The changes to which wood is subject are partly mechanical in their nature, consisting of alterations in the water capacity, and consequent alterations in shape; partly chemical, caused chiefly by the decomposition of the sap, which finally leads to the decay of the wood.

I. Changes In The Water Capacity, And The Changes In Form Which Are Thereby Produced

Newly felled timber contains, as has been said, a large proportion of water - sometimes as much as 50 per cent, of its own weight. After lying for some time in a dry and airy place, it loses about half its amount of water by evaporation. Sawn or split wood, dried for a year or two under cover, still retains 10 to 15 per cent, of water, and only by continuous application of heat, or drying in an oven, can the water in timber be completely expelled.

During the process of drying, timber decreases in volume or shrinks. If exposed again to moisture it increases in volume or swells.

Changes in the volume of timber.

If any given piece of timber were uniform in texture throughout, and if no obstacles in any direction were presented to its expansion, the only result of shrinking or swelling would be alteration in volume ; there would be no change in form. This, however, is seldom the case. Generally speaking, the texture of the wood varies in different parts of the same piece. Again, it is often used under conditions which do not permit it to shrink or swell freely in all directions ; consequently, it shrinks or swells more in one place than in another.

When one part of a piece of timber shrinks more rapidly than an adjacent part, the wood cracks. If, on the other hand, one part swells more than another, or if the adjacent part meets with some obstacle to its expansion, the timber changes in shape - it becomes warped.

The shrinkage of timber stands in close connection with the amount of water contained. The more water it gives off while drying, the more it shrinks. Similarly the warmer and drier the air in which it is placed, the greater the shrinkage.

Some kinds of wood shrink more than others, and the same kind of wood shrinks differently in different directions.

Shrinkage in different directions.

All wood shrinks least in the direction of the fibres' length, and generally so very little that the difference need not be taken into consideration. But the difference caused by shrinking is very great across the fibres, and in tangential section it is two or three times greater than in radial section, or in the plane of the medullary rays. The sap-wood, which contains more water than the heart-wood, always shrinks more than the latter.

The following table, taken from " Karmarsch's Technology," shows the results of experiments made on a number of trees, to ascertain to what extent their timber shrinks. It must be observed that (1) the experiments were made with thin pieces of wood ; (2) that the figures are understood to represent the difference between wood which is either quite green or saturated with water, and that which has been thoroughly well seasoned; and that, therefore, (3) the shrinking of partially seasoned wood is considerably less than is stated in the table. (The same applies of course to the swelling of such wood, when it is again exposed to moisture.)

The last column gives the average degree of shrinkage across the fibres.

Shrinkage of Timber.

Name of tree.

In length. Per cent.

Degree of Shrinkage.

Average across the fibres.

Per cent.

Across the fibres in the direction of-

The medullary rays.

Per cent.

The annual layers.

Per cent.

The common alder .....

0.369

2.91

5.07

3.99

The elm ......

0.124

2.94

6.22

4.58

The apple........

0.109

3.00

739

5.19

The common ash (young) ...

0.821

4.05

6.56

5.30

The common birch .....

0.222

3.86

9.30

6.58

The common beech .....

0.200

5.03

8.06

6.54

The hornbeam ......

0.400

6.66

10.90

8.78

Ebony........

0.010

2.13

4.07

3.10

The oak (young)......

0.400

3.90

7.55

5.72

The oak (old)......

0.130

3.13

7.78

5.45

The Scotch fir ......

0.120

304

5.72

4.38

The spruce fir......

0.076

2.41

6.18

429

The lime........

0.208

7.79

11.50

9.64

The common larch .....

0.075

2.17

6.32

4.24

The maple........

0.072

3.35

6.59

4.97

Mahogany ......

0.110

1.09

1.79

1.44

Lignum vitae ......

0.625

5.18

7.50

6.34

The pear ......

0.228

3.94

12.70

8.32

The rowan........

0.190

2.11

8.88

5.49

The common walnut.....

0.223

3.53

625

4.89

As is seen from the above table, the degree of shrinkage in the direction of the length of the wood is so slight that it may be left entirely out of consideration. In the direction of the breadth, however, it varies from 2 per cent, to 9 per cent. In radial section, the general average is 5 per cent.; for fir and pine 3 per cent.; for birch 4 per cent. In tangential section, where shrinkage is greatest, it varies from 2 per cent, to 13 per cent., the general average for wood in common use being 7 per cent.; for fir and pine 6 per cent.; for birch 9 per cent.

General results afforded by the above table.

Fig. 4. Shrinkage in planks.

Fig. 4. Shrinkage in planks.

When a tree stem is sawn up into planks by parallel longitudinal cuts, the planks shrink as is shown in Fig. 4. The broadest portion shown, which includes the pith, shrinks least in breadth, most in thickness ; least nearest the pith, most near the sides. The outermost plank, however, shrinks most in breadth - in the direction of the annual layers - and least in thickness. The planks lying between shrink differently on different sides, and become concave to the pith, and convex on the other side.

Of trees in most general use, beech, lime, hornbeam, and pear shrink most; birch, apple, white-beam, walnut, ash, and oak shrink considerably; alder, maple, Scotch fir, elm, spruce fir, and larch shrink in a medium degree. Mahogany shrinks least of all timbers.

Cracks occur in timber, because, as indicated above, it is seldom uniform in texture, and it is therefore liable to shrink in different degrees during seasoning. The parts nearest the sap-wood shrink more rapidly than the heart-wood, and cracks, which run almost invariably in the direction of the medullary rays, are the result. The more rapidly wood dries the more it cracks, consequently timber should always be dried very slowly to prevent the formation of cracks. If it is tolerably uniform in texture, it may, with proper treatment, be kept entirely free from cracks.

The swelling, or expansion of timber, takes place when it is exposed to damp air or water, and is in direct relation to its shrinkage. When a piece of dried wood is immersed in water, it swells until it occupies the same volume as it occupied in its fresh condition, after which no further expansion takes place. Its amount of water, however, and consequently its weight, are greater than in its fresh condition, because the vessels originally filled with air are now filled with water.

The warping of timber depends on differences in the nature of its texture, and on other circumstances which cause changes in form both when it shrinks and when it swells. For example, a plank will become twisted or curved if one side only is exposed to the sun without being turned. Thin, flat pieces of wood become convex or concave, according as one or other side is exposed to damp or to drying influences.