Naturally, the heavier the material to be dried, the more difficulty is experienced from checking, which has its most active time in the spring when the sap is rising. In fact the main period of danger in material checking comes with the March winds and the April showers, and not infrequently in the South it occurs earlier than that. In other words, as soon as the sap begins to rise, the timber shows signs of checking, and that is the time to take extra precautions by careful piling and protection from the sun. When the hot days of summer arrive the tendency to check is not so bad, but stock will sour from the heat, stain from the sap, mildew from moisture, and fall a prey to wood-destroying insects.
It has been proven in a general way that wood will season more slowly in winter than in summer, and also that the water content during various months varies. In the spring the drying-out of wood cut in October and November will take place more rapidly.
SECTION XI
KILN-DRYING OF WOOD
Advantages of Kiln-drying over Air-drying
Some of the advantages of kiln-drying to be secured over air-drying in addition to reducing the shipping weight and lessening quantity of stock are the following:
- 1. Less material lost.
- 2. Better quality of product.
- 3. Prevention of sap stain and mould.
- 4. Fixation of gums and resins.
- 5. Reduction of hygroscopicity.
This reduction in the tendency to take up moisture means a reduction in the "working" of the material which, even though slight, is of importance.
The problem of drying wood in the best manner divides itself into two distinct parts, one of which is entirely concerned with the behavior of the wood itself and the physical phenomena involved, while the other part has to do with the control of the drying process.