22 BULLETIN 607, U. S. DEPARTMENT OF AGRICULTURE. 



lower pressure gave better penetrations than shortening the time of 

 treatment and increasing the pressure. 



The temperature of the preservative was found to be very impor- 

 tant. In the case of a mixture of tar and creosote containing 50 per 

 cent of by-product coke-oven tar an increase in the temperature 

 from 160° to 200° F. increased the absorption two and one-half 

 times and penetration one and two-thirds times. A nearly similar 

 result was obtained with another tar and creosote mixture containing 

 75 per cent of by-product coke-oven tar. 



Since the tests were necessarily limited to a comparatively small 

 number of tar and creosote mixtures and also to a small number of 

 tests on each preservative, using only one species of wood, the rela- 

 tions brought out can not be considered as definitely established. 

 The results may prove of value, however, in indicating some of the 

 factors that bear an important relation to the injection of preservatives 

 into wood. 



In general, it might be expected from these results that if tar, 

 either alone or in mixture with creosote, were used in the preserva- 

 tion of wood the difficulty of injection would increase as the percent- 

 age of tar and free carbon in the mixture was increased. The prac- 

 tice of filtering free carbon from tar and creosote mixtures, which is 

 sometimes resorted to, would seem to offer a means of improving the 

 penetrating properties of the preservative. 



In these experiments those tars which contained normally the lower 

 amounts of free carbon appeared to have better penetrating prop- 

 erties than those containing the higher amounts, even after the free 

 carbon was removed. This suggests that mixtures containing low- 

 carbon tars should prove easier to inject into the wood. 



The results of the experiments in which the time of treatment, 

 intensity of pressure, and temperature of the preservative were 

 varied indicate that the pressure period should be as long as possible, 

 the hi tensity of pressure being regulated to obtain the desired absorp- 

 tion, while the preservative should be at as high a temperature as it 

 is practicable to work without injuring the wood. From 190° to 220° 

 F. may be satisfactorily used in treating longleaf pine paving blocks. 



The viscosity measurements made on the tar and creosote mixtures 

 containing tar from different sources indicate that the viscosity may 

 not always be a true index of the penetrating properties of the mix- 

 ture and should probably not be relied upon for this purpose. 



The quality of the wood used has a marked influence on the absorp- 

 tion and penetration. It has very frequently been observed that the 

 springwood of rapid-growth southern pine is hard to penetrate. In 

 general, better treatments may be obtained in wood which has narrow 

 annual rings. 



