﻿DECAYS AND DECOLORATIONS IN AIRPLANE WOODS. 



19 



any importance, since the injury occurred alone with only a very 

 short lightning ring and on the stream lining where high strength 

 is not requisite. Lightning rings are more readily detected on ;i 

 member before it is sanded. In some cases the seriousness of the 

 defect can be determined after planing but before the piece is 

 shaped. This is usually possible when the defect runs entirely 

 through the piece. 



In white fir the lightning rings are easily detected both on cross 

 section or on the radial face. The normal color of the summer wood 

 is a light brown, while the lightning ring is a pronounced brown 

 or purplish brown, which stands out strongly against the whitish 

 sapwood or heartwood: Abundant resin ducts occur in these rings. 



Lightning rings in incense cedar are dark brown in color, standing 

 out plainly in the white sapwood, but are not so apparent, although 

 still recognizable, 

 against the reddish 

 brown heartwood. 

 Resin ducts do not ac- 

 comp any lightning 

 rings in cedar. 



Sitka spruce wood 

 is rather susceptible 

 to the effects of elec- 

 tricity. The light- 

 ning rings appear as 

 light to dark brown 

 lines in the pale pink- 

 ish heartwood or white 

 sapwood. Rings are 

 found which appear 

 to be chiefly composed 

 of resin ducts ; in fact, 

 when viewed on the end section, it is seen that the resin ducts are so 

 numerous that they almost coalesce. This condition is illustrated in 

 Figure 5. Furthermore, spruce wood is peculiarly susceptible to dis- 

 coloration by lightning injury. Often in connection with a lightning- 

 ring a reddish brown discoloration is found, somewhat tinged with 

 purple. This discoloration rarely extends radially more than 3 or 4 

 inches from the lightning ring toward the pith, but m?ij extend 2 feet 

 beyond the limits of the ring in a vertical direction. Wood so dis- 

 colored is not weakened. Furthermore, the color is not sufficiently 

 intense to detract from its value for any purpose, particularly since 

 the discoloration when varnished appears merely as a darker tone 

 of the normal heartwood. 



The lightning rings found in Douglas fir are red-brown in color, 

 darker than the summer wood and consequently are quite apparent 

 in the white sapwood and orange-red or yellowish heartwood. These 

 rings are practically composed of resin ducts. The ducts are smaller 

 than in Sitka spruce. 



The reader must not get the impression from what has been written 

 that lightning rings are a feature of every piece of wood, but they 

 do occur and must be taken into account. 



Fig. 5. — Cross section from an unfinished elevator beam, 

 showing a lightning ring in Sitka spruce. The row of 

 resin ducts extending entirely across the section can be 

 seen in the summer wood of the fifth annual ring from 

 the bottom. The defect ran the entire length of the 

 beam. 



