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 intensity 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. 
