Part 14 (2/2)

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 pa.s.s 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 pa.s.ses 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 s.p.a.ces, the cells of green wood are enclosed by membranes and the water must pa.s.s 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 circ.u.mstances 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.

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