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.
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.
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 remainedopen for the exit of the moisture from the interior of the piece, and this trouble would have been avoided. (See also article following.)
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, butare 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, becomesone 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 asthe 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 thatabsolutely dry woodcannot 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, 11⁄2inches 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 leachedout, 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.
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 hasbeen 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.
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 foras 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 theALL) 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.
Seasoningand kiln-drying is so important a process in the manufacture of woods that a need is keenly felt for fuller information regarding it, based upon scientific study of the behavior of various species at different mechanical temperatures and under different mechanical drying processes. The special precautions necessary to prevent loss of strength or distortion of shape render the drying of wood especially difficult.
All wood when undergoing a seasoning process, either natural (by air) or mechanical (by steam or heat in a dry kiln), checks or splits more or less. This is due to the uneven drying-out of the wood and the consequent strains exerted in opposite directions by the wood fibres in shrinking. This shrinkage, it has been proven, takes place both end-wise and across the grain of the wood. The old tradition that wood does not shrink end-wise has long since been shattered, and it has long been demonstrated that there is an end-wise shrinkage.
In some woods it is very light, while in others it is easily perceptible. It is claimed that the average end shrinkage, taking all the woods, is only about 11⁄2per cent. This, however, probably has relation to the average shrinkage on ordinary lumber as it is used and cut and dried. Now if we depart from this and take veneer, or basket stock, or even stave bolts where they are boiled, causing swelling both end-wise and across the grain or in dimension, after they are thoroughly dried, there is considerably more evidence of end shrinkage. In other words, a slack barrel stave of elm, say, 28 or 30 inches in length, after beingboiled might shrink as much in thoroughly drying-out as compared to its length when freshly cut, as a 12-foot elm board.
It is in cutting veneer that this end shrinkage becomes most readily apparent. In trimming with scoring knives it is done to exact measure, and where stock is cut to fit some specific place there has been observed a shrinkage on some of the softer woods, like cottonwood, amounting to fully1⁄8of an inch in 36 inches. And at times where drying has been thorough the writer has noted a shrinkage of1⁄8of an inch on an ordinary elm cabbage-crate strip 36 inches long, sawed from the log without boiling.
There are really no fixed rules of measurement or allowance, however, because the same piece of wood may vary under different conditions, and, again, the grain may cross a little or wind around the tree, and this of itself has a decided effect on the amount of what is termed "end shrinkage."
There is more checking in the wood of the broad-leaf (hardwood) trees than in that of the coniferous (softwood) trees, more in sapwood than in heartwood, and more in summer-wood than in spring-wood.
Inasmuch as under normal conditions of weather, water evaporates less rapidly during the early seasoning of winter, wood that is cut in the autumn and early winter is considered less subject to checking than that which is cut in spring and summer.
Rapid seasoning, except after wood has been thoroughly soaked or steamed, almost invariably results in more or less serious checking. All hardwoods which check or warp badly during the seasoning should be reduced to the smallest practicable size before drying to avoid the injuries involved in this process, and wood once seasonedshould never again be exposed to the weather, since all injuries due to seasoning are thereby aggravated.
Seasoning increases the strength of wood in every respect, and it is therefore of great importance to protect the wood against moisture.
An important property rendering drying of wood peculiarly difficult is the changes which occur in the hygroscopic properties of the surface of a stick, and the rate at which it will allow moisture to pass through it. If wood is dried rapidly the surface soon reaches a condition where the transfusion is greatly hindered and sometimes appears almost to cease. The nature of this action is not well understood and it differs greatly in different species. Bald cypress (Taxodium distichum) is an example in which this property is particularly troublesome. The difficulty can be overcome by regulating the humidity during the drying operation. It is one of the factors entering into production of what is called "case-hardening" of wood, where the surface of the piece becomes hardened in a stretched or expanded condition, and subsequent shrinkage of the interior causes "honeycombing," "hollow-horning," or internal checking. The outer surface of the wood appears to undergo a chemical change in the nature of hydrolization or oxidization, which alters the rate of absorption and evaporation in the air.
As the total amount of shrinkage varies with the rate at which the wood is dried, it follows that the outer surface of a rapidly dried board shrinks less than the interior. This sets up an internal stress, which, if the board be afterward resawed into two thinner boards by slicing it through the middle, causes the two halves to cup with their convex surfaces outward. This effect may occur even though the moisture distribution in the board has reached a uniform condition, and the board is thoroughly dry before it is resawed. It is distinct from the well-known "case-hardening" effect spoken of above, which is caused by unequal moisture conditions.
The manner in which the water passes from the interior of a piece of wood to its surface has not as yet been fully determined, although it is one of the most important factors which influence drying. This must involve a transfusion of moisture through the cell walls, since, as already mentioned, except for the open vessels in the hardwoods,free resin ducts in the softwoods, and possibly the intercellular spaces, the cells of green wood are enclosed by membranes and the water must pass through the walls or the membranes of the pits. Heat appears to increase this transfusion, but experimental data are lacking.
It is evident that to dry wood properly a great many factors must be taken into consideration aside from the mere evaporation of moisture.
In some cases there is practically no loss in drying, but more often it ranges from 1 to 3 per cent, and 7 to 10 per cent in refractory woods such as gum. In exceptional instances the losses are as high as 33 per cent.
In air-drying there is little or no control over the process; it may take place too rapidly on some days and too slowly on others, and it may be very non-uniform.
Hardwoods in large sizes almost invariably check.
By proper kiln-drying these unfavorable circumstances may be eliminated. However, air-drying is unquestionably to be preferred to bad kiln-drying, and when there is any doubt in the case it is generally safer to trust to air-drying.
If the fundamental principles are all taken care of, green lumber can be better dried in the dry kiln.
It is clear, from the previous discussion of the structure of wood, that this property is of first importance among those influencing the seasoning of wood. The free water way usually be extracted quite readily from porous hardwoods. The presence of tyloses in white oak makes even this a difficult problem. On the other hand, its more complex structure usually renders the hygroscopic moisture quite difficult to extract.
The lack of an open, porous structure renders the transfusion of moisture through some woods very slow, while the reverse may be true of other species. The point of interest is that all the different variations in structureaffect the drying rates of woods. The structure of the gums suggests relatively easy seasoning.
Shrinkage is a very important factor affecting the drying of woods. Generally speaking, the greater the shrinkage the more difficult it is to dry wood. Wood shrinks about twice as much tangentially as radially, thus introducing very serious stresses which may cause loss in woods whose total shrinkage is large. It has been found that the amount of shrinkage depends, to some extent, on the rate and temperature at which woods season. Rapid drying at high or low temperature results in slight shrinkage, while slow drying, especially at high temperature, increases the shrinkage.
As some woods must be dried in one way and others in other ways, to obtain the best general results, this effect may be for the best in one case and the reverse in others. As an example one might cite the case of Southern white oak. This species must be dried very slowly at low temperatures in order to avoid the many evils to which it is heir. It is interesting to note that this method tends to increase the shrinkage, so that one might logically expect such treatment merely to aggravate the evils. Such is not the case, however, as too fast drying results in other defects much worse than that of excessive shrinkage.
Thus we see that the shrinkage of any given species of wood depends to a great extent on the method of drying. Just how much the shrinkage of gum is affected by the temperature and drying rate is not known at present. There is no doubt that the method of seasoning affects the shrinkage of the gums, however. It is just possible that these woods may shrink longitudinally more than is normal, thus furnishing another cause for their peculiar action under certain circumstances. It has been found that the properties of wood which affect the seasoning of the gums are, in the order of their importance: (1) The indeterminate and erratic grain; (2) the uneven shrinkage with the resultant opposing stresses; (3) the plasticity under high temperature while moist; and (4) the slight apparent lack of cohesion between the fibres. The first, second, and fourth properties are clearly detrimental,while the third may possibly be an advantage in reducing checking and "case-hardening."
The grain of the wood is a prominent factor also affecting the problem. It is this factor, coupled with uneven shrinkage, which is probably responsible, to a large extent, for the action of the gums in drying. The grain may be said to be more or less indeterminate. It is usually spiral, and the spiral may reverse from year to year of the tree's growth. When a board in which this condition exists begins to shrink, the result is the development of opposing stresses, the effect of which is sometimes disastrous. The shrinkage around the knots seems to be particularly uneven, so that checking at the knots is quite common.
Some woods, such as Western red cedar, redwood, and eucalyptus, become very plastic when hot and moist. The result of drying-out the free water at high temperature may be to collapse the cells. The gums are known to be quite soft and plastic, if they are moist, at high temperature, but they do not collapse so far as we have been able to determine.
The cells of certain species of wood appear to lack cohesion, especially at the junction between the annual rings. As a result, checks and ring shakes are very common in Western larch and hemlock. The parenchyma cells of the medullary rays in oak do not cohere strongly and often check open, especially when steamed too severely.
1. Physical data of the properties of wood in relation to heat are meagre.2. Figures on the specific heat of wood are not readily available, though upon this rests not only the exact operation of heating coils for kilns, but the theory of kiln-drying as a whole.3. Great divergence is shown in the results of experiments in the conductivity of wood. It remains to be seen whether the known variation of conductivity with moisture content will reduce these results to uniformity.4. The maximum or highest temperature to which the different species of wood may be exposed without serious loss of strength has not yet been determined.5. The optimum or absolute correct temperature for drying the different species of wood is as yet entirely unsettled.6. The inter-relation between wood and water is as imperfectly known to dry-kiln operators as that between wood and heat.7. What moisture conditions obtain in a stick of air-dried wood?8. How is the moisture distinguished?9. What is its form?10. What is the meaning of the peculiar surface conditions which even in air-dried wood appear to indicate incipient "case-hardening"?11. The manner in which the water passes from the interior of a piece of wood to its surface has not as yet been fully determined.
1. Physical data of the properties of wood in relation to heat are meagre.
2. Figures on the specific heat of wood are not readily available, though upon this rests not only the exact operation of heating coils for kilns, but the theory of kiln-drying as a whole.
3. Great divergence is shown in the results of experiments in the conductivity of wood. It remains to be seen whether the known variation of conductivity with moisture content will reduce these results to uniformity.
4. The maximum or highest temperature to which the different species of wood may be exposed without serious loss of strength has not yet been determined.
5. The optimum or absolute correct temperature for drying the different species of wood is as yet entirely unsettled.
6. The inter-relation between wood and water is as imperfectly known to dry-kiln operators as that between wood and heat.
7. What moisture conditions obtain in a stick of air-dried wood?
8. How is the moisture distinguished?
9. What is its form?
10. What is the meaning of the peculiar surface conditions which even in air-dried wood appear to indicate incipient "case-hardening"?
11. The manner in which the water passes from the interior of a piece of wood to its surface has not as yet been fully determined.
These questions can be answered thus far only by speculation or, at best, on the basis of incomplete data.
Until these problems are solved, kiln-drying must necessarily remain without the guidance of complete scientific theory.
A correct understanding of the principles of drying is rare, and opinions in regard to the subject are very diverse. The same lack of knowledge exists in regard to dry kilns. The physical properties of the wood which complicate the drying operation and render it distinct from that of merely evaporating free water from some substance like a piece of cloth must be studied experimentally. It cannot well be worked out theoretically.
Thechoice of a method of drying depends largely upon the object in view. The principal objects may be grouped under three main heads, as follows:
When wood will stand the temperature without excessive checking or undue shrinkage or loss in strength, the first object is most readily attained by heating the wood above the boiling point in a closed chamber, with a large circulation of air or vapor, so arranged that the excess steam produced will escape. This process manifestly does not apply to many of the hardwoods, but is applicable to many of the softwoods. It is used especially in the northwestern part of the United States, where Douglas fir boards one inch thick are dried in from 40 to 65 hours, and sometimes in as short a time as 24 hours. In the latter case superheated steam at 300 degrees Fahrenheit was forced into the chamber but, of course, the lumber could not be heated thereby much above the boiling point so long as it contained any free water.
This lumber, however, contained but 34 per cent moisture to start with, and the most rapid rate was 1.6 per cent loss per hour.
The heat of evaporation may be supplied either by superheated steam or by steam pipes within the kiln itself.
The quantity of wood it is necessary to carry in stockis naturally reduced when either of the other two objects is attained and, therefore, need not necessarily be discussed.
In drying to prepare for use and to improve quality, careful and scientific drying is called for. This applies more particularly to the hardwoods, although it may be required for softwoods also.
Present practice of kiln-drying varies tremendously and there is no uniformity or standard method.
Temperatures vary anywhere from 65 to 165 degrees Fahrenheit, or even higher, and inch boards three to six months on the sticks are being dried in from four days to three weeks, and three-inch material in from two to five months.
All methods in use at atmospheric pressure may be classified under the following headings. The kilns may be either progressive or compartment, and preliminary steaming may or may not be used with any one of these methods:
Various methods of drying wood under pressures other than atmospheric have been tried. Only a brief mention of this subject will be made. Where the apparatus is available probably the quickest way to dry wood is first to heat it in saturated steam at as high a temperature as the species can endure without serious chemical change until the heat has penetrated to the center, then follow this with a vacuum.
By this means the self-contained specific heat of the wood and the water is made available for the evaporation, and the drying takes place from the inside outwardly, just the reverse of that which occurs by drying by means of external heat.
When the specimen has cooled this process is then to be repeated until it has dried down to fibre-saturation point. It cannot be dried much below this point by this method, since the absorption during the heating operation will then equal the evaporation during the cooling. It may be carried further, however, by heating in partially humidified air, proportioning the relative humidity each time it is heated to the degree of moisture present in the wood.
The point to be considered in this operation is that during the heating process no evaporation shall be allowed to take place, but only during the cooling. In this way surface drying and "case-hardening" are prevented since the heat is from within and the moisture passes from the inside outwardly. However, with some species, notably oak, surface cracks appear as a network of fine checks along the medullary rays.
In the first place, it should be borne in mind that it is the heat which produces evaporation and not the air nor any mysterious property assigned to a "vacuum."
For every pound of water evaporated at ordinary temperatures approximately 1,000 British thermal units of heat are used up, or "become latent," as it is called. This is true whether the evaporation takes place in a vacuum or under a moderate air pressure. If this heat is not supplied from an outside source it must be supplied by the water itself (or the material being dried), the temperature of which will consequently fall until the surrounding space becomes saturated with vapor at a pressure corresponding to the temperature which the water has reached; evaporation will then cease. The pressure of the vapor in a space saturated with water vapor increases rapidly with increase of temperature. At a so-called vacuum of 28 inches, which is about the limit in commercial operations, and in reality signifies an actual pressure of 2 inchesof mercury column, the space will be saturated with vapor at 101 degrees Fahrenheit. Consequently, no evaporation will take place in such a vacuum unless the water be warmer than 101 degrees Fahrenheit, provided there is no air leakage. The qualification in regard to air is necessary, for the sake of exactness, for the following reason: In any given space the total actual pressure is made up of the combined pressures of all the gases present. If the total pressure ("vacuum") is 2 inches, and there is no air present, it is all produced by the water vapor (which saturates the space at 101 degrees Fahrenheit); but if some air is present and the total pressure is still maintained at 2 inches, then there must be less vapor present, since the air is producing part of the pressure and the space is no longer saturated at the given temperature. Consequently further evaporation may occur, with a corresponding lowering of the temperature of the water, until a balance is again reached. Without further explanation it is easy to see that but little water can be evaporated by a vacuum alone without addition of heat, and that the prevalent idea that a vacuum can of itself produce evaporation is a fallacy. If heat be supplied to the water, however, either by conduction or radiation, evaporation will take place in direct proportion to the amount of heat supplied, so long as the pressure is kept down by the vacuum pump.
At 30 inches of mercury pressure (one atmosphere) the space becomes saturated with vapor and equilibrium is established at 212 degrees Fahrenheit. If heat be now supplied to the water, however, evaporation will take place in proportion to the amount of heat supplied, so long as the pressure remains that of one atmosphere, just as in the case of the vacuum. Evaporation in this condition, where the vapor pressure at the temperature of the water is equal to the gas pressure on the water, is commonly called "boiling," and the saturated vapor entirely displaces the air under continuous operation. Whenever the space is not saturated with vapor, whether air is present or not, evaporation will take place, by boiling if no air be present or by diffusion under the presenceof air, until an equilibrium between temperature and vapor pressure is resumed.
Relative humidity is simply the ratio of the actual vapor pressure present in a given space to the vapor pressure when the space is saturated with vapor at the given temperature. It matters not whether air be present or not. One hundred per cent humidity means that the space contains all the vapor which it can hold at the given temperature—it is saturated. Thus at 100 per cent humidity and 212 degrees Fahrenheit the space is saturated, and since the pressure of saturated vapor at this temperature is one atmosphere, no air can be present under these conditions. If, however, the total pressure at this temperature were 20 pounds (5 pounds gauge), then it would mean that there was 5 pounds air pressure present in addition to the vapor, yet the space would still be saturated at the given temperature. Again, if the temperature were 101 degrees Fahrenheit, the pressure of saturated vapor would be only 1 pound, and the additional pressure of 14 pounds, if the total pressure were atmospheric, would be made up of air. In order to have no air present and the space still saturated at 101 degrees Fahrenheit, the total pressure must be reduced to 1 pound by a vacuum pump. Fifty per cent relative humidity, therefore, signifies that only half the amount of vapor required to saturate the space at the given temperature is present. Thus at 212 degrees Fahrenheit temperature the vapor pressure would only be 71⁄2pounds (vacuum of 15 inches gauge). If the total pressure were atmospheric, then the additional 71⁄2pounds would be simply air.
"Live steam" is simply water-saturated vapor at a pressure usually above atmospheric. We may just as truly have live steam at pressures less than atmospheric, at a vacuum of 28 inches for instance. Only in the latter case its temperature would be lower,viz., 101 degrees Fahrenheit.
Superheated steam is nothing more than water vapor at a relative humidity less than saturation, but is usually considered at pressures above atmospheric, and in the absence of air. The atmosphere at, say, 50 per cent relativehumidity really contains superheated steam or vapor, the only difference being that it is at a lower temperature and pressure than we are accustomed to think of in speaking of superheated steam, and it has air mixed with it to make up the deficiency in pressure below the atmosphere.
Two things should now be clear; that evaporation is produced by heat and that the presence or absence of air does not influence the amount of evaporation. It does, however, influence the rate of evaporation, which is retarded by the presence of air. The main things influencing evaporation are, first, the quantity of heat supplied and, second, the relative humidity of the immediately surrounding space.
What this term really signifies is simply water vapor in the absence of air in a condition of less than saturation. Kilns of this type are, properly speaking, vapor kilns, and usually operate at atmospheric pressure, but may be used at greater pressures or at less pressures. As stated before, the vapor present in the air at any humidity less than saturation is really "superheated steam," only at a lower pressure than is ordinarily understood by this term, and mixed with air. The main argument in favor of this process seems to be based on the idea that steam is moist heat. This is true, however, only when the steam is near saturation. When it is superheated it is just as dry as air containing the same relative humidity. For instance, steam at atmospheric pressure and heated to 248 degrees Fahrenheit has a relative humidity of only 50 per cent and is just as dry as air containing the same humidity. If heated to 306 degrees Fahrenheit, its relative humidity is reduced to 20 per cent; that is to say, the ratio of its actual vapor pressure (one atmosphere) to the pressure of saturated vapor at this temperature (five atmospheres) is 1:5, or 20 per cent. Superheated vapor in the absence of air, however, parts with its heat with great rapidity and finally becomes saturated when it has lost all of its ability to cause evaporation. In this respect it is more moist than air when it comes in contact with bodies whichare at a lower temperature. When saturated steam is used to heat the lumber it can raise the temperature of the latter to its own temperature, but cannot produce evaporation unless, indeed, the pressure is varied. Only by the heat supplied above the temperature of saturation can evaporation be produced.
Methods of partially overcoming the shrinkage by impregnation of the cell walls with organic materials closely allied to the wood substance itself are in use. In one of these which has been patented, sugar is used as the impregnating material, which is subsequently hardened or "caramelized" by heating. Experiments which the United States Forest Service has made substantiate the claims that the sugar does greatly reduce the shrinkage of the wood; but the use of impregnation processes is determined rather from a financial economic standpoint than by the physical result obtained.
Another process consists in passing a current of electricity through the wet boards or through the green logs before sawing. It is said that the ligno cellulose and the sap are thus transformed by electrolysis, and that the wood subsequently dries more rapidly.
In many dry kiln operations, especially where the kilns are not designed for treatments with very moist air, the wood is allowed to air-season from several months to a year or more before running it into the dry kiln. In this way the surface dries below its fibre-saturation point and becomes hardened or "set" and the subsequent shrinkage is not so great. Moreover, there is less danger of surface checking in the kiln, since the surface has already passed the danger point. Many woods, however, check severely in air-drying or case-harden in the air. It is thought that such woods can be satisfactorily handled in a humidity-regulated kiln direct from the saw.
Preliminary steaming is frequently used to moisten the surface if case-hardened, and to heat the lumber throughto the center before drying begins. This is sometimes done in a separate chamber, but more often in a compartment of the kiln itself, partitioned off by means of a curtain which can be raised or lowered as circumstances require. This steaming is usually conducted at atmospheric pressure and frequently condensed steam is used at temperatures far below 212 degrees Fahrenheit. In a humidity-regulated kiln this preliminary treatment may be omitted, since nearly saturated conditions can be maintained and graduated as the drying progresses.
Recently the process of steaming at pressures up to 20 pounds gauge in a cylinder for short periods of time, varying from 5 to 20 minutes, is being advocated in the United States. The truck load is run into the cylinder, steamed, and then taken directly out into the air. It may subsequently be placed in the dry kiln if further drying is desired. The self-contained heat of the wood evaporates considerable moisture, and the sudden drying of the boards causes the shrinkage to be reduced slightly in some cases. Such short periods of steaming under 20 pounds pressure do not appear to injure the wood mechanically, although they do darken the color appreciably, especially of the sapwood of the species having a light-colored sap, as black walnut (Juglans nigra) and red gum (Liquidamber styraciflua). Longer periods of steaming have been found to weaken the wood. There is a great difference in the effect on different species, however.
Soaking wood for a long time before drying has been practised, but experiments indicate that no particularly beneficial results, from the drying standpoint, are attained thereby. In fact, in some species containing sugars and allied substances it is probably detrimental from the shrinkage standpoint. If soaked in boiling water some species shrink and warp more than if dried without this treatment.
In general, it may be said that, except possibly for short-period steaming as described above, steaming and soaking hardwoods at temperatures of 212 degrees Fahrenheit or over should be avoided if possible.
It is the old saying that wood put into water shortly after it is felled, and left in water for a year or more, will be perfectly seasoned after a short subsequent exposure to the air. For this reason rivermen maintain that timber is made better by rafting. Herzenstein says: "Floating the timber down rivers helps to wash out the sap, and hence must be considered as favorable to its preservation, the more so as it enables it to absorb more preservative."
Wood which has been buried in swamps is eagerly sought after by carpenters and joiners, because it has lost all tendency to warp and twist. When first taken from the swamp the long-immersed logs are very much heavier than water, but they dry with great rapidity. A cypress log from the Mississippi Delta, which two men could barely handle at the time it was taken out some years ago, has dried out so much since then that to-day one man can lift it with ease. White cedar telegraph poles are said to remain floating in the water of the Great Lakes sometimes for several years before they are set in lines and to last better than freshly cut poles.
It is very probable that immersion for long periods in water does materially hasten subsequent seasoning. The tannins, resins, albuminous materials, etc., which are deposited in the cell walls of the fibres of green wood, and which prevent rapid evaporation of the water, undergo changes when under water, probably due to the action of bacteria which live without air, and in the course of time many of these substances are leached out of the wood. The cells thereby become more and more permeable to water, and when the wood is finally brought into the air the water escapes very rapidly and very evenly. Herzenstein's statement that wood prepared by immersion and subsequent drying will absorb more preservative, and that with greater rapidity, is certainly borne out by experience in the United States.
It is sometimes claimed that all seasoning preparatory to treatment with a substance like tar oil might be done away with by putting the green wood into a cylinder with the oil and heating to 225 degrees Fahrenheit, thus drivingthe water off in the form of steam, after which the tar oil would readily penetrate into the wood. This is the basis of the so-called "Curtiss process" of timber treatment. Without going into any discussion of this method of creosoting, it may be said that the same objection made for steaming holds here. In order to get a temperature of 212 degrees Fahrenheit in the center of the treated wood, the outside temperature would have to be raised so high that the strength of the wood might be seriously injured.
A company on the Pacific coast which treats red fir piling asserts that it avoids this danger by leaving the green timber in the tar oil at a temperature which never exceeds 225 degrees Fahrenheit for from five to twelve hours, until there is no further evidence of water vapor coming out of the wood. The tar oil is then run out, and a vacuum is created for about an hour, after which the oil is run in again and is kept in the cylinders under 100 pounds pressure for from ten to twelve hours, until the required amount of absorption has been reached (about 12 pounds per cubic foot).
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. Lumber has always been seasoned in this way, which is still the best for ordinary purposes.
It is probable for the sake of economy, air-drying will be eliminated in the drying process of the future without loss to the quality of the product, but as yet no effective method has been discovered whereby this may be accomplished, because nature performs certain functions in air-drying that cannot be duplicated by artificial means. Because of this, hardwoods, as a rule, cannot be successfully kiln-dried green or direct from the saw, and must receive a certain amount of preliminary air-drying before being placed in a dry kiln.
The present methods of air-seasoning in use have been determined by long experience, and are probably as goodas they could be made for present conditions. But the same care has not up to this time been given to the seasoning of such timber as ties, bridge material, posts, telegraph and telephone poles, etc. These have sometimes been piled more or less intelligently, but in the majority of cases their value has been too low to make it seem worth while to pile with reference to anything beyond convenience in handling.
In piling material for air-seasoning, one should utilize high, dry ground when possible, and see that the foundations are high enough off the ground, so that there is proper air circulation through the bottom of the piles, and also that the piles are far enough apart so that the air may circulate freely through and around them.
It is air circulation that is desired in all cases of drying, both in dry kilns and out-of-doors, and not sunshine; that is, not the sun shining directly upon the material. The ends also should be protected from the sun, and everything possible done to induce a free circulation of air, and to keep the foundations free from all plant growth.
Naturally, the heavier the material to be dried, the more difficulty is experienced from checking, which has its most active time in the spring when the sap is rising. In fact the main period of danger in material checking comes with the March winds and the April showers, and not infrequently in the South it occurs earlier than that. In other words, as soon as the sap begins to rise, the timber shows signs of checking, and that is the time to take extra precautions by careful piling and protection from the sun. When the hot days of summer arrive the tendency to check is not so bad, but stock will sour from the heat, stain from the sap, mildew from moisture, and fall a prey to wood-destroying insects.
It has been proven in a general way that wood will season more slowly in winter than in summer, and also that the water content during various months varies. In the spring the drying-out of wood cut in October and November will take place more rapidly.
Someof the advantages of kiln-drying to be secured over air-drying in addition to reducing the shipping weight and lessening quantity of stock are the following:
This reduction in the tendency to take up moisture means a reduction in the "working" of the material which, even though slight, is of importance.
The problem of drying wood in the best manner divides itself into two distinct parts, one of which is entirely concerned with the behavior of the wood itself and the physical phenomena involved, while the other part has to do with the control of the drying process.