CHAPTER VI. TAR-CREOSOTE SOLUTIONS. 



Within the last few years tar-creosote solutions have been used 

 more and more in wood preservation. Various claims have been 

 put forward in favor of tar: First, that, as it is the mother liquor of 

 creosote, it contains all the toxic principles of creosote; second, that 

 it retards the evaporation of creosote when mixed with it; third, 

 that it does not reduce penetration intb the wood; and fourth, that 

 it is cheaper than coal-tar creosote. In answer to these arguments 

 it may be said that, although tar is the mother liquor and contains 

 all the toxic principles of coal-tar creosote, the concentration of the 

 toxic elements in coal tar is only about one-fourth of that in coal-tar 

 creosote, because coal-tar creosote is only about one-fourth of the 

 volume of the coal tar from which it is distilled and contains prac- 

 tically all the toxic principles. It has also been shown in this bulletin 

 that coal tar does not retard the evaporation of creosote mixed with 

 it. Furthermore, Bond {17), and Teesdale and MacLean {18) have 

 demonstrated that it is more difficult to penetrate wood with coal- 

 tar solutions than it is with creosote. 



TAR AS A DILUENT. 



The chief value of tar is as a diluent of creosote, although it may 

 have a retarding influence on the rate of solution of the toxic princi- 

 ples into the wood. The use of a diluent in wood preservation is 

 no new thing. In the treatment of wood with zinc chloride a strength 

 of solution is used that will insure a thorough penetration. If the 

 wood is difficult to penetrate, a relatively strong solution is used; 

 but, if it is easy to penetrate, a more dilute solution is used, and 

 more solution is put in, thus insuring a more thorough treatment. 

 In other words, the amount of water or solvent is varied, but the 

 amount of zinc chloride is kept constant. With zinc chloride the 

 factor of safety is only about 2 ; with creosote it is in the neighborhood 

 of 50. The reason for this great difference is that by diluting zinc 

 chloride with water a depth of penetration as great as possible is 

 obtained, and still only a small amount of zinc is used. On the other 

 hand, it is impossible to give a deep oil penetration without using 

 large quantities of oil. However, the usual oil penetration could be 

 obtained, and at the same time creosote could be saved if it were 

 diluted with some other oil, and this would have an effect analogous 

 to that of the water in a zinc-chloride solution. As any good solvent 

 of creosote would serve as well as another for this purpose, the main 



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