REVIEWS 423 



This is consistently greater than 1 for all properties except work to 

 maximum load in static bending, height of drop of hammer in impact 

 bending, and modulus of elasticity, for which it occasionally is slightly 

 less than 1. 



Because the strength of air-dry and kiln-dry wood varies with the 

 moisture content it was, of course, necessary to adjust the strength 

 values of the seasoned wood to a uniform moisture content in order to 

 make a satisfactory comparison. This was done by means of a for- 

 mula derived from the many strength tests previously made, including 

 data in Forest Service Circular 108 and Forest Service Bulletin 70. 



The following general conclusions are reached : 



(1) That wood may have its strength properties, particularly tough- 

 ness or resistance to shock, quite seriously damaged without any visible 

 evidence of such damage. Hence, appearance of the material can not, 

 where maximum strength is essential, be accepted as the sole basis of 

 judgment of the effect of a drying process of wood. 



(2) That the effect of a given process is not the same on all species 

 of wood. 



(3) That apparently a given process may be entirely safe for some 

 but quite detrimental to other material of a species. 



(4) That proper kiln drying produces material fully equal in all 

 strength properties to that resulting from air drying under the most 

 favorable conditions. 



(o) That specification 20500A of the Bureau of Aircraft Production 

 (Table 1 or 2 as specified) can in most cases be expected to produce 

 material fully equal to air-dried. 



(6) That best results (with respect to strength properties) on Doug- 

 las fir will result from the use of somewhat milder drying conditions 

 than those laid down in Specification 20500- A (Table 1). Table 2 of 

 this specification (temperatures 105° F. initial to 135° F. final and 

 relative humidities S.") per cent initial to 40 per cent final) is recom- 

 mended for drying Douglas fir. 



(7) That in some species there is apparently no relation between 

 drying temperatures up to 180° F. and the strength properties of the 

 dry material. Such a conclusion, however, needs further confirma- 

 tion, and temperature higher than those of Specification 20500-A have 

 not been recommended. 



A. K. 



