+44 
INFLUENCE OF TREATMENT PREVIOUS TO TEST. 
(a) Drying in Hot Air, Steam, Saturated Steam, etc. A research is 
under way to investigate the safe limits and the most advantageous con- 
ditions for the commercial processes of drying wood. The immediate 
strength after drying is of course usually greater because of the les- 
sened moisture content. It is now apparent, however, that all processes 
of drying wood, even air-drying, are attended with weakening of struc- 
ture, so that when the dried wood is resoaked there is a loss in strength 
of 10%, and generally more. The drying of white ash (Fraxinus amer- 
icana,), for instance, at 145° F. in either dry air or exhausted steam, or 
in superheated steam at 312°, caused no significant loss in strength in the 
air dry condition, but the resoaked wood was considerably weaker than 
the green wood. Under 20 to 30 pounds of steam applied during 1 to 4 
hours, pine and ash suffer but little loss in static strength after the moist- 
ure from the steam is removed by air drying. At higher steam pressures 
(above 50 Ibs.) large and permanent losses result. An equal amount of 
dry heat is less injurious to wood than moist air or saturated vapor, when- 
ever the temperature exceeds 212° F. The hygroscopicity of the wood in 
the air-dry condition is reduced by the process of drying in steam, dry- 
air or saturated steam. Microscopie study shows that the cell walls split 
open because of the shrinkage of these walls when they begin to dry out. 
The results from the Drying-Strength Study are not sufficiently ad- 
vanced to allow complete conclusions. 
(b) Treatment with Preservatives. Tests at the Louisiana Purchase 
Exposition’ established the safe limit of steaming for seasoned loblolly 
pine to be 30 Ibs. applied for 4 hours, or 20 Ibs. applied for 6 hours. Bur- 
nettized loblolly pine ties exhibited some degree of brittleness under im- 
pact test. Creosote appeared to act upon the strength in the same way as 
water. It retards the seasoning of timber, with beneficial results to its 
physical condition. Present evidence points to steaming, or effect of heat 
in preliminary seasoning, as the only dangerous element of the treating 
process. The proper limits of heat should be determined for different 
species of timber. 
In the case of bridge timbers, of coniferous species, of large size, in- 
complete evidence indicates that the desired penetration of creosote can 
7 Circular 39, Forest Service. Experiments on the Strength of Treated Tim- 
ber,” by W. K. Hatt. 
