Drying of Timber. 71 



for drying, and we could, if kiln drying, prescribe the necessary treat- 

 ment. Until we know more about the diffusion of moisture in wood, 

 formulas for drying are more or less guesswork. 



In general the diffusion constant will vary with the specific 

 gravity of timber. Dense heavy timbers are generally slow in dry- 

 ing, and therefore we may expect a low diffusion constant. The cell 

 walls in conifers are on the whole much thinner than the wails of 

 the fibres of hardwoods. The walls of the tracheides are comparable 

 to the walls of the vessels. 



Tissues, engaged in water conduction, must have relatively thin 

 walls. Water conducting elements have their walls freely pitted, 

 while strength elements such as the libriform fibres are sparsely 

 pitted. Coniferous timber has a higher power of diffusion than most 

 dicotyledonous timbers, because the elements concerned in drying, the 

 tracheides, are the water conducting elements, while in the hardwoods 

 the elements concerned in drying are the fibres or strength elements. 

 The movement of water in a tree is, in some intimate way, closely 

 associated with the cell walls, and if a hardwood consisted of vessels 

 only, we should find this timber drying as quickly as coniferous. 



It is of interest to know how the moisture diffuses through the 

 wood. Moisture in wood has been classified into free water and water 

 of saturation. All water in the timber over or above what is known 

 as the fibre saturation point, is called free water. The fibre satura- 

 tion point is defined as the concentration of water necessary to satu- 

 rate the walls of the cells without there being any water in the 

 cavities of the cells. The term free water is used because it Is 

 assumed that this so-called free water passes out of the wood first 

 when timber is drying. When it has all been lost the timber is then 

 at the fibre saturation point. When the water of saturation begins 

 to be lost, shrinkage logins. 



This theory of the loss of moisture from wood is against the facts. 



The word " free " is unfortunate, as the term implies that the 

 water is free to move. Now if a block of wood containing this 

 so-called water be placed in a saturated atmosphere, the block remains 

 constant in weight. In soils we get free water, which is the water in 

 excess O'f saturation. The water is truly free, because it moves under 

 the force of gravity. The water in the cells of the wood is not free to 

 move under the force of gravity. Instead of calling this cell water 

 free, we may call it Contained Water. There is, however, actual free 

 water in a tree. When a giant eucalypt is felled, water pours from 

 the cut end of the bole. This may be observed even in mid-summer. 

 This water is truly free, for we cannot prevent its loss by merely 

 altering the humidity of the atmosphere. 



Free water occurs in birch, and the phenomenon of weeping is 

 well known. This free water has all been lost by the time the logs 

 get to the mill. We may define Free Water as the water which 

 escapes from the lumber not as vapor, but as a liquid. The loss is 

 due to the force of gravity. Free water, as here defined, must 

 readily escape from the cut ends of the vessels, when full, of such 

 timbers as Eucalptus, for in these the vessels are very large. 



