444 



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 tiUs 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 lbs.) 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. Microscopic 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 7 established the safe limit of steaming for seasoned loblolly 

 pine to be 30 lbs. applied for 4 hours, or 20 lbs. 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, oT 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. 



