﻿DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 17 



white wood of these species, whereas in darker woods the streaks 

 would pass unnoticed. 



Burns or scorches in wood may occur from the use of high-speed 

 saws if the saws are not set properly to provide sufficient clearance. 

 Improperly set planing knives will produce the same effect. Usu- 

 ally such burns, appearing on the face of the piece as dark-brown to 

 blackish blotches, are very superficial and can be planed off. The 

 injurious effect is negligible. Deep burns, extending through a piece 

 one-eighth or even one-fourth of an inch in thickness are rarely 

 encountered and are usually confined to particularly susceptible 

 woods, such as the white pines. These may result when a high-speed 

 sander stops suddenly. The wood is injured and can not be used 

 for highly stressed parts. Burns usually occur in the remanufac- 

 ture of dry lumber and not on green lumber in the mills. 



DISCOLOEATIONS CAUSED BY WOUNDS. 



The term "wounds" as applied to trees includes not only those 

 scars by which the bark is removed from living trees, exposing the 

 sapwood or heartwood with the death of the cambium over the ex- 

 posed surface, but also those injuries by which the cambium is 

 temporarily damaged but not killed. The cambium, which is very 

 susceptible to injury, is the very narrow layer of delicate growing 

 tissue of a tree situated at the junction of the living bark and sap- 

 wood. When this tissue is injured or killed, a healing or callusing 

 process immediately begins which causes a dip or wave in the grain. 

 Consequently, irregularity of grain in a timber often indicates prox- 

 imity to a wound. 



Wounds in living trees result from a variety of causes, among 

 which may be mentioned fire, lightning, insects, birds, and man. 

 All such injuries are usually accompanied by a discoloration of 

 the wood, particularly the sapwood. Such discolorations are most 

 intense in the hardwoods, especially in the sapwood of such species 

 as white ash, hickory, maple, birch, and tulip poplar. 



When the wood of a living tree is exposed to the air it dries out 

 and changes color. In softwoods the change is to a grayish brown 

 or dead-gray color, while in hardwoods the change ranges from a 

 deep brown to almost black, most noticeable in the sapwood. This 

 color change is an oxidation process. Although the wood is not 

 weakened by this change, wound wood of this type . should be 

 avoided, owing to the fact that during its exposure to the air it 

 often becomes infected by wood- destroying fungi and may be weak- 

 ened by incipient decay. 



LIGHTNING WOUNDS. 



The general appearance of lightning injury is readily recognized. 

 Spike tops and stag heads, together with the spiral wounds exist- 

 ing for many feet along the trunks of the trees, are unmistakable. 



Besides such wounds, the cambium is very susceptible to electrical 

 discharge and may be affected for some distance down the tree with- 

 out any outward visible indications. This irritation to the cambium 

 results in the formation of a layer of cells changed in both shape and 

 structure from the normal. Often in the conifers an unusually large 

 number of resin cells or resin ducts are formed within this injured 



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