104 HIGHWAY CONSTRUCTION 



its decomposition, also to fill the pores of the wood with a bituminous 

 substance which excludes both air and moisture, and which is noxious 

 to the lower forms of animal and vegetable life. In adopting this pro- 

 cess, all moisture should be dried out of the pores of the timber. The 

 softer woods, w r hile warm from the drying-house, may be immersed at 

 once in an open tank containing hot creosote oil, when they will absorb 

 about 8 or 9 pounds per cubic foot. For hardwoods, and woods which 

 are required to absorb more than 8 or 9 pounds of creosote per cubic 

 foot, the timber should be placed in an iron cylinder with closed ends, 

 and the creosote, which should be heated to a temperature of about 

 120 F., forced in with a pressure of 170 pounds to the square inch. 

 The heat must be kept up until the process is complete, to prevent the 

 creosote from crystallizing in the pores of the wood. By this means 

 the softer woods will easily absorb from 10 to 12 pounds of oil per cubic 

 foot. 



The most effective method, however, is to exhaust the air from tht 

 cylinder after the timber is inserted ; then to allow the oil to flow in; and 

 when the cylinder is full, to use a force pump with a pressure of 150 to 

 200 pounds per square inch, until the wood has absorbed the requisite 

 quantity of oil, as indicated by a gauge, which should be fitted to the 

 reservoir tank. 



The oil is usually heated by coils of pipe placed in the reservoir, 

 through which a current of steam is passed. 



The quantity of creosote oil recommended to be forced into the 

 wood is from 8 to 12 pounds per cubic foot. Into oak and other hard 

 woods it is difficult to force, even with the greatest pressure, more than 

 2 or 3 pounds of oil. 



The advantages of this process are : The chemical constituents of 

 the oil preserve the fibers of the wood by coagulating the albumen of 

 the sap ; the fatty matters act mechanically by filling the pores and thus 

 exclude water; while the carbolic acid contained in the oil is a powerful 

 disinfectant. 



The life of the wood is extended by any of the above processes, by 

 preserving it from decay ; but such processes have little or no effect on 

 the wear of the blocks under traffic. 



The process of dipping the blocks in coal tar or creosote oil is 

 injurious. Besides affording a cover for the use of defective er sappy 

 wood, it hastens decay, especially of green wood ; it closes up the ex- 



