74 Forestry Quarterly. 



Principles of Drying Lumber at Atmospheric Pressure a^id 

 Humidiiy Diagram. By H. D. Tiemann. Bulletin 104, U. S. 

 Forest Service. Washington, D. C. 1912. Pp. 19. 



Part I is a progress report of the investigations by the Forest 

 Service of the fundamental principles in dr>'ing lumber. The 

 work has all been done on a small scale and two series of experi- 

 ments are now under way to determine how these principles may 

 best be applied in commercial practice. 



The experiments so far made indicate that successful dry-kiln 

 operation requires the obser\^ance of the following points : 



"(i) The timber should be heated through before drying 

 begins. 



"(2) The air should be very humid at the beginning of the dry- 

 ing process, and be made drier only gradually. 



"(3) The temperature of the lumber must be maintained uni- 

 formly throughout the entire pile. For this an exceedingly large 

 circulation of air is essential. 



"(4) Control of the drying process at any given temperature 

 must be secured by controlling the relative humidity, not by de- 

 creasing the circulation. 



"(5) In general, high temperatures permit more rapid drying 

 than do lower ones. The higher the temperature of the lumber 

 the more efficient is the kiln. It is believed that temperatures as 

 high as the boiling point are not injurious to most woods, pro- 

 viding all the other fundamentally important factors are taken 

 care of. Some species, however, may not be able to stand as 

 high temperatures as others. 



"(6) The degree of dryness attained, where strength is the 

 prime requisite, should not exceed that at which the wood is to 

 be used." 



Part II contains a humidity diagram the purpose of which is 

 to enable the dry-kiln operator to determine quickly the humidity 

 conditions and vapor pressures, as well as the changes which take 

 place with changes in temperature. The diagram, which con- 

 sists of two distinct sets of curves on the same sheet, is adapted 

 to the direct solution of a great many problems of this character 

 without recourse to tables of mathematical calculations. It is 

 fully explained both as to theory and its practical application. 



S. J. R. 



