Part 13 (1/2)

The following figures show the gain in weight by absorption of several coniferous woods, air-dry at the start, expressed in per cent of the kiln-dry weight:

ABSORPTION OF WATER BY DRY WOOD --------------------------------------------------------------- | White Pine | Red Cedar | Hemlock | Tamarack --------------------------------------------------------------- Air-dried | 108 | 109 | 111 | 108 Kiln-dried | 100 | 100 | 100 | 100 In water 1 day | 135 | 120 | 133 | 129 In water 2 days | 147 | 126 | 144 | 136 In water 3 days | 154 | 132 | 149 | 142 In water 4 days | 162 | 137 | 154 | 147 In water 5 days | 165 | 140 | 158 | 150 In water 7 days | 176 | 143 | 164 | 156 In water 9 days | 179 | 147 | 168 | 157 In water 11 days | 184 | 149 | 173 | 159 In water 14 days | 187 | 150 | 176 | 159 In water 17 days | 192 | 152 | 176 | 161 In water 25 days | 198 | 155 | 180 | 161 In water 30 days | 207 | 158 | 183 | 166 ---------------------------------------------------------------

Rapidity of Evaporation

The rapidity with which water is evaporated, that is, the rate of drying, depends on the size and shape of the piece and on the structure of the wood. An inch board dries more than four times as fast as a four-inch plank, and more than twenty times as fast as a ten-inch timber. White pine dries faster than oak. A very moist piece of pine or oak will, during one hour, lose more than four times as much water per square inch from the cross-section, but only one half as much from the tangential as from the radial section. In a long timber, where the ends or cross-sections form but a small part of the drying surface, this difference is not so evident. Nevertheless, the ends dry and shrink first, and being opposed in this shrinkage by the more moist adjoining parts, they check, the cracks largely disappearing as seasoning progresses.

High temperatures are very effective in evaporating the water from wood, no matter how humid the air, and a fresh piece of sapwood may lose weight in boiling water, and can be dried to quite an extent in hot steam.

In drying chemicals or fabrics, all that is required is to provide heat enough to vaporize the moisture and circulation enough to carry off the vapor thus secured, and the quickest and most economical means to these ends may be used. While on the other hand, in drying wood, whether in the form of standard stock or the finished product, the application of the requisite heat and circulation must be carefully regulated throughout the entire process, or warping and checking are almost certain to result. Moreover, wood of different shapes and thicknesses is very differently effected by the same treatment.

Finally, the tissues composing the wood, which vary in form and physical properties, and which cross each other in regular directions, exert their own peculiar influences upon its behavior during drying.

With our native woods, for instance, summer-wood and spring-wood show distinct tendencies in drying, and the same is true in a less degree of heartwood, as contrasted with sapwood. Or, again, p.r.o.nounced medullary rays further complicate the drying problem.

Physical Properties that influence Drying

The princ.i.p.al properties which render the drying of wood peculiarly difficult are: (1) The irregular shrinkage; (2) the different ways in which water is contained; (3) the manner in which moisture transfuses through the wood from the center to the surface; (4) the plasticity of the wood substance while moist and hot; (5) the changes which take place in the hygroscopic and chemical nature of the surface; and (6) the difference produced in the total shrinkage by different rates of drying.

The shrinkage is unequal in different directions and in different portions of the same piece. It is greatest in the circ.u.mferential direction of the tree, being generally twice as great in this direction as in the radial direction. In the longitudinal direction, for most woods, it is almost negligible, being from 20 to over 100 times as great circ.u.mferentially as longitudinally.

There is a great variation in different species in this respect.

Consequently, it follows from necessity that large internal strains are set up when the wood shrinks, and were it not for its plasticity it would rupture. There is an enormous difference in the total amount of shrinkage of different species of wood, varying from a shrinkage of only 7 per cent in volume, based on the green dimensions, in the case of some of the cedars to nearly 50 per cent in the case of some species of eucalyptus.

When the free water in the capillary s.p.a.ces of the wood fibre is evaporated it follows the laws of evaporation from capillary s.p.a.ces, except that the pa.s.sages are not all free pa.s.sages, and much of the water has to pa.s.s out by a process of transfusion through the moist cell walls. These cell walls in the green wood completely surround the cell cavities so that there are no openings large enough to offer a pa.s.sage to water or air.

The well-known ”pits” in the cell walls extend through the secondary thickening only, and not through the primary walls. This statement applies to the tracheids and parenchyma cells in the conifer (gymnosperms), and to the tracheids, parenchyma cells, and the wood fibres in the broad-leaved trees (angiosperms); the vessels in the latter, however, form open pa.s.sages except when clogged by ingrowth called tyloses, and the resin ca.n.a.ls in the former sometimes form occasional openings.

By heating the wood above the boiling point, corresponding to the external pressure, the free water pa.s.ses through the cell walls more readily.

To remove the moisture from the wood substance requires heat in addition to the latent heat of evaporation, because the molecules of moisture are so intimately a.s.sociated with the molecules, minute particles composing the wood, that energy is required to separate them therefrom.

Carefully conducted experiments show this to be from 16.6 to 19.6 calories per grain of dry wood in the case of beech, long-leaf pine, and sugar maple.

The difficulty imposed in drying, however, is not so much the additional heat required as it is in the rate at which the water transfuses through the solid wood.

SECTION VIII

ADVANTAGES IN SEASONING

Three most important advantages of seasoning have already been made apparent:

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 s.h.i.+pped 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.