ABSORPTION AND PENETRATION OF COAL TAR AND CREOSOTE. 21 



SUMMARY. 



The results of this investigation may be briefly summarized as fol- 

 lows: 



The addition of coal tar to coal-tar creosote increased materially 

 the difficulty of injection into heart longleaf pine. The resistance to 

 impregnation was increased as the amount of tar was increased. 

 Resistance to impregnation was greater to coal tars of high than to 

 those of low free-carbon content. This was the case even when the 

 free carbon was removed from these tars, indicating that the character 

 of the bitumens as well as the free carbon influenced impregnation. 

 The tests indicate that coal tars produced at relatively low tempera- 

 tures penetrate better than those produced at relatively high tem- 

 peratures. The size of the free-carbon particles was also found to be 

 a factor in penetration, as those tars in which a large proportion of 

 the particles were relatively small produced mats which were more 

 nearly impervious to the passage of the preservative than those tars 

 in which these particles were larger. 



The relative viscosities of mixtures of coal tar creosote and coal 

 tars from different sources are not necessarily a true index of their 

 ability to penetrate wood. In these tests the viscosities of mixtures 

 containing different tars did not appear to have any definite relation 

 to the ease or difficulty of penetration. 



For a mixture of a given tar and creosote the following three factors 

 appear to be important in their relation to penetrations and absorp- 

 tions: 



1. The composition and character of the bitumens. 



2. The amount of the free carbon in the tar. 



3. The condition or size of the free carbon particles. 



In the treatment of paving blocks the most general practice at 

 present is to inject about 16 pounds of the preservative per cubic 

 foot of wood. This absorption is not usually difficult to obtain in 

 air-seasoned longleaf pine with coal tar creosote at relatively moderate 

 pressures and temperatures, on account of the short length of the 

 blocks (usually not over 4 inches). In these tests it was found that 

 when coal tar was added to coal tar creosote, it was possible to obtain 

 an absorption of 16 pounds per cubic foot by increasing the pressure, 

 the temperature, and the time of treatment, the amount of the in- 

 crease required depending upon the kind and amount of tar added. 

 There was, however, a tendency to obtain less uniform penetrations 

 with those mixtures containing the higher amounts of tar. Increasing 

 the intensity of pressure was of greater importance in obtaining the 

 desired absorption than increasing the time of treatment, while with 

 a given absorption lengthening the time of treatment and using a 



