


THE AIR SEASONING OF WESTERN SOFTWOOD LUMBER 3 
A great variety of air-seasoning methods have come into use dur- 
ing the long period of development in the lumber industry. Obser- 
vations of much value have been accumulated by those of experience 
in the industry; and yet to-day, after many years of air-seasoning 
practice, a wide difference of opinion exists among operators as to 
the relative value of many of the basic principles involved. There 
is urgent need for authentic and usable information based on careful 
studies. 
It is obvious that the solution of the air-seasoning problem can not 
be found in any set of “cut-and-dried” rules. ‘The chief aim of 
this bulletin is to present those general principles which can be 
applied by the lumberman in the manner that will best meet his own 
specific conditions and problems. No attempt is made to present 
the detailed data in substantiation of the conclusions given. The 
conclusions are based on surveys and study of current practice and 
on intensive air-seasoning investigations by the Forest Service within 
the four western lumber-producing regions.? 
IMPORTANT PRINCIPLES OF WOOD DRYING AND THEIR 
GENERAL APPLICATION 
OCCURRENCE OF MOISTURE IN WOOD 
Moisture in wood, or sap, is chiefly water with small percentages 
of organic and mineral matter present in soluble form. In the sap- 
wood these materials are largely sugars, but in the heartwood they 
are principally tannins, resins, and dyestuffs. For all practical 
purposes in the drying of wood sap can be considered as water, 
since only very small quantities of the other materials pass off in 
evaporation.* 
3 Acknowledgment is made by the authors to other members of the Forest Service, par- 
ticularly those of the Forest Products Laboratory, who have contributed in a large 
measure to present knowledge relating to the principles of drying wood. Acknowledgment 
is also made to C. Burdette Green, formerly a member of the Forest Service, for his very 
considerable contributions of air-seasoning data from California. 
4The amount of moisture in wood, or the moisture content, is expressed in terms of 
percentage of the oven-dry weight of the wood. Thus, if the moisture content of a green 
board is 71 per cent, there are by weight 71 parts of water to 100 parts of oven-dry wood. 
Similarly, should the moisture content of a board happen to be exactly 100 per cent, the 
weight of the moisture and that of the oven-dry wood would be equal. The average 
moisture content of a lot of lumber may be determined in the following manner: 
Select representative pieces—about 1 out of every 100 to 500 pieces—with a fair repre- 
sentation of both heartwood and sapwood. 
At a point about 2 feet from one end of each piece, cut out a section three-fourths to 
1 inch wide, making the cut at a place free from knots, rot, pitch streaks, or other defects. 
Trim off all slivers from this section or sample. Sy 
Weigh the samples immediately and carefully on a delicate balance. This is the 
original weight. 
Place samples in an oven heated to 212° F., or, if an oven is not available, on hot steam 
pipes; but do not scorch or bake them. : 4 
When samples have reached a constant weight, as can be determined by repeated weigh- 
ing, remove them from the oven. (After a little experience the time required to reach 
constant weight can be estimated, and thus repeated weighings may be avoided. Twenty- 
four hours should be the maximum time necessary with softwoods.) This final weight is 
the oven-dry weight. ; c f 4 
Subtract the oven-dry weight from the original weight. The difference is the loss in 
moisture. 
Divide the difference by the oven-dry weight and multiply by 100. This gives the per- 
centage of moisture contained in the wood based on the oven-dry weight. 
EXAMPLB 
Original weight =284.7 grams. 
Oven-dry weight—180.2 grams. 
284.7 grams—180.2 grams=104.5 grams, or the moisture lost. 
104.5 +180.2=.580 X 100=58.0 per cent moisture originally in the wood. 
(For convenience and accuracy the gram is often used as the unit of measurement, but 
other units, such as the ounce, may be employed.) 
