THE SEASONING OF WOOD. 



27 



culation of the air passing over the lumber should give satisfactory 

 results. If checking begins during the drying process, the humidity 

 should be increased until it stops. Steam jets in a kiln are often useful 

 for this purpose. In changing the humidity the circulation should 

 not be reduced. A large body of 

 moving air is necessary in order 

 to keep a uniform temperature 

 clear through to the center of 

 each piece of wood in the pile and 

 at the same time supply the heat 

 required for evaporation. If suf- 

 ficient circulation is not secured, 

 the supply of heat for both pur- 

 poses will be lacking and the 

 material will not dry uniformly. 

 Figure 18 shows the conditions 

 in a kiln during a run with refer- 

 ence to temperature, humidity, 

 and moisture in the wood. It 

 will be noted that the humidity 

 is kept high at first and lowered 

 gradually. The temperature is 

 held at a certain level for some 

 time and then raised. The mois- 

 ture is lowered gradually to a 

 final condition of less than 5 per 

 cent. 



The maximum rate of drying 

 at a given temperature is reached 

 when moisture is evaporated 

 from the surface of the wood just 

 as fast as it is transmitted from 

 the interior. This rate is fixed 

 by the rate of transmission of 

 moisture within the wood and 

 varies with different woods. 



The temperature of drying ap- 

 parently influences the rate of 

 transmission of moisture within 

 the wood. The higher the temperature of the wood the more rapid is 

 the rate of transmission of the moisture, and hence the rate at which 

 the moisture may be evaporated safely. This, of course, applies only 

 to temperatures below those which might result in injury to the wood. 



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