August 25, 1917 



The Warping of Wood 



Hu Maxwkll 



Editor's Note 



SoniP (tf the most vexatious problems that come to the \v Iworker relate to the shrinking, 



nd checkinjr of wood. Those distortions result when limiber parts with some of its moisture 



receives mrvr*' than it can stow away in the cavities with which wood abounds. The dry kiln is -,-. 



tus most fri'(jii,.ntly put in use to rid the manufacturer of the nuisance of the warping arid checking ofYumber. 

 Many of the problems have not yet been satisfactorily mastered, but progress in that direction has been great 

 in recent years. 



ARTICLE TWELVE 



Board sidewalks in villages which have not passed that stage of 

 civic progress, often show lumber that is warped in an extreme degree. 

 The boards are three or four feet long and are nailed across sleepers 

 or stringers, and are curled in the shape of new moons laid on their 

 backs. The stranger who is lacking in the special kind of experience 

 required, can get over such a sidewalk with difficulty. Every plank 

 tilts and rocks as he steps on it; but practice has made the native 

 villager perfect in that variety of pedestrian gymnastics, and he is 

 able to navigate such a sidewalk without apparent inconvenience. 



The old roof of oak shingles furnishes another example of excessive 

 warping. Oak shingles are seldom made in these days, because the oak 

 is worth more for staves and lumber, and shingles are now made of 

 other woods. But the genuine article may still be occasionally seen 

 in rural districts where old roofs liavc survivcil from former years. 

 During long spells of dry weather 

 the old oak shingles curl up at 

 the loose end, giving the roof the 

 appearance of the ruffled feathers 

 of an irritable fowl. 



Tan bark stripped from tree 

 trunks by the peelers, and laid out 

 to dry in the sun, furnishes 

 examples of excessive warping. 

 Broad slabs of bark, during the 

 process of drying, may roll up 

 tightly in the form of cigars. 



Other and various instances of 

 the warping of wood may be met 

 with nearly anywhere that the 

 material is put to use. It is fre- 

 quent in lunil;er yards, particu- 

 larly if the piling of the lumber 

 has not been carefully attendetl 



to. Boards bending upward at the sides and the ends, if free to 

 move, may rise partly from the ground. Finished work may warp 

 as badly as rough boards, for the mere process of planing, sawing, and 

 fitting wood does not guarantee it agaiust warping. Possibly a door 

 refuses to latch because one side has twisted out of plumb, or a window 

 sash will not slide up and down as it was meant to do, or a table top 

 presents a distorted appearance, of possibly a piano top will do the 

 same, and all sorts of paneling may become curled and warped. 



When wood behaves in this manner it is doing nothing new. The 

 handle of the stone hatchet of the paleolithic man warped as badly, 

 and in the same way, as the ax handle of the modern lumberman. 

 Wood has not changed. Modern methods of working it have not 

 increased or lessened the ninterial's natural tendencies to twist or 

 pull out of shape. The modern boat builder who is compelled to reject 

 a warped stanchion is confronted by precisely the same proposition 

 as confounded Noah when he discovered that a king post of the ark 

 had warped and had pulled the rooftree out of line. 

 W.\RPiNG IN Wood Due to Shrinkage. 

 The warping of any material is due to stresses, usually to the pull 

 of a force that is not counteracted by an equal pull in the opposite 

 direction. In wood this pull is commonly caused by shrinkage, and 

 the shrinkage is the result of drying. 



Wood is not the only material that warps. In the old cemetery at 

 Frederick, Md., are many graves, each covered with a marble slab, 



ROOF OF WARPED SHINGLES 



The curl in these shingles is due to the more rapid drying of the upper than 



the lower side after rains. North t'arolina. 



on four pofts, like a talle top. Some of them have been in position 

 for a hundred years. The pull of gravity has warjied some of these 

 slabs so that the middle is several inches below the ends which rest 

 on the posts. The stone slabs are curved in much the same manner 

 as the hoards of the sidewalk mentioned in a former paragraph; but 

 the stress that warped them was not due to shrinkage but to gravity — 

 to a pull in one direction with nothing on the opposite side to offset 

 it. Though due to a different cause from that commonly producing 

 lumber warping, the result is comparable to the warpage of wood. 



Wood will warp in the same way, but when the stress is outside 

 rather than in, the name of the result is usually "bending" instead 

 of warping. On some of the western mountains where wet snow in 

 winter accumulates to depths of from five to twenty feet, the hitching 

 racks for horses in front of moimtain stores (which are abaniloned 

 in winter! present an interesting appearance after the snow melts in 



the spring. The racks consist of 

 beams about eight inches square 

 resting on posts twenty feet 

 apart. The weight of the snow, 

 liearing down for weeks at a time, 

 benils the beams to the shape of 

 the sidewalk boards or the grave 

 slabs. Again it is the result of 

 stress in one direction only. 



These homely examples contain 

 the key to all kinds of wood warp- 

 age. The processes may be very 

 sinijde or extremely complex, but 

 the causes are the same — stress 

 in one direction with nothing to 

 offset it. 



Checks anil cracks in wood may 

 be regarded as a kind of safety 

 valve to relieve the stresses and 

 prevent warping. The wood pulls apart and a crack opens, because 

 the wood will split sooner than it will bend. Cheeks vary in size from 

 those so small that a microscope is required to reveal them, to others 

 large enough to furnish hiding places for bats. Very large cracks 

 sometimes open in the ends of hardwood logs (seldom in softwoods). 

 Oak, hickory and chestnut are frequently damaged by such cracks; 

 but the extreme is seen in blue gum or eucalyptus that grows in Cali- 

 fornia, the logs of which sometimes split wide open from end to end. 

 Cause and Effect. 

 A study of the warping of wood, considered in detail, involves an 

 examination of the minute structure of the material. This might 

 be carried far into the special field of the microscope, but it is not 

 necessary to do it in order to cover the ground outlined in this article. 

 A few simple facts will suffice. 



In the first place, the structure of wood is very complex. It has 

 been remarked that a modern skyscraper building, with its rooms, 

 halls, elevators, windows, chutes, floors, and dividing walls, is not 

 more complex in structure than a half cubic inch of wood, if magnified 

 to thi' size of the buUding. Wood is composed primarily of cells and 

 other elements, some of which lie with their long axes up and down 

 the trunk, others at right angles to them, extending from the bark 

 toward the center of the tree. Those extending up ami down are the 

 most important in the majority of woods, because most numerous, 

 the largest in size, and most vital to the tree's existence. The elements 



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