20 



HARDWOOD RECORD 



August 23, 1917 



extending from the bark inward belong to the medullary rays. All 

 woods have these rays, and all woods likewise have fiber running the 

 long way, up and down the trunk. These fibers or cells are the strings 

 which draw, when wood is in the process of drying, and the pull may 

 warp the wood or cause it to check. 



The pull or stress is produced by the drying, and the consequent 

 shrinking, of the wood. When a certain percentage of the water in 

 green or wet wood goes out, the individual cells become smaller by 

 the thinning or contracting of their walls. Every cell so shrinking 

 pulls a little, and when sucli a force is multiplied by millions, it may 



SF/VIMV'A'EU. 



W.VRPED T.ABLE TOP. 



Caused by the More Itapid Drying of the Upper Than the Lower Side. 



Common in Kitolien Furniture Suljje'ct to Frequent Wetting. 



become powerful enough to produce warping in a plauk, panel, board, 

 or other piece of wood. 



A piece of wood contracts sidewise but not much endwise. That i,'* 

 because the individual cells composing the piece shrink sidewise but 

 very little endwise. The shrinkage of a plank or beam is only a multi- 

 plication of the shrinkage of individual cells or fibers. Most of the 

 cells composing a stick run lengthwise. There is, however, some 

 lengthwise shrinkage in wood, and a good deal more of it in some 

 species than in others. Oak has it to a considerable extent, whicli 

 accounts for the curling up of oak shingles and for the new-moon 

 shape of oak boards in sidewalks. There is less end shrinkage in soft 

 woods than in hardwoods. The pioneer farmer know that the white 

 pine shingles with which he roofed his buildings did not curl up; and 

 experience taught boards of aldermen in little tott'ns that sidewalks of 

 hemlock, pine, and spruce did not warp badly, though these wood.s 

 decayed sooner than oak; and sometimes the new-moon sidewalk of 

 oak that lasted well was preferred to the straight planks of softwooil 

 which looked better but fell to pieces sooner. 



Endwise Shrinkage 

 The sidewise shrinkage of a board or other piece of wood is common 

 and well-known, but there is some shrinkage endwise and a reason 

 for it. The cells which run the long way of the lumber, while they are 

 drying, contract a little in length and contribute in proportion to 

 the pull lengthwise. But the main lengthwise pull comes from the 

 contraction of that system of cells forming the medullary rays. These 

 cells, as stated before, have their longest axes extending from the bark 

 inward toward the tree's center. When they contract sidewise, their 

 pull is lengthwise with the stick and tends to make it shorter. Cells 

 of that kind are pretty numerous, but few in comparison with those 

 extending up and down the tree. Hermann von Schrenk estimated that 

 western yellow pine has 25,000 medullary rays per square inch. Each 

 ray is composed of bundles of cells and the cells have their long axes 

 extending from the bark toward the heart. When these cells contract 

 sidewise their pull shortens the piece of timber. Perhaps most end- 

 wise shrinking of wood is duo to the sidewise contraction of the 

 medullary cells. As might be expected, woods with the largest and 

 most numerous medullary lays, such as o«k, beech, and sycamore. 



show considerable tendency to shrink lengthwise. Wagon felloes of 

 oak shrink so much lengthwise in the hot dry summers in the South- 

 west that tires can scarcely be kept on the wheels ; while Osage orange 

 felloes in the same region have so Uttle endwise shrinkage that it can 

 scarcely be noticed. The difference in the behavior of oak and Osage 

 orange may be accounted for by the difiference in size and number of 

 the medullary rays of the woods. 



The stress of shrinkage appears never to be suflBcient lengthwise 

 to cause checks running across the grain, though checks lengthwise 

 may occur in any wood. Theoretically, at least, a stress or pull might 

 be strong enough to produce checks in wood across the grain. Some 

 of the so-called African mahoganies occasionally show ' ' breaks ' ' 

 which have all the appearances of checks across the grain. It is 

 supposed they are caused, not by the stress of shrinkage, but by the 

 violent swaying of the trees by high winds. 



The Economic Aspects 

 The shrinkage of wood and all the consequences, particularly warp- 

 ing, cheeking, and hollow-horning, are considered much more impor- 

 tant now than they once were. That is because the utilization of wood 

 is more highly developed than it was a few generations ago. The 

 early farmer did not much care whether his fence boards warped or 

 not; but the modern farmer does not want his dining table or piano 

 to warp. In former times it did not matter much about the rough 

 lumber used for the barn, but the panel work in the modern residence 

 must not check and crack. The whole order of uses has undergone 

 a change, and is stUl undergoing changes, and will continue to do so, 

 and the manufacturer of wooden commodities must recognize the 

 changes and conform to their demands. Nearly every article made 

 of wood is entering higher classes, demamliug better material and 

 better work. 



E.VSIER TO SPLIT THAN TO WARP. 



The Log Was Too Thick and Strong to Bend. So the Wood 

 I'nder the Stress of Excessive Shrinlingc. 



'ulled .Vp.Trt 



One of the most important changes in woodworking is that time 

 is now an element where formerly it was relatively unimportant. 

 The drying of lumber must be done quickly. The makers of old-time 

 hat blocks and gunstocks could wait six years for their wood to air 

 dry; but the modern furniture manufacturer is not willing to wait so 

 long, and he hurries the seasoning process by means of a dry kUn. 

 Much time is saved, but at the same time risk of spoiling lumber is 

 incurred. The old slow way of taking moisture out of lumber by open 

 air exposure generally left the boards straight and sound, but the 

 modern kUn has brought the shrinking, warping, and cheeking prob- 

 lems to the front, and the foremost dry kiln makers are doing their 

 best to solve those problems. The kiln 's purjiosc is to dry wood 

 quickly, but it must be done well. 



