Seasoning and 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 11/2 per 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 being boiled 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 fully 1/8 of an inch in 36 inches. And at times where drying has been thorough the writer has noted a shrinkage of 1/8 of 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 seasoned should 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.

Changes rendering Drying difficult

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.

Losses Due to Improper Kiln-drying

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.

Properties of Wood that affect Drying

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 structure affect 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.

Unsolved Problems in Kiln-drying

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.

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