Part 15 (1/2)

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 furnis.h.i.+ng another cause for their peculiar action under certain circ.u.mstances. 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

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 pa.s.ses 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.

SECTION X

HOW WOOD IS SEASONED

Methods of Drying

The choice of a method of drying depends largely upon the object in view. The princ.i.p.al objects may be grouped under three main heads, as follows:

1. To reduce s.h.i.+pping weight.

2. To reduce the quant.i.ty necessary to carry in stock.