AMERICAN FOREST TREES 11 



Long exposure to dry air takes out a large percentage of the moisture 

 which green wood holds, and* the lumber is known as air-dry. But 

 some of the original moisture remains, and air at climatic temperature 

 is unable to expel it. The greater heat of a drykiln drives away some 

 more of it, but a quantity yet remains. The lumber is then kiln-dry. 

 Greater heat than the drykiln 's is secured in an oven, and a little 

 more of the wood's moisture is expelled; but the only method of driving 

 all the moisture out is to heat the wood sufficiently to break down its 

 structure, and reduce it to charcoal. 



Wood warps in the process of drying unless it seasons equally on 

 all sides. It curls or bends toward the side which dries most rapidly. 

 Dry wood may warp if exposed to dampness, if one side is more exposed 

 and receives more moisture than another. It curls or bends toward 

 the dryer side. 



Warping is primarily due to the more rapid contraction or ex- 

 pansion of wood cells on one side of the piece than on the other. 

 Saturated cells are larger than dry ones. 



Moisture in wood affects its strength, the dryer the stronger, at 

 least within certain limits. Architects and builders carefully study the 

 seasoning of timber, because it is a most important factor in then* busi- 

 ness. The moisture which most affects a wood's strength is that ab- 

 sorbed in the cell walls, rather than that contained in the cell cavities 

 themselves. 



Some woods check or split badly in seasoning unless attended with 

 constant care. Checking is due chiefly to lack of uniformity in season- 

 ing. One part of the stick dries faster than another, the dryer fibers 

 contract, and the pull splits the wood. The checks may be small, 

 even microscopic, or they may develop yawning cracks such as some- 

 times appear in the ends of hickory and black walnut logs. Greenwood 

 checks worse in summer than in winter, because the weather is warmer, 

 the wood's surface dries faster, and the strain on the fibers is greater. 

 Phases of the moon have no influence on the seasoning, checking, warp- 

 ing, or lasting properties of timber. 



Stiffness, Elasticity, and Strength Rules for measuring the stiffness 

 of timber are involved in mathematical formulas; but the practical 

 quality of stiffness is not difficult to understand. Wood which does not 

 bend easily is stiff. If it springs back to its original position after the 

 removal of the force which bends it, the wood is elastic. The greatest 

 load it can sustain without breaking, is the measure of its strength. The 

 load required to produce a certain amount of bending is the measure of 

 its stiffness. Flexibility, a term much used by certain classes of workers 

 in wood, is the opposite of stiffness. A brittle wood is not necessarily 



