Drying of Timber. 73 



timbers examined shrinkage follows the gradient. This can be 

 readily seen if blocks of timber be coated on four sides so that the 

 moisture can only escape in one direction, preferably the radial. 

 Shrinkage can be measured under such conditions. A block of beech 

 2" X 2" X 3'', drying at 40°C., gave the following amounts of shrink- 

 age at the times and positions given. The 2x2 face was tangential, 

 and the shrinkage occurred in the tangential direction. 







Table II. 













Slirinkaj-e at 



each distance 



from 



end. 





Days 



iin. 



iin. 



1 in. 



1 in. 





H in. 



2 



.02 cms. 



.02 cms. 



.01 cms. 



. 









5 



.05 



.03 



.01 









. — 



7 



.08 



.06 



.02 



.01 





— 



11 



.11 



.08 



.05 



.04 





.03 



19 



13 



.11 



.10 



.10 





.10 



Amount of shrinkage in a beech block when drying. 



In Fig. 4 are given the results of drying a series of similar oak 

 blocks under the same conditions — 40°C. and 50 per cent, humidity. 

 The blocks were approximately 2'' x 2" x 3'', and the longest side was 

 in the radial direction. The four long faces were sealed, and the two 

 ^" X 2" faces were exposed. Blocks were cut up at the times shown on 

 the graphs. The distribution of the moisture was obtained as before. 

 It will be seen that the gradient was steep at the commencement. It 

 may be argued that such a condition indicates case hardening, but as 

 a matter of fact no such condition existed. It will be shown later 

 that the conditions of drying were very favourable, and that these 

 same conditions permit of a greater amount of shrinkage than lower 

 temperatures and higher humidities. The graphs indicate that drying 

 is accompanied by a moisture gradient, and that no such condition 

 ,as fibre saturation is reached. 



Instead of a piece of timber losing its contained water until fibre 

 saturation is reached, we may say that as soon as a surface of green 

 timber is exposed to the air, it immediately tries to come into equili- 

 brium with the air moisture. The vapour tension of the moisture in 

 the wood is greater than the vapour pressure of the air, and moisture 

 is lost. This loss is made good from the contained water of the cells 

 immediately next to the surface. This water passes out through 

 the outer cell wall. As soon as this water is lost a gradient begins to 

 be established. The walls begin then to lose moisture, and as wood 

 is hygroscopic the cell walls draw moisture from the next layer of 

 cells. The contained water in the cells passes to the outside by means 

 of the cell walls, not through the cavities of the empty cells. This 

 process goes on from cell to cell. The steepness of the gradient 

 depends partly on the rate at which the moisture is being lost, and 

 partly on the rate at which moisture can move through the wood 

 to the evaporating face. 



