The kiln drying of lumber; a practical and theoretical treatise

Front matter

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Contents

CHAPTER PAGE

I. Introduction 1

II. The Structure and Properties op Wood 10

CHAPTER I Intkoduction the objects of deying wood and the need op a

TECHNICAL KNOWLEDGE OF KHjN DRYING

"Wood, next to stone, is doubtless the earliest material used by man and has served bis needs more than any other substance. In fact, it is safe to say that civilization could not exist were it not for the forests and the v/ood derived therefrom. Kingdoms have risen and fallen and become extinct, as the forests have flourished, been wantonly destroyed and perished. In China, for example, in the Shan-Si and in the Chi-Li Provinces, and near Fou-Ping, vast regions, once thickly populated, are now desolate wastes or wholly uninhabitable through the prodigal destruction of their greatest source of prosperity — the forests which once clothed the steep mountains, preventing floods and erosion, the washing away of the fertile soils, and which furnished fuel and building material in abundance. In the Southern Alps and in the Pyrenees Mountains, in Europe and Greece, and in Northern Syria and Palestine, and in Northern Africa, the same results have followed the loss of the forests. Through heroic efforts and vast expense France and Italy have been reestablishing the habitability of the land by replanting.

But it is not with the living tree that our present subject has to do — all of that belongs to the province of Forestry — ^but rather with the product after the tree has been cut down and sawed into lumber or other forms of wood. Lumbering and Forestry are not antitheses, but both should accomplish the same result, the perpetuation of the forest for future use, whatever that may be. Lumber is a crop, as are vegetables, and the forest may be utilized, and at the same time conserved for future use.

Table I. — Consumption of Wood in Cubic Feet Per Capita in Various Countries Before the War ( Including Firewood ) .

From Forest Service Bull. 83, Forest Resources of the World, by Raphael Zon.

Cu. Ft. Cu. Ft.

United States 260.0 Japan 30.0

Canada 192'.0 France 24.6

Norway 125.0 Denmark 19.8

Sweden 120.0 Belgium 17.7

Finland 91.5 United Kingdom 14.0

Russia 63.0 Holland 13.1

Austria-Hungary 57.0 Italy 13.0

Switzerland 38.0 British India 0.8

Germany 36.6

To give an idea numerically of the intimate relation which wood bears to every individual, the table above is inserted, which shows, before the Grreat War, what

INTRODUCTION 3

the relative consumption of wood per capita has been in various countries.

It is evident that wood in the living tree contains a great deal of moisture. In fact, the walls of the cells of which the wood is composed, as we shall see later on, are saturated and never become dry in the natural state. Trees contain anywhere from 30 to over 200 per cent, of the dry weight of the wood in water. This moisture renders the wood very heavy and unsuitable for use for most purposes. It must be got rid of before the wood will burn for fuel, and before lumber is used for houses or furniture or any manufactured article. For some uses, as piles, concrete forms, etc., wet wood is desirable, but in the vast majority of cases it must be dry.

When a tree is cut down and sawed into lumber, this will gradually dry in the air, if piled so the air can get at it, to a certain degree depending upon the average humidity of the climate, and is then called in general terms ''air dry" (Fig. 1). This condition will range from 8 to 18 per cent, of water, based on the perfectly dry weight, according to the climate and time of year. In the neighborhood of New York City this condition will average about 14 per cent, during the summer time, if protected from the rain and dampness. It takes a long time for wood to become air dry, depending upon the species, the thickness of the boards, the initial amount of moisture, and the manner of exposure and the climate. One-inch white oak, for instance, will require from one and a half to three years to thoroughlyair season; white pine, on the other hand, may thoroughly dry in four months. If left in the log it will require many years for the water to evaporate, especially if the bark remains on. In fact, except for very durable species, as cypress or white oak, the wood is apt to completely decay before it becomes dry, in the log form.

All wood in drying shrinks and is, therefore, liable to warp and check. Moreover, the strength of the wood substance greatly increases as it dries below the '* fibre saturation point, ' ' which is the name given to the condition of the wood when the cell walls become saturated with moisture (about 25 to 30 per cent, of the dry weight).

Briefly, leaving out the use of wood as fuel, the objects of drying wood may be summed up as follows :

1. To render it suitable for the purpose for which it is to be used.

2. To lessen the weight for handling and shipping purposes.

3. To prevent decay.

4. To increase its hardness and strength, and to prevent subsequent shrinkage and ''working" after it has been placed in a building or manufactured article.

INTRODUCTION 5

For many purposes, sucli as use in heated buildings, cooperage, gun stocks, athletic goods, etc., "air dried" wood is not sufficiently dry, and it must be further dried by means of heat in a dry kiln. Furthermore, many woods, such as red gum, western larch, and southern oaks, seldom air dry without great losses in warping, checking, case hardening, and honeycombing. These losses may be greatly reduced or entirely eliminated by properly kiln drying the green material in kilns where the humidity and circulation and temperature can be maintained at the most suitable amounts. For some purposes, as flooring, cabinet work, etc., the ''working," that is, the twisting of the wood due to swelling and shrinking under changes of the atmosphere, may be slightly reduced by thorough kiln drying.

The principal objects, therefore, of kiln drying instead of the slow process of air drying may be enumerated under the following headings:

1. To improve the condition of the wood for the purpose for which used.

2. To reduce losses which occur in air drying.

3. To reduce the time necessary to carry stock in the yard.

4. To reduce shipping weights without waiting for the long time required to air dry the material.

All kinds of methods have been tried and several hundred patents taken out for apparatus, processes, and kilns, for drying wood. Many of these have not proved to be a success. The processes include preliminary treatments of steaming at atmospheric pressure and at pressures up to forty or even sixty pounds gauge, soaking in various solutions, drying in vacuum, in compressed air, in alternating compressed air and vacuum, in compressed air and steam, in superheated steam, and in gases such as waste products of combustion from chimneys, and even electrical treatments. Temperatures vary all the way from 60° F. up to 300° F. and even 400° F. in gases other than air.

Apparatus of all kinds have been tried, also, including steel cylinders, accordion-like heaters, ovens, and drying rooms of all shapes and sizes.

While some species, such as pine, fir, basswood and mahogany, are easy to dry, many others, as oak, gum, and notably eucalyptus (blue gum), are exceedingly difficult to dry, requiring totally different treatment.

A great amount of lumber is being kiln dried chiefly for the purpose of reducing shipping costs and for eliminating the investment necessary to carry a large stock on hand. This is particularly true with respect to the "West Coast timbers, especially Douglas fir, and also of southern yellow pine. For this purpose the quickest possible method of drying consistent with not seriously injuring the sales value of the stock is sought

INTRODUCTION 7

after, and higli temperatures and superheated steam are largely used.

In 1913 it is estimated that the total cut of lumber in the United States (exclusive of poles, posts, hewn ties, and firewood) (sawed into lumber or timber at mills) was 38,388,110,000 board feet, and 88,984,000 additional was imported. The various ways in which this was used are given below :

Wood-using factories 24,576,557 M

Exports of lumber and timber (planks, sawed ties, etc.) . . 3,185,864 M Consumed by railroads:

Sawed ties 1,215,597 M

Bridge and trestle timbers 841,859 M

Miscellaneous sawed wood 28,8i04 M

Building lumber, studding, joists, rafters, and rough construction 8,628,413 M

Total less imports 38,477,094 M

Out of the 24,576,557M used in factories, an estimate made according to 54 distinct wood-using industries shows that approximately 15,116,000M softwoods and 4,725,000M hardwoods were kiln dried before using. The hardwoods are mostly first air dried and then put through the kiln. Many of the softwoods are dried directly from the saw. Assuming an average value of $16 for softwoods and of $21 for hardwoods, this figures out a value of $241,856,000 for softwoods which are annually kiln dried and of $99,225,000 for hardwoods.

The official estimate for the total lumber cut for 1915 is 37,013,2943^^, which is not greatly different from that of 1913, so it is fair to assume that the figures hold throughout very closely for the present conditions. If the present losses in preliminary air drying, which amount to considerably over 12 per cent, for hardwoods and 5 per cent, for softwoods, could be reduced to 2 per cent, by the best methods of kiln drying direct from the green condition, there is a possible annual saving of $17,178,000 alone, not including lumber which is not now being kiln dried.

The value of a technical knowledge of kiln drying is, therefore, self-evident, and it is the purpose of this book to give the reader the best information available on this important subject. The United States is acknowledged to be in advance of all other countries in this particular phase of the wood problem.

Thus far, I have touched only upon the losses which it is possible to obviate to a large extent. Let us consider some of the positive advantages to be gained by better methods of drying.

In the first place, it would enable architects, engineers, and wood-users to specify what they required in regard to this condition of the wood with a degree of exactness, as is now done with other materials. How would it seem for an architect, for instance, to order edge grain western larch flooring for his building and to state that it should contain between 5 and 7 per cent, moisture, that it should have been manu-

INTRODUCTION 9

factured only when it contained 5 per cent., that it should not be casehardened beyond a certain degree determinable by a simple test, be free from all checks or honeycombing, and that it should not have been heated beyond 160° in the dry kiln while it was moist in order to avoid brittleness. Who would accept such an order to-day? Yet when lumber dealers can come up to such standards, which is by no means a Utopian idea, there will be little necessity to ^ ^boom" the lumber markets, for the customer will get just what he wants and be satisfied.

Chapter II

The Stkucture and Propeeties op "Wood * A TECHNICAL discussion of the minute anatomy of wood as revealed by the microscope need not be given here, as it can be found in botanical works, but some idea of the structure and the way in which different species differ from one another is deisirable for a clear understanding of the manner in which it behaves in drying. In general, v/ood is built up of individual ^'cells'' of various shapes and sizes which have been formed by the '' cambium" or the region of active growth, consisting of a thin layer of soft, succulent tissue lying between the inner bark and the wood proper. Every year this cambium deposits a new layer of wood upon the outer circumference of the central cylinder, thus forming the ''annual rings" or the "grain" of the wood. In most trees the several last-formed outer layers remain active and conduct water and mineral substances from the roots upwards to the leaves. This is the "sapwood" of the tree. The inner layers of the sapwood, after they have functioned in this way for a number of years, finally be-

*For a more complete discussion of the properties of wood, the reader is referred to " The Economic Woods of the United States," by Samuel J. Record, Wiley & Sons, 1912; also to " Wood," by G. S. Boulger, Arnold, 1908. A complete list of references is given in Record's book.

come inactive and are converted successively into lieartwood, which, so far as the living functions of the tree are concerned, is dead wood, but it still acts as a mechanical support to the tree. Certain chemical waste products are deposited within the cell walls from the living protoplasm in the sapwood as it becomes converted into heartwood, thus giving the usual darker color to this portion. In some species, as white beech, heartwood is never formed, and in others, as balsam fir, no change in color takes place. Several distinct kinds of cells are formed by the cambium layer in some species of wood, which are as a rule long and tapering, some being almost hair-like in shape and others much wider in proportion to length. These cells are arranged vertically in the tree, but in addition they are interspersed by numerous horizontal groups of thin-walled, pith-like cells arranged radially in the trunk in strands shaped somewhat like twoedged swords placed edgewise. These are the *' medullary rays" (silver grain in quarter-sawed oak), and serve to conduct food and moisture from the sapwood and the bark to the growing cambium layer. It is on account of these medullary rays that wood dries more rapidly in the radial direction than in the tangential. Nearly all woods may be classed into two principal groups, having very marked distinctions, with respect to the elements of which they are composed. The so-called ''hardwoods," or broad-leaved trees, more correctly the *' ' angiosperms, " are the more complex in structure, whereas the commonly called "softwoods," or needle-leaved trees, the conifers, or more correctly the "gymnosperms," are comparatively simple. The former are called porous woods on account of the tubular openings which they contain, known as vessels. These vessels are entirely lacking in the gymnosperms. In some woods, as red oak, these vessels are completely open passages, so that air may readily be blown through a stick of the green wood many feet in length. In others, such as white oak, for example, the tubes are blocked by ingrowths of thin-walled cells known as tyloses. It is impossible on this account to force any appreciable amount of air through a piece of fresh white oak even an inch in length. This is why white oak is such an excellent wood for tight cooperage, whereas red oak would make a leaky barrel.

In addition to the vessels, hardwoods (angiosperms) contain several distinct types of cells.

Fig. 2 is a sketch from the actual microscopic views showing a small block of one of the simplest of the hardwoods, tulip or yellow poplar, as it would appear if highly magnified. TT is a horizontal surface cut across the grain. EE is a radial surface (quarter-sawed) and TG- is a tangential surface (plain sawed). AE is one annual ring, S being the first formed layer

Fig. 2.— Highly magnified block of wood (hardwood).

or the springwood composed of large thin-walled cells, and SM the later formed layer of small thick-walled cells or summerwood. It is the relative thickness of the cell walls which gives the density and hardness to certain woods, as hickory, compared to thin-walled woods, as basswood, and which makes the hard part of the rings in yellow pine. MR and MR are medullary rays. V is a vessel cut in section and showing its remarkable end splicing on to the cell below by means of a scalariform grating. These grating-splices are shown in three places marked SC. They differ in form in different woods, in some, as birch, being simple, elliptical, or circular openings. The vessels communicate with adjacent vessels by means of numerous small partial openings called "bordered pits." All cells retain their individual walls, which become thickened as the cell develops from the cambium layer by successive depositions laid down on the inside by the living protoplasm. Adjacent walls between two cells are, therefore, composed of five distinct layers, the original thin primary walls of the two contiguous cells and an intermediate substance by which they are joined together. The two primary walls, together with the cementing substance, are collectively known as the "middle lamella." On either side of this "middle lamella" are the thickened walls of the two contiguous cells. The pits are openings in the thickened walls, but do not pass through the middle lamella. While the pits serve as communicating regions for the protoplasm, they must not be considered as openings in the

Fig. 3. — Comparative sizes of the wood elements of red gum. F, wood fiber; D duct or vessel (trachea) ; M, medullary ray cells. (Forest Service Bulletin 58. Drawn by the author.)

sense of punctures in the walls. Numbers of these ''bordered" pits are shown on the walls of the vessels in the figure.

The other cells, which in this species are almost all ''wood fibers" with thick walls and small cavities, are what give the rigidity and hardness to the wood. They are shown at F and ML. These wood fibers also have pits which are very small and slit-like. One is shown

Fig. 4.

Fig. 5.

Fig. 4. — Highly magnified portions of the elements of red gum wood. D, end of a duct or vessel (trachea), showing the peculiar grating; F, pointed end of a fiber; M, short medullary ray cells. The narrow, slit-like pits are seen in the walls of each element.

Fig. 5. — Cross section of red gum wood, showing the structure. F, wood fiber; D, duct or vessel (trachea) ; M. medullary ray cells.

at P. Wood fibers vary in average lengths in different species from one to two millimeters.

Some idea of the relative sizes of the various cells in red gum, which is almost identical to tulip in structure, is shown in Figs. 3, 4, and 5 (from Forest Service Bulletin 58), which are sketches made by the writer directly from the microscope.

Oak is considerably more complex in structure than the woods described above, but for a more detailed description the reader is referred to works on wood structure.^

The gymnosperms or coniferous "softwoods," on the other hand, are much simpler in structure than the tulip just described. In the firs, for instance, the vertical cells are practically all of one kind, called tracheids, and are of approximately the same widths in the circumferential directions, though differing considerably in thickness, measured radially, from the large, thin-walled springwood cells to the small, thick-walled summerwood cells. In section they are nearly rectangular and are arranged in very even and uniform radial rows. The medullary rays are very small, numerous and uniformly arranged. This remarkable simplicity of structure gives to this class of woods certain properties which are lacking or less strongly marked in the other class. They are remarkably stiff in proportion to their weight, which renders the conifers especially suitable for building and structural purposes. Also, their resonance properties are good, so that they are used for sounding boards, etc., in musical instruments. In the pines especially, and also in spruce and larch, there are in addition to the tracheids many tubular openings called "resin ducts," in which the resin is stored and

* " The Oak," by Marshal Ward.

out of which it flows when the tree is injured or tapped. These resin ducts are not cells but are openings between the cells. The tracheids all have numerous, bordered pits. These bordered pits are of peculiar structure, deserving notice in considering the drying or impregnation of wood. The middle lamella extends straight across the round opening of the pits of the two contiguous cells like the membrane of a drum, the openings being always in exact aligimient. Imagine two soup plates placed face to face with their rims touching, a membrane stretched across between them and round holes cut in the bottom of each, and a very good conception will be had of the shape of the bordered pits. The membrane has a thickened circular disk at its center which is larger in diameter than the two holes in the ''borders." When wood dries this thickened disk, called the "torus," becomes pressed to one side tightly against the border, to which it firmly adheres, thus effectively sealing the opening of the pit. This phenomenon occurs chiefly in the springwood of the annual rings, since the pits in the summerwood portions do not as a rule become closed in this manner. Just how these remarkable structures behave in the conduction of water up the tree is not known, but they must have an important function in this regard, as they are present in all the water-conducting tissue.

In the pulp and paper industry these tracheids are indiscriminately spoken of as ''wood fibers." But the true wood fibers belong to the hardwoods and are of quite a different form.

Trees native of tropical countries often do not have the annual rings, since their growth is more or less continuous, but even in hot countries irregular rings are sometimes formed, due to periodic seasons of active growth.

There is also another kind of wood not included in the description given above, which occurs in the palm trees, bamboo, rattan, sugar cane, etc., in which the water-conducting vessels, wood fibers, and inner bark occur grouped together in small strands interspersed all through the stem with pith tissue between. The wood is not formed by concentric rings laid down circumferentially from a cambium layer, but growth takes place by an increase in the numbers of these strands. The strands are about the size of broom straws and interlace throughout the stem in the form of a network. These trees, however, are not commonly used as lumber and need not receive further consideration here. They used to be called "endogens" on account of the way in which the growth of the stem took place, and the others with concentric layers, "exogens." The endogens belong also under the angiosperms, but the seeds are monocotyledonous

Fig. 6. — Cress section of red oak highly magnitied. V, springwood vessel; S, summerwood vessel; M, M, medullary ray.

Fig. 7. — Cross section of yellow pine highly magnified. A, springwood tracheida; B, summerwood tracheids; D, resin duct; M, medullary ray.

(sprouting like corn), whereas the seeds of the exogens are dicotyledonous (sprouting like beans).

Figure 6 is the cross section of a piece of red oak showing a large vessel at V, a small one at S, dense wood fibers at B and a medullary ray MM. Figure 7 is a cross section of a piece of pine magnified the same amount, showing a resin duct D, summerwood tracheids B, springwood tracheids A, and medullary ray M.

All wood substance, irrespective of species, weighs about the same amount, its specific gravity being 1.56. Thus it will sink in water. The buoyancy of wood is due to its air spaces, and the lighter the wood the thinner are its cell walls. It is also probable that the strength of the wood substance itself is nearly the same in all species.

It is supposed that the cell walls are composed of minute particles almost molecular in size, and that moisture enters between these particles by molecular attraction or cohesion. As the amount of moisture increases the particles are jDushed apart until their mutual attractive force counterbalances the cohesion attraction of the moisture, when they will absorb no more. This accounts for the hygroscopicity of wood and its power to absorb moisture from air which is only partly saturated. It is merely a hypothesis, as the particles are too small to be seen by any microscope. In this respect wood is a colloidal substance. The shape and arrangement of these hypothetical particles may account for the way in which wood swells differently in different directions.

Chemically wood substance is exceedingly complex and has never been completely determined. It is basically the same in all species, but is impregnated with different substances. The basic substance is cellulose (CeHioOg)^, and with this is combined lignin, forming what is called lignocellulose. Lignin, however, has never been isolated by itself. Cotton is pure cellulose. In the manufacture of sulphite pulp, the lignin is extracted and nearly pure cellulose is left. The proportional composition of cellulose is the same as for starch and its component relationship to sugar is seen by the equation

Starch + water = sugar

The conversion which can be accomplished, however, is not quite so simple as the above equation might seem to indicate.

In addition to the wall substance, wood may contain within the cells, gums, resins, combined acids, starches, volatile and essential oils, and very little albuminoids. The sapwood, particularly, is rich in gums, starches, and sugars. There are also traces of many very complex organic compounds present in most

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The foregoing Table (II) gives the analyses for mineral content by Dr. Eudolf Weber and by Hartig. In using a table of this kind it must be understood that great variations in the ash and ash content occur in the same species and even in different parts of the same tree. The non-mineral elements are about the same in all woods, being in the proportions :

Carbon 49 per cent, by we

Oxygen 6 per cent, by weight.

Hydrogen 44 per cent, by weight.

Ash, etc 1 per cent, by weight.

The ash content may vary from 0.3 per cent, to over 3 per cent.

Chapter III

Common Peactices in Drying Present practice in kiln drying varies enormously and there is no ''standard" method. Even with the same species and for the same purpose all kinds and conditions are met with. Temperatures vary anywhere from 60° F. to above the boiling point, and inch lumber, from one to eight months air seasoned, is dried in from thirty-six hours to six weeks, thicker material from two to five months. As a general thing, hardwoods are dried at a much lower temperature than softwoods, and for some softwoods, notably Douglas fir and southern yellow pine, temperatures above the boiling point of water are used. Such softwoods are usually dried directly from the saw in the perfectly green condition, as speed of drjdng is the main object aimed at, but with hardwoods, especially with the dense, heavy species, as oak, hickory, mahogany, etc., it is usual to allow them first to air dry by open piling in the yards or sheds from a month to one or two years before placing in the dry kiln. The reason for this will be explained in Chapter IV, ''How Wood Dries." Some of the softer "hardwoods" as basswood, poplar, yellow poplar, etc., are also occasionally dried directly from the saw. It is possible, however, to obtain the

24 THE KILN DEYING OF LUMBER

best results with all species by doing away with the air drying entirely and placing the wood in the kiln directly from the saw. This can only be accomplished, however, by using a proper method of drying best suited to the species, as will be shown later on. In air drying there is little or no control over the conditions of the air, which change according to the weather, so that the drying is likely to take place too rapidly on some days and too slowly on others. The result is liable to be surface and end checking, casehardening and honeycombing (Fig. 8). In a properly operated dry kiln, in which the conditions are all under control, these injuries may be avoided. Some species, indeed, can not be successfully air dried under ordinary conditions, as they will honeycomb (internal checking) very badly. This is true of the southern oaks, especially those white oaks containing large amounts of moisture, like the willow oak. It is also true of the blue gum or Eucalyptus glohulus.

Another case which may be mentioned where air drying is injurious is black walnut gun stock blanks, which are cut into form while green and subsequently dried. This wood in this form will invariably end check in* air drying unless the ends are coated with an impervious covering. In fact, most of the hardwoods if cut into blanks while green, if more than an inch in thickness, will seriously check in air drying. This is true of maple last blocks, for instance.

POORLY KILN DRIED SOUTHERN RED AND WHITE OAK WiGOH

-ELLOES

THE SAME.

WELL KILN DRIED IN HUMIDITY REGULATED

D5Y KILH

nght are kiln-riried oak wagon eUoes 3 - i^Phill^^' m section, the four specimens to the lower one white oak. ^^ '''•* *"'^^«=s m section, the top ones being red and the