﻿DECAYS AND DISCOLOEATIONS IN AIKPLANK WOODS. 31 



as liable to decay as before. To be sure, kiln drying is much better 

 than air drying, since the high temperatures employed in the fori net 

 process are probably fatal to the hyphso of some decay-producing 

 fungi, while under the latter conditions the fungi may merely re- 

 main dormant until suitable moisture conditions are again restored. 

 However, since wood-destroying fungi are common around and in 

 yards and wood-working factories, the chances are that kiln-dried 

 lumber will be reinfected, and if it becomes moist again decay will 

 begin. 



Shipping green or even partially air- dried lumber on long voy- 

 ages through tropical seas in the hold of a vessel offers a chance 

 for a heavy loss through decay. The close humid air of the ship's 

 hold becomes a perfect forcing chamber for wood-destroying fungi 

 when warm latitudes are reached. Shipments of Douglas fir leav- 

 ing the Pacific coast perfectly sound have contained a considerable 

 percentage of decayed lumber when unloaded at a South African 

 port (36, p. 36). Indirect reports indicate that the same condition 

 resulted during the World War in some shipments of Sitka spruce 

 routed to Europe through the Panama Canal and the Mediterranean 

 Sea. 



Durability of tvood. — Resistance to decay, or as it is termed 

 " durability," is a factor that should no longer be neglected in 

 selecting woods for airplane construction. Airplanes are being 

 more and more exposed to unfavorable weather conditions as their 

 use extends, conditions which in some instances are highly favorable 

 to decay. Furthermore, certain conditions created by the construc- 

 tion of an airplane promote decay. For example, in the interior of 

 the wings the relative humidity may be much higher than that of 

 the surrounding air, and there is often considerable condensation 

 of moisture. In addition, the temperature is slightly higher. All 

 these factors are favorable to the development of wood-destroying 

 fungi. 



Within any species durability increases with the increase in 

 specific gravity. Consequently, the fact that only wood with high 

 specific gravity is used for aircraft not only increases strength but 

 serves to increase durability. However, it is well known that differ- 

 ent species vary widely in their durability. Unfortunately, spruce 

 is not at all durable. Neither are basswood and birch. Douglas fir 

 is fairly durable, as is also white oak. But the cedars are remarkable 

 for their inherent durability, and among these Port Orford cedar 

 compares favorably with spruce in all its strength properties and is 

 only slightly heavier. Consequently, this wood can not be too 

 highly recommended for use in aircraft where resistance to decay 

 must be considered. Sapwood must not be used under such cir- 

 cumstances, for no matter what the species is it decays easily. 



Contrary to existing belief, the resin content of wood is of slight 

 importance in relation to durability (74, p. 153-154; 75, p. 66-68). 

 Resin itself has no poisonous effect on the growth of fungous 

 hyphae, and its only beneficial effect in increasing durability is its 

 waterproofing action on wood. This is so slight, however, if the 

 normal resin content of softwoods is considered, as to be practically 

 negligible. If wood is rendered more durable through a sufficient 

 increase in its resin content to have a decided waterproofing effect, 



