term land or overflow pitch is applied to the deposits of 

 asphaltum found outside the lake. These deposits form extensive 

 beds of variable thickness, and are covered with from a few to several 

 feet of earth ; they are considered by some authorities to be formed from 

 pitch which has overflowed from the lake; by others to be of entirely 

 different origin. The name cheese pitch is given to such portions of the 

 land pitch as more nearly resemble that obtained from the lake. 



The term iron pitch is used to designate large and isolated masses 

 of extremely hard asphaltum found both within and without the bor- 

 ders of the lake. It is supposed to have been formed by the action of 

 heat caused by forest fires, which, sweeping over the softer pitch, re- 

 moved its more volatile constituents. 



. The name epuree is given to asphaltum refined on the island of 

 Trinidad. The process is conducted in a very crude manner, in large, 

 open, cast-iron sugar boilers. 



The characteristics of crude Trinidad asphaltum, both lake and 

 land, are as follows: It is composed of bitumen mixed with fine sand, 

 clay, and vegetable matter. Its specific gravity varies according to the 

 impurities present, but is usually about 1.28. Its color, when fieshly 

 excavated, is a brown, which changes to black on exposure to the at- 

 mosphere. When freshly broken, it emits the usual bituminous odor- 

 It is porous, containing gas cavities, and in consistency resembles 

 cheese. If left long enough in the sun, the surface will soften and melt, 

 and will finally flow into a more or less compact mass. 



Refined Trinidad Asphaltum. The crude asphaltum is refined 

 or purified by melting it in iron kettles or stills by the application of 

 indirect heat. 



The operation of refining proceeds as follows : During the heat- 

 ing, the water and lighter oils are evaporated; the asphaltum is lique- 

 fied; the vegetable matter rises to the surface, and is skimmed off; the 

 earthy and siliceous matters settle to the bottom; and the liquid asphal- 

 tum is drawn off into old cement or flour barrels. 



When the asphaltum is refined without agitation, the residue 

 remaining in the still forms a considerable percentage of the crude 

 material, frequently amounting to 12 per cent; and it was at one time 

 considered that the greater the amount of this residue the better the 



