DEFECTS OF WOODS 415 



Warping is the result of unequal drying or shrinkage. If 

 one side of a board is fully exposed to the air or heat while 

 the other side is less exposed, the side most exposed will dry 

 more quickly and, of course, shrink. This unequal shrinkage 

 causes the board to warp or curl somewhat on that side. 

 To prevent this occurrence, lumber is usually piled with 

 sticks between each course or layer, so that the air may 

 penetrate to the under side of each board. In some of the 

 modern kiln-dryers, the lumber is dried under pressure, the 

 piling sticks being replaced by steam pipes. When the lumber 

 is all piled, the whole mass is clamped together, so that 

 warping is impossible. Some wood has a natural tendency 

 to warp even before it is dried at all, and unless fastened, 

 cannot be kept straight. Such lumber comes from the 

 outside cuts obtained in flat sawing. In many cases it is 

 of commercial use, but is of an inferior quality, and should 

 not be used in the modern shop. The natural tendency in 

 warping is for the annual rings to straighten out. Therefore 

 boards cut nearest the heart are the best. 



478. Cause of Rottenness. Little plants, called fungi, 

 and insects attack wood in many ways and cause it to rot. 

 Some fungi kill the roots of trees; some grow upward from 

 the ground into the trees and change the sound wood of 

 the trunks to a useless rotten mass; and the minute 

 spores (or seeds) of others float through the air and come 

 in contact with that part of the tree which is above ground. 

 Wherever wood is exposed, there is danger that dis- 

 ease may form. Consequently all wounds, such as those 

 made in pruning, should be covered with some sub- 

 stance like paint or tar to exclude the air and the spores it 

 carries. 



