182 



IMPREGNATION 



(e) Results obtained : —With the modern processes of creosote impregnation, the drawbacks adhering 

 to the original method are obviated. 



The material to be impregnated is no more exposed to high temperatures, prior to impregnation, 

 since the strength of the timber is found to be greatly impaired by high temperatures. Preliminary steaming 

 operations, also, have been found to reduce the strength of the wood by 25 per cent when a steam 

 pressure of 20 pounds was applied for 10 hours to green shortleaf pine. Disastrous results of this character 

 are now avoided. 



As an illustration of the effect on tie impregnation with European railroads, where impregnated ties 

 have been used for many years, the following tabular statistics are given: — 



Heartwood is not so permeable and hence not so impregnable as sapwood. Maple, birch, beech, 

 spruce, sappy pine, &c., are more benefited by impregnation than are white oak, longleaf pine, &c. 

 Generally the duration of the life of impregnated ties is increased at the following ratio: -Beech, 400 per 

 cent; yellow pine and oak, 200 per cent; spruce, 50 per cent. 



Obviously, every additional pound of preservative pressed into the fibre has a lesser effect on the 

 life of the wood than the preceding pound. For every woody species the limit must be found at which 

 additional impregnation proves unremunerative. 



(f) The PROCESSES: — The leading processes in vogue with railway engineers of the United States are the 

 Card process, the Lowry process, and the Rueping process, described in Bulletin 131 of the American 

 Railway Engineering and Maintenance of Way Association. 



1. The Card process. The timber is thoroughly air-seasoned. 



It is laid on small steel cars and run, a number of cars at a time, into a large horizontal 

 steel cylinder. The steel cylinder is sealed hermetically. A vacuum of from 22 to 26 inches 

 is secured, by the vacuum pump, in the steel cylinder and is maintained for one hour. 



Thereupon the impregnating liquid, consisting of 80 per cent of solution of chloride of zinc 

 and 20 per cent of creosote oil, is admitted at a temperature of not less than 180 degrees. A 

 pressure of 125 pounds per square inch is obtained and held within the cylinder for from three 

 to five hours. 



Thereupon the liquid is withdrawn, and a final vacuum dries off the timber before the cars 

 are run out of the steel cylinder. 



If it is desired to treat material that is not fully seasoned, a previous steaming process 

 may be used. 



in order to keep the chloride of zinc solution and the creosote oil thoroughly mixed during 

 the process of treatment, it is first agitated in the measuring tank by forcing air through pipes 

 into the liquid. 



Within the cylinder, the liquid is continually agitated by a centrifugal pump, drawing the 

 liquid from the top of the cylinder at three points, viz., from the center and from both ends. 

 After passing into the centrifugal pump, the liquid is discharged through a pipe which lays 

 along the ful! length of the bottom of the cylinder and is perforated through its length. The 

 pump should handle about 1,500 gallons of the mixture per minute. 



