Three most important advantages of seasoning have already been made apparent:
1. Seasoned timber lasts much longer than unseasoned. Since the decay of timber is due to the attacks of wood-destroying fungi, and since the most important condition of the growth of these fungi is water, anything which lessens the amount of water in wood aids in its preservation.
2. In the case of treated timber, seasoning before treatment greatly increases the effectiveness of the ordinary methods of treatment, and seasoning after treatment prevents the rapid leaching out of the salts introduced to preserve the timber.
3. The saving in freight where timber is shipped from one place to another. Few persons realize how much water green wood contains, or how much it will lose in a comparatively short time. Experiments along this line with lodge-pole pine, white oak, and chestnut gave results which were a surprise to the companies owning the timber.
Freight charges vary considerably in different parts of the country; but a decrease of 35 to 40 per cent in weight is important enough to deserve everywhere serious consideration from those in charge of timber operations.
When timber is shipped long distances over several roads, as is coming to be more and more the case, the saving in freight will make a material difference in the cost of lumber operations, irrespective of any other advantages of seasoning.
Prevention of Checking and Splitting
Under present methods much timber is rendered unfit for use by improper seasoning. Green timber, particularly when cut during January, February, and March, when the roots are most active, contains a large amount of water. When exposed to the sun and wind or to high temperatures in a drying room, the water will evaporate more rapidly from the outer than from the inner parts of the piece, and more rapidly from the ends than from the sides. As the water evaporates, the wood shrinks, and when the shrinkage is not fairly uniform the wood cracks and splits.
When wet wood is piled in the sun, evaporation goes on with such unevenness that the timbers split and crack in some cases so badly as to become useless for the purpose for which it was intended. Such uneven drying can be prevented by careful piling, keeping the logs immersed in a log pond until wanted, or by piling or storing under an open shed so that the sun cannot get at them.
Experiments have also demonstrated that injury to stock in the way of checking and splitting always develops immediately after the stock is taken into the dry kiln, and is due to the degree of humidity being too low.
The receiving end of the kiln should always be kept moist, where the stock has not been steamed before being put into the kiln, as when the air is too dry it tends to dry the outside of the stock first—which is termed “case-hardening”—and in so doing shrinks and closes up the pores. As the material is moved down the kiln (as in the case of “progressive kilns”), it absorbs a continually increasing amount of heat, which tends to drive off the moisture still present in the center of the piece, the pores on the outside having been closed up, there is no exit for the vapor or steam that is being rapidly formed in the center of the piece. It must find its way out in some manner, and in doing so sets up strains, which result either in checking or splitting. If the humidity had been kept higher, the outside of the piece would not have dried so quickly, and the pores would have remained open for the exit of the moisture from the interior of the piece, and this trouble would have been avoided. (See also article following.)
Shrinkage of Wood
Since in all our woods, cells with thick walls and cells with thin walls are more or less intermixed, and especially as the spring-wood and summer-wood nearly always differ from each other in this respect, strains and tendencies to warp are always active when wood dries out, because the summer-wood shrinks more than the spring-wood, and heavier wood in general shrinks more than light wood of the same kind.
If a thin piece of wood after drying is placed upon a moist surface, the cells on the under side of the piece take up moisture and swell before the upper cells receive any moisture. This causes the under side of the piece to become longer than the upper side, and as a consequence warping occurs. Soon, however, the moisture penetrates to all the cells and the piece straightens out. But while a thin board of pine curves laterally it remains quite straight lengthwise, since in this direction both shrinkage and swelling are small. If one side of a green board is exposed to the sun, warping is produced by the removal of water and consequent shrinkage of the side exposed; this may be eliminated by the frequent turning of the topmost pieces of the piles in order that they may be dried evenly.
As already stated, wood loses water faster from the ends than from the longitudinal faces. Hence the ends shrink at a different rate from the interior parts. The faster the drying at the surface, the greater is the difference in the moisture of the different parts, and hence the greater the strains and consequently also the greater amount of checking. This becomes very evident when freshly cut wood is placed in the sun, and still more when put into a hot, dry kiln. While most of these smaller checks are only temporary, closing up again, some large radial checks remain and even grow larger as drying progresses. Their cause is a different one and will presently be explained. The temporary checks not only appear at the ends, but are developed on the sides also, only to a much smaller degree. They become especially annoying on the surface of thick planks of hardwoods, and also on peeled logs when exposed to the sun.
So far we have considered the wood as if made up only of parallel fibres all placed longitudinally in the log. This, however, is not the case. A large part of the wood is formed by the medullary or pith rays. In pine over 15,000 of these occur on a square inch of a tangential section, and even in oak the very large rays, which are readily visible to the eye, represent scarcely a hundredth part of the number which a microscope reveals, as the cells of these rays have their length at right angles to the direction of the wood fibres.
If a large pith ray of white oak is whittled out and allowed to dry, it is found to shrink greatly in its width, while, as we have stated, the fibres to which the ray is firmly grown in the wood do not shrink in the same direction. Therefore, in the wood, as the cells of the pith ray dry they pull on the longitudinal fibres and try to shorten them, and, being opposed by the rigidity of the fibres, the pith ray is greatly strained. But this is not the only strain it has to bear. Since the fibres shrink as much again as the pith ray, in this its longitudinal direction, the fibres tend to shorten the ray, and the latter in opposing this prevents the former from shrinking as much as they otherwise would.
Thus the structure is subjected to two severe strains at right angles to each other, and herein lies the greatest difficulty of wood seasoning, for whenever the wood dries rapidly these fibres have not the chance to “give” or accommodate themselves, and hence fibres and pith rays separate and checking results, which, whether visible or not, are detrimental in the use of the wood.
The contraction of the pith rays parallel to the length of the board is probably one of the causes of the small amount of longitudinal shrinkage which has been observed in boards. This smaller shrinkage of the pith rays along the radius of the log (the length of the pith ray), opposing the shrinkage of the fibres in this direction, becomes one of the causes of the second great trouble in wood seasoning, namely, the difference in the shrinkage along the radius and that along the rings or tangent. This greater tangential shrinkage appears to be due in part to the causes just mentioned, but also to the fact that the greatly shrinking bands of summer-wood are interrupted along the radius by as many bands of porous spring-wood, while they are continuous in the tangential direction. In this direction, therefore, each such band tends to shrink, as if the entire piece were composed of summer-wood, and since the summer-wood represents the greater part of the wood substance, this greater tendency to tangential shrinkage prevails.
The effect of this greater tangential shrinkage effects every phase of woodworking. It leads to permanent checks and causes the log or piece to split open on drying. Sawed in two, the flat sides of the log become convex; sawed into timber, it checks along the median line of the four faces, and if converted into boards, the latter checks considerably from the end through the center, all owing to the greater tangential shrinkage of the wood.
Briefly, then, shrinkage of wood is due to the fact that the cell walls grow thinner on drying. The thicker cell walls and therefore the heavier wood shrinks most, while the water in the cell cavities does not influence the volume of the wood.
Owing to the great difference of cells in shape, size, and thickness of walls, and still more in their arrangement, shrinkage is not uniform in any kind of wood. This irregularity produces strains, which grow with the difference between adjoining cells and are greatest at the pith rays. These strains cause warping and checking, but exist even where no outward signs are visible. They are greater if the wood is dried rapidly than if dried slowly, but can never be entirely avoided.
Temporary checks are caused by the more rapid drying of the outer parts of any stick; permanent checks are due to the greater shrinkage, tangentially, along the rings than along the radius. This, too, is the cause of most of the ordinary phenomena of shrinkage, such as the difference in behavior of the entire and quartered logs, “bastard” (tangent) and rift (radial) boards, etc., and explains many of the phenomena erroneously attributed to the influence of bark, or of the greater shrinkage of outer and inner parts of any log.
Once dry, wood may be swelled again to its original size by soaking in water, boiling, or steaming. Soaked pieces on drying shrink again as before; boiled and steamed pieces do the same, but to a slightly less degree. Neither hygroscopicity, i.e., the capacity of taking up water, nor shrinkage of wood can be overcome by drying at temperatures below 200 degrees Fahrenheit. Higher temperatures, however, reduce these qualities, but nothing short of a coaling heat robs wood of the capacity to shrink and swell.
Rapidly dried in a kiln, the wood of oak and other hardwoods “case-harden,” that is, the outer part dries and shrinks before the interior has a chance to do the same, and thus forms a firm shell or case of shrunken, commonly checked wood around the interior. This shell does not prevent the interior from drying, but when this drying occurs the interior is commonly checked along the medullary rays, commonly called “honeycombing” or “hollow-horning.” In practice this occurrence can be prevented by steaming or sweating the wood in the kiln, and still better by drying the wood in the open air or in a shed before placing in the kiln. Since only the first shrinkage is apt to check the wood, any kind of lumber which has once been air-dried (three to six months for one-inch stuff) may be subjected to kiln heat without any danger from this source.
Kept in a bent or warped condition during the first shrinkage, the wood retains the shape to which it has been bent and firmly opposes any attempt at subsequent straightening.
Sapwood, as a rule, shrinks more than heartwood of the same weight, but very heavy heartwood may shrink more than lighter sapwood. The amount of water in wood is no criterion of its shrinkage, since in wet wood most of the water is held in the cavities, where it has no effect on the volume.
The wood of pine, spruce, cypress, etc., with its very regular structure, dries and shrinks evenly, and suffers much less in seasoning than the wood of broad-leaved (hardwood) trees. Among the latter, oak is the most difficult to dry without injury.
Desiccating the air with certain chemicals will cause the wood to dry, but wood thus dried at 80 degrees Fahrenheit will still lose water in the kiln. Wood dried at 120 degrees Fahrenheit loses water still if dried at 200 degrees Fahrenheit, and this again will lose more water if the temperature be raised, so that absolutely dry wood cannot be obtained, and chemical destruction sets in before all the water is driven off.
On removal from the kiln, the dry wood at once takes up moisture from the air, even in the driest weather. At first the absorption is quite rapid; at the end of a week a short piece of pine, 11⁄2 inches thick, has regained two thirds of, and, in a few months, all the moisture which it had when air-dry, 8 to 10 per cent, and also its former dimensions. In thin boards all parts soon attain the same degree of dryness. In heavy timbers the interior remains more moist for many months, and even years, than the exterior parts. Finally an equilibrium is reached, and then only the outer parts change with the weather.
With kiln-dried woods all parts are equally dry, and when exposed, the moisture coming from the air must pass through the outer parts, and thus the order is reversed. Ordinary timber requires months before it is at its best. Kiln-dried timber, if properly handled, is prime at once.
Dry wood if soaked in water soon regains its original volume, and in the heartwood portion it may even surpass it; that is to say, swell to a larger dimension than it had when green. With the soaking it continues to increase in weight, the cell cavities filling with water, and if left many months all pieces sink. Yet after a year’s immersion a piece of oak 2 by 2 inches and only 6 inches long still contains air; i.e., it has not taken up all the water it can. By rafting or prolonged immersion, wood loses some of its weight, soluble materials being leached out, but it is not impaired either as fuel or as building material. Immersion, and still more boiling and steaming, reduce the hygroscopicity of wood and therefore also the troublesome “working,” or shrinking and swelling.
Exposure in dry air to a temperature of 300 degrees Fahrenheit for a short time reduces but does not destroy the hygroscopicity, and with it the tendency to shrink and swell. A piece of red oak which has been subjected to a temperature of over 300 degrees Fahrenheit still swells in hot water and shrinks in a dry kiln.
Expansion of Wood
It must not be forgotten that timber, in common with every other material, expands as well as contracts. If we extract the moisture from a piece of wood and so cause it to shrink, it may be swelled to its original volume by soaking it in water, but owing to the protection given to most timber in dwelling-houses it is not much affected by wet or damp weather. The shrinkage is more apparent, more lasting, and of more consequence to the architect, builder, or owner than the slight expansion which takes place, as, although the amount of moisture contained in wood varies with the climate conditions, the consequence of dampness or moisture on good timber used in houses only makes itself apparent by the occasional jamming of a door or window in wet or damp weather.
Considerable expansion, however, takes place in the wood-paving of streets, and when this form of paving was in its infancy much trouble occurred owing to all allowances not having been made for this contingency, the trouble being doubtless increased owing to the blocks not being properly seasoned; curbing was lifted or pushed out of line and gully grids were broken by this action. As a rule in street paving a space of one or two inches wide is now left next to the curb, which is filled with sand or some soft material, so that the blocks may expand longitudinally without injuring the contour or affecting the curbs. But even with this arrangement it is not at all unusual for an inch or more to have to be cut off paving blocks parallel to the channels some time after the paving has been laid, owing to the expansion of the wood exceeding the amounts allowed.
Considerable variation occurs in the expansion of wood blocks, and it is noticeable in the hardwoods as well as in the softwoods, and is often greater in the former than in the latter.
Expansion takes place in the direction of the length of the blocks as they are laid across the street, and causes no trouble in the other direction, the reason being that the lengthway of a block of wood is across the grain, of the timber, and it expands or contracts as a plank does. On one occasion, in a roadway forty feet wide, expansion occurred until it amounted to four inches on each side, or eight inches in all. This continual expansion and contraction is doubtless the cause of a considerable amount of wood street-paving bulging and becoming filled with ridges and depressions.
Elimination of Stain and Mildew
A great many manufacturers, and particularly those located in the Southern States, experience a great amount of difficulty in their timber becoming stained and mildewed. This is particularly true with gum wood, as it will frequently stain and mould in twenty-four hours, and they have experienced so much of this trouble that they have, in a great many instances, discontinued cutting it during the summer season.
If this matter were given proper attention they should be able to eliminate a great deal of this difficulty, as no doubt they will find after investigation that the mould has been caused by the stock being improperly piled to the weather.
Freshly sawn wood, placed in close piles during warm, damp weather in the months of July and August, presents especially favorable conditions for mould and stain. In all cases it is the moist condition and retarded drying of the wood which causes this. Therefore, any method which will provide for the rapid drying of the wood before or after piling will tend to prevent the difficulty, and the best method for eliminating mould is (1) to provide for as little delay as possible between the felling of the tree, and its manufacture into rough products before the sap has had an opportunity of becoming sour. This is especially necessary with trees felled from April to September, in the region north of the Gulf States, and from March to November in the latter, while the late fall and winter cutting should all be worked up by March or April. (2) The material should be piled to the weather immediately after being sawn or cut, and every precaution should be taken in piling to facilitate rapid drying, by keeping the piles or ricks up off the ground. (3) All weeds (and emphasis should be placed on the ALL) and other vegetation should be kept well clear of the piles, in order that the air may have a clear and unobstructed passage through and around the piles, and (4) the piles should be so constructed that each stick or piece will have as much air space about it as it is possible to give to it.
If the above instructions are properly carried out, there will be little or no difficulty experienced with mould appearing on the lumber.
INDEX: Seasoning of Wood