34 



Hardwood Record — Veneer & Panel Section 



June 25, 1921 



Controlling Moisture in Lumber and Veneers 



Jlci'-f>resident, Forest Products En^hieeriri;^ Company, (Jhica^o 



The title of the subject, which your secretary has permitted me 

 to select, indicates to my mind by far the greatest cause for the 

 many difficulties and astoundinj^ losses encountered in the man- 

 ufacture and use of lumber and veneers. 



Unquestionably the greatest stimulus toward the solution of 

 these important problems was created by the government through 

 the U. S. Forest Products Laboratory at Madison, Wis. Un- 

 controvertible facts now make possible a full solution of the 

 many problems encountered, and provide a fundamental knowl- 

 edge with which to proceed and prove in practice much that 

 has been unknowingly and unjustly termed "impractical theory." 



In order to make this discussion as clear as possible to every- 

 one, I have deemed it advisable to construct a foundation of 

 facts, some of which may seem more or less elementary to 

 those among you who have studied this important subject. 



It is a well known fact that a single fibre of wood will not change 

 its shape to any appreciable extent unless it imbibes moisture 

 when it will expand, or unless it gives up moisture, in which 

 event it will shrink. It follows then that the degree of expan- 

 sion or shrinkage of the single fibre is multiplied in almost direct 

 ratio to the number of fibres in a board or piece of veneer of 

 any particular size. The thermal expansion and contraction Js 

 so slight that for our purpose we can ignore it entirely. 



We first must deal with the natural, inherent moisture of 

 our timber as it is manufactured into lumber or veneers. 



In manufacturing veneers this problem is more simple, since 

 you receive your logs nearly green from the tree and, therefore, 

 practically all shrinkage resulting from evaporation of moisture 

 is within your control. To take advantage of this requires 

 thorough knowledge of what takes place in lumber and veneers 

 while the moisture is being reduced. 



Surprising Moisture Content 



Many manufacturers express astonishment when they learn 

 that some species of wood contain 200 per cent moisture in their 

 green state. Certain species contain even more, 250 per cent 

 being near the limit. 



This moisture is contained in two known forms: Free and 

 Fibre moisture. The former is contained in the open cells or 

 pores of the wood and constitutes the bulk of the total moisture. 

 The latter is contained in the actual fibrous structure of the 

 w^ood and with the exception of a few species of wood can be 

 safely stated to be about thirty per cent. 



A sharp distinction must be drawn between Free and Fibre 

 moisture since all the shrinkage occurring in the elimination of 

 moisture occurs during the reduction of Fibre Moisture, and after 

 the Free Moisture has been evaporated. This is quite general, 

 but there are, however, a few exceptions to the rule with which 

 w^e will not deal here. 



Expansion of dry lumber and veneers can only take place when 

 the moisture content is raised, but again, no further expansion 

 takes place when the Fibre Saturation, that is thirty per cent, 

 has been reached. 



We now reach the conclusion that all so-called "working" in 

 wood takes place during changes of moisture content betw^een 

 oven dry and up to thirty per cent, or Fibre Saturation, with the 

 exceptions previously mentioned. 



Just what chemical and certain physical changes take place 

 during moisture changes is not definitely known, but we are really 

 more interested in the structural changes and this has now been 

 solved to a near state of perfection. 



*A talk before the .Watiomtl Fencer & Panel Manufacturers' Association in 

 conz'ention at Chicago, June 14, 19^1. 



Ihe fundamental principles of reducing and controlling moisture 

 in veneers are identical with those applied to lumber, the only 

 practical difference being that lumber requires more time and in 

 approximate ratio to thickness. 



In working lumber you purchase your w^aste by cutting off the 

 checked ends of boards and planing a 4/4" piece do\vn to 5/8", 

 if it will not make I 1/16" or 3 4" core or other dimension stock. 

 With veneers you lose through checking and principally through 

 severe "buckling" and consequent breakage. 



Guessing Wastes Veneers 



In the past w^e have talked of kiln drying lumber in terms of 

 time in days required to dry a certain kind and thickness, with- 

 out knowing the moisture content of the stock before placing it 

 in the kiln, either guessing at the final moisture content, or, as 

 in many other cases, making erroneous tests. Very few man- 

 ufacturers are doing this correctly at the present time. Much 

 less is being done in the control of moisture in veneers. They 

 are usually dried by mere guess work, and herein lies perhaps 

 the greatest source of loss. 



It is decidedly incorrect to reduce veneers to an oven-dry 

 state, because they cannot possibly remain flat when they again 

 imbibe moisture. Considering the fact that any piece of lumber 

 or veneer is made up of innumerable fibres and that these are 

 distributed very non-uniformly, at the same time accepting for 

 the moment the theory that any one fibre has approximately the 

 same power to imbibe and give up moisture as another, it be- 

 comes quite apparent that board and, more especially, a sheet 

 or piece of veneer, cannot possibly remain flat during moisture 

 changes unless it is held or rather stressed into a flat position. 

 Naturally the particular spot in an over- dried piece containing 

 the greatest number of fibres, in other words the most dense spot. 

 will take on the greatest amount of moisture and thereby very 

 obviously v^'ill produce local expansion, more commonly know^n 

 as a "buckle." 



No doubt you have all noticed a greater depression around a 

 knot in a dried board. This is undoubtedly due to a greater 

 density of fibres around the knot, each having given up its ow^n 

 share of moisture and consequently proportionately greater shrink- 

 age has occurred. This is a somewhat exaggerated example of 

 the non- uniform density of timber and, for that matter, a sheet 

 of veneer may have no knots at all and yet "buckle" beyond a 

 point to which it would be practical to use it without further 

 treatment. 



Directions of Shrinkage 



Other causes of unequal shrinkage are due to the natural 

 general formation of fibres in the tree. A very good example 

 of this can be observed in any telephone line pole. This shrink- 

 age is evident in the form of large checked openings on the outer 

 surface w^hich grow smaller toward the heart of the tree. A 

 single board, cut from any particular portion in the tree and 

 left to dry out without piling or providing some other method 

 to hold it in a straight position while drying, will shrink very 

 nearly as indications would suggest in the telephone line pole. 

 This indicates two directions of shrinkage w^hich \ve need to con- 

 sider here ; they are commonly known as radial and tangential. 

 For practical purposes longitudinal shrinkage need not be con- 

 sidered except that it must be recognized in panel constructions 

 known as concealed or exposed crossings and in all banded types. 

 The radial direction in a quartersa^vn board is with its width, 

 while the tangential direction is through its thickness. Lumber 

 (Continued <ni piiffc 3S) 



