It is evident that to dry wood properly a great many factors must be taken into consideration aside from the mere evaporation of moisture.
Losses Due to Improper Kiln-drying
In some cases there is practically no loss in drying, but more often it ranges from 1 to 3 per cent, and 7 to 10 per cent in refractory woods such as gum. In exceptional instances the losses are as high as 33 per cent.
In air-drying there is little or no control over the process; it may take place too rapidly on some days and too slowly on others, and it may be very non-uniform.
Hardwoods in large sizes almost invariably check.
By proper kiln-drying these unfavorable circumstances may be eliminated. However, air-drying is unquestionably to be preferred to bad kiln-drying, and when there is any doubt in the case it is generally safer to trust to air-drying.
If the fundamental principles are all taken care of, green lumber can be better dried in the dry kiln.
Properties of Wood that affect Drying
It is clear, from the previous discussion of the structure of wood, that this property is of first importance among those influencing the seasoning of wood. The free water way usually be extracted quite readily from porous hardwoods. The presence of tyloses in white oak makes even this a difficult problem. On the other hand, its more complex structure usually renders the hygroscopic moisture quite difficult to extract.
The lack of an open, porous structure renders the transfusion of moisture through some woods very slow, while the reverse may be true of other species. The point of interest is that all the different variations in structure affect the drying rates of woods. The structure of the gums suggests relatively easy seasoning.