Seasoning is ordinarily understood to mean drying. When exposed to the sun and air, the water in green wood rapidly evaporates. The rate of evaporation will depend on: (1) the kind of wood; (2) the shape and thickness of the timber; and (3) the conditions under which the wood is placed or piled.
Pieces of wood completely surrounded by air, exposed to the wind and the sun, and protected by a roof from rain and snow, will dry out very rapidly, while wood piled or packed close together so as to exclude the air, or left in the shade and exposed to rain and snow, will dry out very slowly and will also be subject to mould and decay.
But seasoning implies other changes besides the evaporation of water. Although we have as yet only a vague conception as to the exact nature of the difference between seasoned and unseasoned wood, it is very probable that one of these consists in changes in the albuminous substances in the wood fibers, and possibly also in the tannins, resins, and other incrusting substances. Whether the change in these substances is merely a drying-out, or whether it consists in a partial decomposition is at yet undetermined. That the change during the seasoning process is a profound one there can be no doubt, because experience has shown again and again that seasoned wood fiber is very much more permeable, both for liquids and gases than the living, unseasoned fiber.
One can picture the albuminous substances as forming a coating which dries out and possibly disintegrates when the wood dries. The drying-out may result in considerable shrinkage, which may make the wood fiber more porous. It is also possible that there are oxidizing influences at work within these substances which result in their disintegration. Whatever the exact nature of the change may be, one can say without hesitation that exposure to the wind and air brings about changes in the wood, which are of such a nature that the wood becomes drier and more permeable.
When seasoned by exposure to live steam, similiar changes may take place; the water leaves the wood in the form of steam, while the organic compounds in the walls probably coagulate or disintegrate under the high temperature.
The most effective seasoning is without doubt that obtained by the uniform, slow drying which takes place in properly constructed piles outdoors, under exposure to the winds and the sun and under cover from the rain and snow, and is what has been termed “air-seasoning.” By air-seasoning oak and similiar hardwoods, nature performs certain functions that cannot be duplicated by any artificial means. Because of this, woods of this class cannot be successfully kiln-dried green from the saw.
In drying wood, the free water within the cells passes through the cell walls until the cells are empty, while the cell walls remain saturated. When all the free water has been removed, the cell walls begin to yield up their moisture. Heat raises the absorptive power of the fibers and so aids the passage of water from the interior of the cells. A confusion in the word “sap” is to be found in many discussions of kiln-drying; in some instances it means water, in other cases it is applied to the organic substances held in a water solution in the cell cavities. The term is best confined to the organic substances from the living cell. These substances, for the most part of the nature of sugar, have a strong attraction for water and water vapor, and so retard drying and absorb moisture into dried wood. High temperatures, especially those produced by live steam, appear to destroy these organic compounds and therefore both to retard and to limit the reabsorption of moisture when the wood is subsequently exposed to the atmosphere.
Air-dried wood, under ordinary atmospheric temperatures, retains from 10 to 20 per cent of moisture, whereas kiln-dried wood may have no more than 5 per cent as it comes from the kiln. The exact figures for a given species depend in the first case upon the weather conditions, and in the second case upon the temperature in the kiln and the time during which the wood is exposed to it. When wood that has been kiln-dried is allowed to stand in the open, it apparently ceases to reabsorb moisture from the air before its moisture content equals that of wood which has merely been air-dried in the same place, and under the same conditions, in other words kiln-dried wood will not absorb as much moisture as air-dried wood under the same conditions.
Difference between Seasoned and Unseasoned Wood
Although it has been known for a long time that there is a marked difference in the length of life of seasoned and of unseasoned wood, the consumers of wood have shown very little interest in its seasoning, except for the purpose of doing away with the evils which result from checking, warping, and shrinking. For this purpose both kiln-drying and air-seasoning are largely in use.
The drying of material is a subject which is extremely important to most industries, and in no industry is it of more importance than in the lumber trade. Timber drying means not only the extracting of so much water, but goes very deeply into the quality of the wood, its workability and its cell strength, etc.
Kiln-drying, which dries the wood at a uniformly rapid rate by artificially heating it in inclosed rooms, has become a part of almost every woodworking industry, as without it the construction of the finished product would often be impossible. Nevertheless much unseasoned or imperfectly seasoned wood is used, as is evidenced by the frequent shrinkage and warping of the finished articles. This is explained to a certain extent by the fact that the manufacturer is often so hard pressed for his product that he is forced to send out an inferior article, which the consumer is willing to accept in that condition rather than to wait several weeks or months for an article made up of thoroughly seasoned material, and also that dry kilns are at present constructed and operated largely without thoroughgoing system.
Forms of kilns and mode of operation have commonly been copied by one woodworking plant after the example of some neighboring establishment. In this way it has been brought about that the present practices have many shortcomings. The most progressive operators, however, have experimented freely in the effort to secure special results desirable for their peculiar products. Despite the diversity of practice, it is possible to find among the larger and more enterprising operators a measure of agreement, as to both methods and results, and from this to outline the essentials of a correct theory. As a result, properly seasoned wood commands a high price, and in some cases cannot be obtained at all.
Wood seasoned out of doors, which by many is supposed to be much superior to kiln-dried material, is becoming very scarce, as the demand for any kind of wood is so great that it is thought not to pay to hold it for the time necessary to season it properly. How long this state of affairs is going to last it is difficult to say, but it is believed that a reaction will come when the consumer learns that in the long run it does not pay to use poorly seasoned material. Such a condition has now arisen in connection with another phase of the seasoning of wood; it is a commonly accepted fact that dry wood will not decay nearly so fast as wet or green wood; nevertheless, the immense superiority of seasoned over unseasoned wood for all purposes where resistance to decay is necessary has not been sufficiently recognized. In the times when wood of all kinds was both plentiful and cheap, it mattered little in most cases how long it lasted or resisted decay. Wood used for furniture, flooring, car construction, cooperage, etc., usually got some chance to dry out before or after it was placed in use. The wood which was exposed to decaying influences was generally selected from those woods which, whatever their other qualities might be, would resist decay longest.
To-day conditions have changed, so that wood can no longer be used to the same extent as in former years. Inferior woods with less lasting qualities have been pressed into service. Although haphazard methods of cutting and subsequent use are still much in vogue, there are many signs that both lumbermen and consumers are awakening to the fact that such carelessness and wasteful methods of handling wood will no longer do, and must give way to more exact and economical methods. The reason why many manufacturers and consumers of wood are still using the older methods is perhaps because of long custom, and because they have not yet learned that, though the saving to be obtained by the application of good methods has at all times been appreciable, now, when wood is more valuable, a much greater saving is possible. The increased cost of applying economical methods is really very slight, and is many times exceeded by the value of the increased service which can be secured through its use.
Manner of Evaporation of Water
The evaporation of water from wood takes place largely through the ends, i.e., in the direction of the longitudinal axis of the wood fibers. The evaporation from the other surfaces takes place very slowly out of doors, and with greater rapidity in a dry kiln. The rate of evaporation differs both with the kind of timber and its shape; that is, thin material will dry more rapidly than heavier stock. Sapwood dries faster than heartwood, and pine more rapidly than oak or other hardwoods.
Tests made show little difference in the rate of evaporation in sawn and hewn stock, the results, however, not being conclusive. Air-drying out of doors takes from two months to a year, the time depending on the kind of timber, its thickness, and the climatic conditions. After wood has reached an air-dry condition it absorbs water in small quantities after a rain or during damp weather, much of which is immediately lost again when a few warm, dry days follow. In this way wood exposed to the weather will continue to absorb water and lose it for indefinite periods.
When soaked in water, seasoned woods absorb water rapidly. This at first enters into the wood through the cell walls; when these are soaked, the water will fill the cell lumen, so that if constantly submerged the wood may become completely filled with water.
The following figures show the gain in weight by absorption of several coniferous woods, air-dry at the start, expressed in per cent of the kiln-dry weight:
Absorption of Water by Dry Wood
| White Pine | Red Cedar | Hemlock | Tamarack | |
| Air-dried | 108 | 109 | 111 | 108 |
| Kiln-dried | 100 | 100 | 100 | 100 |
| In water 1 day | 135 | 120 | 133 | 129 |
| In water 2 days | 147 | 126 | 144 | 136 |
| In water 3 days | 154 | 132 | 149 | 142 |
| In water 4 days | 162 | 137 | 154 | 147 |
| In water 5 days | 165 | 140 | 158 | 150 |
| In water 7 days | 176 | 143 | 164 | 156 |
| In water 9 days | 179 | 147 | 168 | 157 |
| In water 11 days | 184 | 149 | 173 | 159 |
| In water 14 days | 187 | 150 | 176 | 159 |
| In water 17 days | 192 | 152 | 176 | 161 |
| In water 25 days | 198 | 155 | 180 | 161 |
| In water 30 days | 207 | 158 | 183 | 166 |
Rapidity of Evaporation
The rapidity with which water is evaporated, that is, the rate of drying, depends on the size and shape of the piece and on the structure of the wood. An inch board dries more than four times as fast as a four-inch plank, and more than twenty times as fast as a ten-inch timber. White pine dries faster than oak. A very moist piece of pine or oak will, during one hour, lose more than four times as much water per square inch from the cross-section, but only one half as much from the tangential as from the radial section. In a long timber, where the ends or cross-sections form but a small part of the drying surface, this difference is not so evident. Nevertheless, the ends dry and shrink first, and being opposed in this shrinkage by the more moist adjoining parts, they check, the cracks largely disappearing as seasoning progresses.
High temperatures are very effective in evaporating the water from wood, no matter how humid the air, and a fresh piece of sapwood may lose weight in boiling water, and can be dried to quite an extent in hot steam.
In drying chemicals or fabrics, all that is required is to provide heat enough to vaporize the moisture and circulation enough to carry off the vapor thus secured, and the quickest and most economical means to these ends may be used. While on the other hand, in drying wood, whether in the form of standard stock or the finished product, the application of the requisite heat and circulation must be carefully regulated throughout the entire process, or warping and checking are almost certain to result. Moreover, wood of different shapes and thicknesses is very differently effected by the same treatment. Finally, the tissues composing the wood, which vary in form and physical properties, and which cross each other in regular directions, exert their own peculiar influences upon its behavior during drying. With our native woods, for instance, summer-wood and spring-wood show distinct tendencies in drying, and the same is true in a less degree of heartwood, as contrasted with sapwood. Or, again, pronounced medullary rays further complicate the drying problem.
Physical Properties that influence Drying
The principal properties which render the drying of wood peculiarly difficult are: (1) The irregular shrinkage; (2) the different ways in which water is contained; (3) the manner in which moisture transfuses through the wood from the center to the surface; (4) the plasticity of the wood substance while moist and hot; (5) the changes which take place in the hygroscopic and chemical nature of the surface; and (6) the difference produced in the total shrinkage by different rates of drying.
The shrinkage is unequal in different directions and in different portions of the same piece. It is greatest in the circumferential direction of the tree, being generally twice as great in this direction as in the radial direction. In the longitudinal direction, for most woods, it is almost negligible, being from 20 to over 100 times as great circumferentially as longitudinally.
There is a great variation in different species in this respect. Consequently, it follows from necessity that large internal strains are set up when the wood shrinks, and were it not for its plasticity it would rupture. There is an enormous difference in the total amount of shrinkage of different species of wood, varying from a shrinkage of only 7 per cent in volume, based on the green dimensions, in the case of some of the cedars to nearly 50 per cent in the case of some species of eucalyptus.
When the free water in the capillary spaces of the wood fiber is evaporated it follows the laws of evaporation from capillary spaces, except that the passages are not all free passages, and much of the water has to pass out by a process of transfusion through the moist cell walls. These cell walls in the green wood completely surround the cell cavities so that there are no openings large enough to offer a passage to water or air.
The well-known “pits” in the cell walls extend through the secondary thickening only, and not through the primary walls. This statement applies to the tracheids and parenchyma cells in the conifer (gymnosperms), and to the tracheids, parenchyma cells, and the wood fibers in the broad-leaved trees (angiosperms); the vessels in the latter, however, form open passages except when clogged by ingrowth called tyloses, and the resin canals in the former sometimes form occasional openings.
By heating the wood above the boiling point, corresponding to the external pressure, the free water passes through the cell walls more readily.
To remove the moisture from the wood substance requires heat in addition to the latent heat of evaporation, because the molecules of moisture are so intimately associated with the molecules, minute particles composing the wood, that energy is required to separate them therefrom.
Carefully conducted experiments show this to be from 16.6 to 19.6 calories per grain of dry wood in the case of beech, long-leaf pine, and sugar maple.
The difficulty imposed in drying, however, is not so much the additional heat required as it is in the rate at which the water transfuses through the solid wood.
INDEX: Seasoning of Wood