422 JOURNAL OF FORESTRY 



runs and 26 species of wood. These tests were undertaken in co- 

 operation with the War and Navy Departments for the purpose of 

 determining whether Signal Corps Specification 20o00A, "General Kiln 

 Drying Process for Airplane Stock," as already prepared by the For- 

 est Products Laboratory from its extensive studies in kiln-drying, was 

 entirely safe for drying material in which the maximum strength must 

 be reserved. It was also desirable to find out whether more rapid 

 processes than those allowed by Specification 20o00A might possibly 

 be developed. 



Two methods of procedure were followed : One was to compare the 

 strength of kiln-dried material with results previously obtained from 

 tests on green and air-dry material of the same species, but obviously 

 of different trees. On the basis of this comparison tentative con- 

 clusions were drawn as to whether kiln drying had produced as fa- 

 vorable a change in properties as had been found, from previous tests, 

 to result from air drying. This method gave quick results but the 

 conclusions drawn could not be so accurate or dependable as was de- 

 sirable because the test pieces in the two series of tests were from 

 dififerent trees, and not directly comparable. 



The other procedure was to select and test green, air-dry, and kiln- 

 dry matched pieces from the same tree and even from the same portion 

 of the tree. This took considerably longer because of the time neces- 

 sary to allow for thorough air seasoning. But the results were more 

 satisfactory of the species dealt with in the report. Sitka spruce, 

 Douglas fir, white pine, and white ash were tested in this way, while 

 tests on kiln-dried material of the other 22 species are compared, in 

 the publication, only with tests on material from previous shipments, 

 since the results of tests of matched kiln-dried and air-dried material 

 were not available when the publication was prepared. 



In comparing test data on kiln-dried material with previously tested 



air-dried material it was found that more reliable conclusions could be 



drawn by comparing the amounts of change in properties which took 



place in drying the material from the green condition, instead of by 



'comparing the strength values themselves directly. The change in the 



properties due to drying was termed the "improvement ratio." It is 



expressed by the formula 



Strength value for kiln or air dried material 



Improvement ratio=^ ^ , , — j -. — -. :~7- 



Same strength value for matched green material 



