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Scientific Lumber Drying 



Editor's Note 



Tlie foIIowiiK." pnpor was read bv Z. Clni-k Tliwlnc ot tlio firaiul Hapids Vniiooi' Works, Craiid Itapids. Mich., 

 iK'Tore tho inoctlii;; of the National Veneer and Panel Manufacturers' Associalion at L'bieago on December 10. Com- 

 ment on tills paper will be found on page 10 of this Issue. 



A thorough unilorstamling of the fibrous and cellular construc- 

 tion of iumlior and its laws of growth is essential to a clear under- 

 standing of the correct and scientific i)rinciplcs of drying luml)cr. 

 Those who try to reduce lumber drying to a few definite rules and 

 practices fail to realize that every tree possesses individuality as 

 does every human being. 



In other words, the range of variation between the white oak 

 of Michigan and the so-called delta or swamp oak of the Missis- 

 sijipi valley is so great that no general description or rules for 

 handling can be made to apply to both accurately. In the same 

 way there is a marked variation between the famous "cork" 

 pine of Michigan and Wisconsin and the present substitutes for 

 tho same in western pine of the Pacific coast. It is therefore 

 inevitable that any underlying principle governing the drying of 

 any particular kind of lumber must be applied with that judgment 

 and <liscretion that is prompted by the character, the locality and 

 particular variety of wood to be handled. 



In general, commercial varieties of lumber have been divided 

 by scientific investigators into three groups. 



First — non-porous. These are so called because a cross-section 

 of the wood under the microscope gives no indication of pores, 

 capillaries, ducts, or sap canals similar to the well-known sap canals 

 of the oak. In other words there is uniformity in the cross-section due 

 to absence of these canals, which sometimes makes it difficult to 

 identify even the annular growth rings. Under this head are 

 practically all evergreens or pines, technically called conifers. 



Skcond — The diffuse-porous are intermediate between the non- 

 porous and the ring-porous below. In other words, the sap canals 

 are small and uniformly distributed throughout a cross-section. 

 They can be observed under the microscope in cross-sections that 

 have been well smoothed and prepared for inspection. Examples 

 of this class are the maple and gum. 



Third — Eing-porous. This is the most easily described and dis- 

 tinguishable class for the reason that the sap canals are large 

 and grouped together in a conspicuous ariangement. Oak and 

 chestnut are typical of this class of woods. It is perhaps well in 

 passing to note that this last classification will be the most difficult 

 to dry because it lacks the uniformity in texture and solidity that 

 the two other classes possess. 



The medullary or pith rays, which are most conspicuous in woods 

 like oak, are pockets containing food material for nourishing the 

 tree. They are practically on a radial line from the heart to the 

 bark of the tree. These pith rays are more or less perceptible in a 

 number of commercial woods but in no other case have they reached 

 the stage of beauty that they give to white oak when properly 

 quarter-sawn. 



The growth rings of a tree are distinguished, not by the sap 

 canals, but by the difference in size of the sap cells between the 

 spring and summer growth. It is evident to any one that the 

 growth of the tree, which always occurs just under the bark, will 

 be more rapid when there is an adequate supply of water in the 

 ground, or in the spring time. It will be equally obvious that as 

 the season progresses into midsummer there w-ill be less and less 

 water in the ground and as a consequence growth will be limited 

 and the size of the sap cells will be greatly reduced. This may 

 be observed with the naked eye in the cross-section of nearly all 

 woods, and is very clear under the microscope. 



The sap cells of a tree are to be distinguished from the sap 

 canals. The sap canal in oak trees, for example, is that open space 

 about the size of a pin through which free sap may move upward 

 from the roots to the trunk and branches above. The duets are 

 sometimes twenty to thirty feet long and have cross and transverse 

 connections with each other in order that they may continually 

 22 



carry the sap upward. Nature provides small check or flap valves 

 to prevent the sap from flowing downward. 



The sap cells, strictly speaking, are part of the fiber of the wopd, 

 containing a limited amount of moisture, and are only from one- 

 sixteenth to one-fourth inch long. The sap cells also serve to carry ( 

 sap upwards and are provided with more frequent transverse con- 

 nections and cheek valves than are the sap canals. The sap cells 

 are long and narrow, shaped somewhat like ;in Imliaii canoe with 

 ends overlapping. 



In longleaf pine the cells are unusuallj- long and there is encour- 

 agement to believe that this wood will eventually make a paper 

 that will rival the paper made from Canadian spruce, for toughness. 

 If you take a i^iece of jJaper and tear it carefully in such a way as 

 to rip the fibers, and examine the torn edge under a microscope, 

 you will observe short fine hairs which constitute the wall of the 

 sap cell and which are the valuable strength giving part of the 

 tree for either woodworking or the preparation of pulp for arti- 

 ficial wood products. It is evident, therefore, that the portion 

 of the wood to be preserved for useful ends is the fiber that 

 constitutes the cell wall. 



It has been determined by experiment that in some instances, 

 notably in such woods as Cottonwood and spruce, the total mois- 

 ture content may be as high as 150 per cent; the greater portion 

 of which may be termed "free" moisture, forming the contents 

 of the sap canals. To render this more explicit: presuming that 

 the net weight of lumber when bone dry would be 100, the total 

 weight of the lumber when green would be 250, two-fifths of this 

 consisting of solid material and three-fifths moisture. 



The cell moisture is the liquid contents of the cellular fibrous 

 structure of the lumber, which will consist of about 30 per cent 

 of the 150 per cent total moisture content. 



Practically all the shrinkage that takes place in the drying of 

 lumber occurs after the "free" or sap canal moisture has evap- 

 orated and is caused by the contraction of the cell walls during the 

 removal of their contents. 



In passing it may be well to mention that the part of the tree 

 that is most alive, or the part that is growing, is directly under 

 the bark. For this reason the heart of the tree always contains 

 less life than the circumference, in fact the heart of the tree is 

 the point at which death of the cell first occurs. The mere ref- 

 erence to the giant sequoias of California, which have famous hol- 

 low spaces in their centers, is sufficient evidence to show that life 

 and perfection in the heart of the tree is not at all essential to its 

 ultimate growth and development. On the other hand, it is a 

 well-known fact that the girdling of a tree, a little deeper than 

 the sapwood, will kill the tree. It is, therefore, evident that 

 the sapwood will always contain a greater amount of moisture 

 than the heartwood and will be subject to more shrinkage and 

 swelling in drying and will prove to be the strongest wood for 

 use where tensile strength is necessary. The heartwood, however, 

 will be the choice wood when it comes to the securing of rich 

 grain and color effects. This is partieularl}' noteworthy in the 

 red gums where the heart has a very rich color. 



To go more directly to the subject under discussion — scientific 

 lumber dryilig — it will be interesting to call attention to the 

 various methods for drying lumber which have served woodwork- 

 ers and sawmills during the last hundred years, and show a dis- 

 tinct evolution from the crude to the refined. This evolution at 

 every step shows an increasing efficiency and growing comprehen- 

 sion of the treatment that is necessary to rapidly and safely 

 remove the moisture from the wood without injuring the fiber. 



The first method of drying lumber is as old as the world — air- 

 drying in one form or another has been practiced since the first 



