Chapter VII: Introduction (2)

The insect injuries to forest products may be classed according to the stage of manufacture of the material. Thus round timber with the bark on, such as poles, posts, mine props, and sawlogs, is subject to serious damage by the same class of insects as those mentioned above, particularly by the round-headed borers, timber worms, and ambrosia beetles. Manufactured unseasoned products are subject to damage from ambrosia beetles and other wood borers. Seasoned hardwood lumber of all kinds, rough handles, wagon stock, etc., made partially or entirely of sapwood, are often reduced in value from 10 to 90 per cent by a class of insects known as powder-post beetles. Finished hardwood products such as handles, wagon, carriage and machinery stock, especially if ash or hickory, are often destroyed by the powder-post beetles. Construction timbers in buildings, bridges and trestles, cross-ties, poles, mine props, fence posts, etc., are sometimes seriously injured by wood-boring larvæ, termites, black ants, carpenter bees, and powder-post beetles, and sometimes reduced in value from 10 to 100 per cent. In tropical countries termites are a very serious pest in this respect.

MARINE WOOD-BORER INJURIES

Vast amounts of timber used for piles in wharves and other marine structures are constantly being destroyed or seriously injured by marine borers. Almost invariably they are confined to salt water, and all the woods commonly used for piling are subject to their attacks. There are two genera of mollusks, _Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria, Chelura_, and _Sphoeroma_, that do serious damage in many places along both the Atlantic and Pacific coasts.

These mollusks, which are popularly known as "shipworms," are much alike in structure and mode of life. They attack the exposed surface of the wood and immediately begin to bore. The tunnels, often as large as a lead pencil, extend usually in a longitudinal direction and follow a very irregular, tangled course. Hard woods are apparently penetrated as readily as soft woods, though in the same timber the softer parts are preferred. The food consists of infusoria and is not obtained from the wood substance. The sole object of boring into the wood is to obtain shelter.

Although shipworms can live in cold water they thrive best and are most destructive in warm water. The length of time required to destroy an average barked, unprotected pine pile on the Atlantic coast south from Chesapeake Bay and along the entire Pacific coast varies from but one to three years.

Of the crustacean borers, _Limnoria_, or the "wood louse," is the only one of great importance, although _Sphoeroma_ is reported destructive in places. _Limnoria_ is about the size of a grain of rice and tunnels into the wood for both food and shelter. The galleries extend inward radially, side by side, in countless numbers, to the depth of about one-half inch. The thin wood partitions remaining are destroyed by wave action, so that a fresh surface is exposed to attack. Both hard and soft woods are damaged, but the rate is faster in the soft woods or softer portions of a wood.

Timbers seriously attacked by marine borers are badly weakened or completely destroyed. If the original strength of the material is to be preserved it is necessary to protect the wood from the borers. This is sometimes accomplished by proper injection of creosote oil, and more or less successfully by the use of various kinds of external coatings.[38] No treatment, however, has proved entirely satisfactory.

[Footnote 38: See Smith, C. Stowell: Preservation of piling against marine wood borers. Cir. 128, U.S. Forest Service, 1908, pp. 15.]

FUNGOUS INJURIES[39]

[Footnote 39: See Von Schrenck, H.: The decay of timber and methods of preventing it. Bul. 14, U.S. Bu. Plant Industry, Washington, D.C., 1902. Also Buls. 32, 114, 214, 266.

Meineoke, E.P.: Forest tree diseases common in California and Nevada, U.S. Forest Service, Washington, D.C., 1914.

Hartig, R.: The diseases of trees. London and New York, 1894.]

Fungi are responsible for almost all decay of wood. So far as known, all decay is produced by living organisms, either fungi or bacteria. Some species attack living trees, sometimes killing them, or making them hollow, or in the case of pecky cypress and incense cedar filling the wood with galleries like those of boring insects. A much larger variety work only in felled or dead wood, even after it is placed in buildings or manufactured articles. In any case the process of destruction is the same. The mycelial threads penetrate the walls of the cells in search of food, which they find either in the cell contents (starches, sugars, etc.), or in the cell wall itself. The breaking down of the cell walls through the chemical action of so-called "enzymes" secreted by the fungi follows, and the eventual product is a rotten, moist substance crumbling readily under the slightest pressure. Some species remove the ligneous matter and leave almost pure cellulose, which is white, like cotton; others dissolve the cellulose, leaving a brittle, dark brown mass of ligno-cellulose. Fungi (such as the bluing fungus) which merely stain wood usually do not affect its mechanical properties unless the attacks are excessive.

It is evident, then, that the action of rot-causing fungi is to decrease the strength of wood, rendering it unsound, brittle, and dangerous to use. The most dangerous kinds are the so-called "dry-rot" fungi which work in many kinds of lumber after it is placed in the buildings. They are particularly to be dreaded because unseen, working as they do within the walls or inside of casings. Several serious wrecks of large buildings have been attributed to this cause. It is stated[40] that in the three years (1911-1913) more than $100,000 was required to repair damage due to dry rot.

[Footnote 40: Dry rot in factory timbers, by Inspection Dept. Associated Factory Mutual Fire Insurance Cos., 31 Milk Street, Boston, 1913.]

Dry rot develops best at 75°F. and is said to be killed by a temperature of 110°F.[41] Fully 70 per cent humidity is necessary in the air in which a timber is surrounded for the growth of this fungus, and probably the wood must be quite near its fibre saturation condition. Nevertheless _Merulius lacrymans_ (one of the most important species) has been found to live four years and eight months in a dry condition.[42] Thorough kiln-drying will kill this fungus, but will not prevent its redevelopment. Antiseptic treatment, such as creosoting, is the best prevention.

[Footnote 41: Falck, Richard: Die Meruliusfaüle des Bauholzes, Hausschwammforschungen, 6. Heft., Jena, 1912.]

[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]

All fungi require moisture and air[43] for their growth. Deprived of either of these the fungus dies or ceases to develop. Just what degree of moisture in wood is necessary for the "dry-rot" fungus has not been determined, but it is evidently considerably above that of thoroughly air-dry timber, probably more than 15 per cent moisture. Hence the importance of free circulation of air about all timbers in a building.

[Footnote 43: A culture of fungus placed in a glass jar and the air pumped out ceases to grow, but will start again as soon as oxygen is admitted.]

Warmth is also conducive to the growth of fungi, the most favorable temperature being about 90°F. They cannot grow in extreme cold, although no degree of cold such as occurs naturally will kill them. On the other hand, high temperature will kill them, but the spores may survive even the boiling temperature. Mould fungus has been observed to develop rapidly at 130°F. in a dry kiln in moist air, a condition under which an animal cannot live more than a few minutes. This fungus was killed, however, at about 140° or 145°F.[44]

[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in Berkeley, U.S. Forest Service.]

The fungus (_Endothia parasitica_ And.) which causes the chestnut blight kills the trees by girdling them and has no direct effect upon the wood save possibly the four or five growth rings of the sapwood.[45]

[Footnote 45: See Anderson, Paul J.: The morphology and life history of the chestnut blight fungus. Bul. No. 7, Penna. Chestnut Tree Blight Com., Harrisburg, 1914, p. 17.]

PARASITIC PLANT INJURIES.[46]

[Footnote 46: See York, Harlan H.: The anatomy and some of the biological aspects of the "American mistletoe." Bul. 120, Sci. Ser. No. 13, Univ. of Texas, Austin, 1909.

Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166, U.S. Bu. Plant Ind., Washington, 1910.

Meinecke, E.P.: Forest tree diseases common in California and Nevada. U.S. Forest Service, Washington, 1914, pp. 54-58.]

The most common of the higher parasitic plants damaging timber trees are mistletoes. Many species of deciduous trees are attacked by the common mistletoe (_Phoradendron flavescens_). It is very prevalent in the South and Southwest and when present in sufficient quantity does considerable damage. There is also a considerable number of smaller mistletoes belonging to the genus _Razoumofskya (Arceuthobium)_ which are widely distributed throughout the country, and several of them are common on coniferous trees in the Rocky Mountains and along the Pacific coast.

One effect of the common mistletoe is the formation of large swellings or tumors. Often the entire tree may become stunted or distorted. The western mistletoe is most common on the branches, where it produces "witches' broom." It frequently attacks the trunk as well, and boards cut from such trees are filled with long, radial holes which seriously damage or destroy the value of the timber affected.

LOCALITY OF GROWTH

The data available regarding the effect of the locality of growth upon the properties of wood are not sufficient to warrant definite conclusions. The subject has, however, been kept in mind in many of the U.S. Forest Service timber tests and the following quotations are assembled from various reports:

"In both the Cuban and longleaf pine the locality where grown appears to have but little influence on weight or strength, and there is no reason to believe that the longleaf pine from one State is better than that from any other, since such variations as are claimed can be found on any 40-acre lot of timber in any State. But with loblolly and still more with shortleaf this seems not to be the case. Being widely distributed over many localities different in soil and climate, the growth of the shortleaf pine seems materially influenced by location. The wood from the southern coast and gulf region and even Arkansas is generally heavier than the wood from localities farther north. Very light and fine-grained wood is seldom met near the southern limit of the range, while it is almost the rule in Missouri, where forms resembling the Norway pine are by no means rare. The loblolly, occupying both wet and dry soils, varies accordingly." Cir. No. 12, p. 6.

" ... It is clear that as all localities have their heavy and their light timber, so they all share in strong and weak, hard and soft material, and the difference in quality of material is evidently far more a matter of individual variation than of soil or climate." _Ibid._, p.22

"A representative committee of the Carriage Builders' Association had publicly declared that this important industry could not depend upon the supplies of southern timber, as the oak grown in the South lacked the necessary qualities demanded in carriage construction. Without experiment this statement could be little better than a guess, and was doubly unwarranted, since it condemned an enormous amount of material, and one produced under a great variety of conditions and by at least a dozen species of trees, involving, therefore, a complexity of problems difficult enough for the careful investigator, and entirely beyond the few unsystematic observations of the members of a committee on a flying trip through one of the greatest timber regions of the world.

"A number of samples were at once collected (part of them supplied by the carriage builders' committee), and the fallacy of the broad statement mentioned was fully demonstrated by a short series of tests and a more extensive study into structure and weight of these materials. From these tests it appears that pieces of white oak from Arkansas excelled well-selected pieces from Connecticut, both in stiffness and endwise compression (the two most important forms of resistance)." Report upon the forestry investigations of the U.S.D.A. 1877-1898, p. 331. See also Rep. of Div. of For., 1890, p. 209.

"In some regions there are many small, stunted hickories, which most users will not touch. They have narrow sap, are likely to be birdpecked, and show very slow growth. Yet five of these trees from a steep, dry south slope in West Virginia had an average strength fully equal to that of the pignut from the better situation, and were superior in toughness, the work to maximum load being 36.8 as against 31.2 for pignut. The trees had about twice as many rings per inch as others from better situations.

"This, however, is not very significant, as trees of the same species, age, and size, growing side by side under the same conditions of soil and situation, show great variation in their technical value. It is hard to account for this difference, but it seems that trees growing in wet or moist situations are rather inferior to those growing on fresher soil; also, it is claimed by many hickory users that the wood from limestone soils is superior to that from sandy soils.

"One of the moot questions among hickory men is the relative value of northern and southern hickory. The impression prevails that southern hickory is more porous and brash than hickory from the north. The tests ... indicate that southern hickory is as tough and strong as northern hickory of the same age. But the southern hickories have a greater tendency to be shaky, and this results in much waste. In trees from southern river bottoms the loss through shakes and grub-holes in many cases amounts to as much as 50 per cent.

"It is clear, therefore, that the difference in northern and southern hickory is not due to geographic location, but rather to the character of timber that is being cut. Nearly all of that from southern river bottoms and from the Cumberland Mountains is from large, old-growth trees; that from the north is from younger trees which are grown under more favorable conditions, and it is due simply to the greater age of the southern trees that hickory from that region is lighter and more brash than that from the north." Bul. 80, pp. 52-55.

SEASON OF CUTTING

It is generally believed that winter-felled timber has decided advantages over that cut at other seasons of the year, and to that cause alone are frequently ascribed much greater durability, less liability to check and split, better color, and even increased strength and toughness. The conclusion from the various experiments made on the subject is that while the time of felling may, and often does, affect the properties of wood, such result is due to the weather conditions rather than to the condition of the wood.

There are two phases of this question. One is concerned with the physiological changes which might take place during the year in the wood of a living tree. The other deals with the purely physical results due to the weather, as differences in temperature, humidity, moisture, and other features to be mentioned later.

Those who adhere to the first view maintain that wood cut in summer is quite different in composition from that cut in winter. One opinion is that in summer the "sap is up," while in winter it is "down," consequently winter-felled timber is drier. A variation of this belief is that in summer the sap contains certain chemicals which affect the properties of wood and does not contain them in winter. Again it is sometimes asserted that wood is actually denser in winter than in summer, as part of the wood substance is dissolved out in the spring and used for plant food, being restored in the fall.

It is obvious that such views could apply only to sapwood, since it alone is in living condition at the time of cutting. Heartwood is dead wood and has almost no function in the existence of the tree other than the purely mechanical one of support. Heartwood does undergo changes, but they are gradual and almost entirely independent of the seasons.

Sapwood might reasonably be expected to respond to seasonal changes, and to some extent it does. Just beneath the bark there is a thin layer of cells which during the growing season have not attained their greatest density. With the exception of this one annual ring, or portion of one, the density of the wood substance of the sapwood is nearly the same the year round. Slight variations may occur due to impregnation with sugar and starch in the winter and its dissolution in the growing season. The time of cutting can have no material effect on the inherent strength and other mechanical properties of wood except in the outermost annual ring of growth.

The popular belief that sap is up in the spring and summer and is down in the winter has not been substantiated by experiment. There are seasonal differences in the composition of sap, but so far as the amount of sap in a tree is concerned there is fully as much, if not more, during the winter than in summer. Winter-cut wood is not drier, to begin with, than summer-felled--in reality, it is likely to be wetter.[47]

[Footnote 47: See Record, S.J.: Sap in relation to the properties of wood. Proc. Am. Wood Preservers' Assn., Baltimore, Md., 1913, pp. 160-166.

Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am. Ry. Eng. Assn., 1913, p. 214.]

The important consideration in regard to this question is the series of circumstances attending the handling of the timber after it is felled. Wood dries more rapidly in summer than in winter, not because there is less moisture at one time than another, but because of the higher temperature in summer. This greater heat is often accompanied by low humidity, and conditions are favorable for the rapid removal of moisture from the exposed portions of wood. Wood dries by evaporation, and other things being equal, this will proceed much faster in hot weather than in cold.

It is a matter of common observation that when wood dries it shrinks, and if shrinkage is not uniform in all directions the material pulls apart, causing season checks. (See Fig. 27.) If evaporation proceeds more rapidly on the outside than inside, the greater shrinkage of the outer portions is bound to result in many checks, the number and size increasing with the degree of inequality of drying.

In cold weather, drying proceeds slowly but uniformly, thus allowing the wood elements to adjust themselves with the least amount of rupturing. In summer, drying proceeds rapidly and irregularly, so that material seasoned at that time is more likely to split and check.

There is less danger of sap rot when trees are felled in winter because the fungus does not grow in the very cold weather, and the lumber has a chance to season to below the danger point before the fungus gets a chance to attack it. If the logs in each case could be cut into lumber immediately after felling and given exactly the same treatment, for example, kiln-dried, no difference due to the season of cutting would be noted.

WATER CONTENT[48]

[Footnote 48: See Tiemann, H.D.: Effect of moisture upon the strength and stiffness of wood. Bul. 70, U.S. Forest Service, Washington, D.C., 1906; also Cir. 108, 1907.]

Water occurs in living wood in three conditions, namely: (1) in the cell walls, (2) in the protoplasmic contents of the cells, and (3) as free water in the cell cavities and spaces. In heartwood it occurs only in the first and last forms. Wood that is thoroughly air-dried retains from 8 to 16 per cent of water in the cell walls, and none, or practically none, in the other forms. Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry.

The general effect of the water content upon the wood substance is to render it softer and more pliable. A similar effect of common observation is in the softening action of water on rawhide, paper, or cloth. Within certain limits the greater the water content the greater its softening effect.

Drying produces a decided increase in the strength of wood, particularly in small specimens. An extreme example is the case of a completely dry spruce block two inches in section, which will sustain a permanent load four times as great as that which a green block of the same size will support.

The greatest increase due to drying is in the ultimate crushing strength, and strength at elastic limit in endwise compression; these are followed by the modulus of rupture, and stress at elastic limit in cross-bending, while the modulus of elasticity is least affected. These ratios are shown in Table XV, but it is to be noted that they apply only to wood in a much drier condition than is used in practice. For air-dry wood the ratios are considerably lower, particularly in the case of the ultimate strength and the elastic limit. Stiffness (within the elastic limit), while following a similar law, is less affected. In the case of shear parallel to the grain, the general effect of drying is to increase the strength, but this is often offset by small splits and checks caused by shrinkage.

|----------------------------------------------------------------------| | TABLE XV | |----------------------------------------------------------------------| | EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD, SHOWN IN | | RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM | | THE GREEN CONDITION TO KILN-DRY (3.5 PER CENT) | | (Forest Service Bul. 70, p. 89) | |----------------------------------------------------------------------| | KIND OF STRENGTH | Longleaf | Spruce | Chestnut | | | pine | | | |----------------------------+-------------+-------------+-------------| | | (1) (2) | (1) (2) | (1) (2) | | | | | | | Crushing strength parallel | | | | | to grain | 2.89 2.60 | 3.71 3.41 | 2.83 2.55 | | Elastic limit in | | | | | compression | | | | | parallel to grain | 2.60 2.34 | 3.80 3.49 | 2.40 2.26 | | Modulus of rupture in | | | | | bending | 2.50 2.20 | 2.81 2.50 | 2.09 1.82 | | Stress at elastic limit in | | | | | bending | 2.90 2.55 | 2.90 2.58 | 2.30 2.00 | | Crushing strength at right | | | | | angles to grain | | 2.58 2.48 | | | Shearing strength parallel | | | | | to grain | 2.01 1.91 | 2.03 1.95 | 1.55 1.47 | | Modulus of elasticity in | | | | | compression parallel to | | | | | grain | 1.63 1.47 | 2.26 2.08 | 1.43 1.29 | | Modulus of elasticity in | | | | | bending | 1.59 1.35 | 1.43 1.23 | 1.44 1.21 | |----------------------------------------------------------------------| | NOTE.--The figures in the first column show the relative increase in | | strength between a green specimen and a kiln-dry specimen of equal | | size. The figures in the second column show the relative increase of | | strength of the same block after being dried from a green condition | | to 3.5 per cent moisture, correction having been made for shrinkage. | | That is, in the first column the strength values per actual unit of | | area are used; in the second the values per unit of area of green | | wood which shrinks to smaller size when dried. | | | | See also Cir. 108, Fig. 1, p. 8. | |----------------------------------------------------------------------|

The moisture content has a decided bearing also upon the manner in which wood fails. In compression tests on very dry specimens the entire piece splits suddenly into pieces before any buckling takes place (see Fig. 9.), while with wet material the block gives way gradually, due to the buckling or bending of the walls of the fibres along one or more shearing planes. (See Fig. 14.) In bending tests on wet beams, first failure occurs by compression on top of the beam, gradually extending downward toward the neutral axis. Finally the beam ruptures at the bottom. In the case of very dry beams the failure is usually by splitting or tension on the under side (see Fig. 17.), without compression on the upper, and is often sudden and without warning, and even while the load is still increasing. The effect varies somewhat with different species, chestnut, for example, becoming more brittle upon drying than do ash, hemlock, and longleaf pine. The tensile strength of wood is least affected by drying, as a rule.

In drying wood no increase in strength results until the free water is evaporated and the cell walls begin to dry[49]. This critical point has been called the _fibre-saturation point_. (See Fig. 24.) Conversely, after the cell walls are saturated with water, any increase in the amount of water absorbed merely fills the cavities and intercellular spaces, and has no effect on the mechanical properties. Hence, soaking green wood does not lessen its strength unless the water is heated, whereupon a decided weakening results.

[Footnote 49: The wood of _Eucalyptus globulus_ (blue gum) appears to be an exception to this rule. Tiemann says: "The wood of blue gum begins to shrink immediately from the green condition, even at 70 to 90 per cent moisture content, instead of from 30 or 25 per cent as in other species of hardwoods." Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913, p. 313.]

The strengthening effects of drying, while very marked in the case of small pieces, may be fully offset in structural timbers by inherent weakening effects due to the splitting apart of the wood elements as a result of irregular shrinkage, and in some cases also to the slitting of the cell walls (see Fig. 25). Consequently with large timbers in commercial use it is unsafe to count upon any greater strength, even after seasoning, than that of the green or fresh condition.

In green wood the cells are all intimately joined together and are at their natural or normal size when saturated with water. The cell walls may be considered as made up of little particles with water between them. When wood is dried the films of water between the particles become thinner and thinner until almost entirely gone. As a result the cell walls grow thinner with loss of moisture,--in other words, the cell shrinks.

It is at once evident that if drying does not take place uniformly throughout an entire piece of timber, the shrinkage as a whole cannot be uniform. The process of drying is from the outside inward, and if the loss of moisture at the surface is met by a steady capillary current of water from the inside, the shrinkage, so far as the degree of moisture affected it, would be uniform. In the best type of dry kilns this condition is approximated by first heating the wood thoroughly in a moist atmosphere before allowing drying to begin.

In air-seasoning and in ordinary dry kilns this condition too often is not attained, and the result is that a dry shell is formed which encloses a moist interior. (See Fig. 26.) Subsequent drying out of the inner portion is rendered more difficult by this "case-hardened" condition. As the outer part dries it is prevented from shrinking by the wet interior, which is still at its greatest volume. This outer portion must either check open or the fibres become strained in tension. If this outer shell dries while the fibres are thus strained they become "set" in this condition, and are no longer in tension. Later when the inner part dries, it tends to shrink away from the hardened outer shell, so that the inner fibres are now strained in tension and the outer fibres are in compression. If the stress exceeds the cohesion, numerous cracks open up, producing a "honey-combed" condition, or "hollow-horning," as it is called. If such a case-hardened stick of wood be resawed, the two halves will cup from the internal tension and external compression, with the concave surface inward.

For a given surface area the loss of water from wood is always greater from the ends than from the sides, due to the fact that the vessels and other water-carriers are cut across, allowing ready entrance of drying air and outlet for the water vapor. Water does not flow out of boards and timbers of its own accord, but must be evaporated, though it may be forced out of very sappy specimens by heat. In drying a log or pole with the bark on, most of the water must be evaporated through the ends, but in the case of peeled timbers and sawn boards the loss is greatest from the surface because the area exposed is so much greater.

The more rapid drying of the ends causes local shrinkage, and were the material sufficiently plastic the ends would become bluntly tapering. The rigidity of the wood substance prevents this and the fibres are split apart. Later, as the remainder of the stick dries many of the checks will come together, though some of the largest will remain and even increase in size as the drying proceeds. (See Fig. 27.)

A wood cell shrinks very little lengthwise. A dry wood cell is, therefore, practically of the same length as it was in a green or saturated condition, but is smaller in cross section, has thinner walls, and a larger cavity. It is at once evident that this fact makes shrinkage more irregular, for wherever cells cross each other at a decided angle they will tend to pull apart upon drying. This occurs wherever pith rays and wood fibres meet. A considerable portion of every wood is made up of these rays, which for the most part have their cells lying in a radial direction instead of longitudinally. (See Frontispiece.) In pine, over 15,000 of these occur on a square inch of a tangential section, and even in oak the very large rays which are readily visible to the eye as flakes on quarter-sawed material represent scarcely one per cent of the number which the microscope reveals.

A pith ray shrinks in height and width, that is, vertically and tangentially as applied to the position in a standing tree, but very little in length or radially. The other elements of the wood shrink radially and tangentially, but almost none lengthwise or vertically as applied to the tree. Here, then, we find the shrinkage of the rays tending to shorten a stick of wood, while the other cells resist it, and the tendency of a stick to get smaller in circumference is resisted by the endwise reaction or thrust of the rays. Only in a tangential direction, or around the stick in direction of the annual rings of growth, do the two forces coincide. Another factor to the same end is that the denser bands of late wood are continuous in a tangential direction, while radially they are separated by alternate zones of less dense early wood. Consequently the shrinkage along the rings (tangential) is fully twice as much as toward the centre (radial). (See Table XIV.) This explains why some cracks open more and more as drying advances. (See Fig. 27.)

Although actual shrinkage in length is small, nevertheless the tendency of the rays to shorten a stick produces strains which are responsible for some of the splitting open of ties, posts, and sawed timbers with box heart. At the very centre of a tree the wood is light and weak, while farther out it becomes denser and stronger. Longitudinal shrinkage is accordingly least at the centre and greater toward the outside, tending to become greatest in the sapwood. When a round or a box-heart timber dries fast it splits radially, and as drying continues the cleft widens partly on account of the greater tangential shrinkage and also because the greater contraction of the outer fibres warps the sections apart. If a small hardwood stem is split while green for a short distance at the end and placed where it can dry out rapidly, the sections will become bow-shaped with the concave sides out. These various facts, taken together, explain why, for example, an oak tie, pole, or log may split open its entire length if drying proceeds rapidly and far enough. Initial stresses in the living trees produce a similar effect when the log is sawn into boards. This is especially so in _Eucalyptus globulus_ and to a less extent with any rapidly grown wood.

The use of S-shaped thin steel clamps to prevent large checks and splits is now a common practice in this country with crossties and poles as it has been for a long time in European countries. These devices are driven into the butts of the timbers so as to cross incipient checks and prevent their widening. In place of the regular S-hook another of crimped iron has been devised. (See Fig. 28.) Thin straps of iron with one tapered edge are run between intermeshing cogs and crimped, after which they may be cut off any length desired. The time for driving S-irons of either form is when the cracks first appear.

The tendency of logs to split emphasizes the importance of converting them into planks or timbers while in a green condition. Otherwise the presence of large checks may render much lumber worthless which might have been cut out in good condition. The loss would not be so great if logs were perfectly straight-grained, but this is seldom the case, most trees growing more or less spirally or irregularly. Large pieces crack more than smaller ones, quartered lumber less than that sawed through and through, thin pieces, especially veneers, less than thicker boards.

In order to prevent cracks at the ends of boards, small straps of wood may be nailed on them or they may be painted. This method is usually considered too expensive, except in the case of valuable material. Squares used for shuttles, furniture, gun-stocks, and tool handles should always be protected at the ends. One of the best means is to dip them into melted paraffine, which seals the ends and prevents loss of moisture there. Another method is to glue paper on the ends. In some cases abroad paper is glued on to all the surfaces of valuable exotic balks. Other substances sometimes employed for the purpose of sealing the wood are grease, carbolineum, wax, clay, petroleum, linseed oil, tar, and soluble glass. In place of solid beams, built-up material is often preferable, as the disastrous results of season checks are thereby largely overcome or minimized.

TEMPERATURE

The effect of temperature on wood depends very largely upon the moisture content of the wood and the surrounding medium. If absolutely dry wood is heated in absolutely dry air the wood expands. The extent of this expansion is denoted by a coefficient corresponding to the increase in length or other dimensions for each degree rise in temperature divided by the original length or other dimension of the specimen. The coefficient of linear expansion of oak has been found to be .00000492; radial expansion, .0000544, or about eleven times the longitudinal. Spruce expands less than oak, the ratio of radial to longitudinal expansion being about six to one. Metals and glass expand equally in all directions, since they are homogeneous substances, while wood is a complicated structure. The coefficient of expansion of iron is .0000285, or nearly six times the coefficient of linear expansion of oak and seven times that of spruce[50].

[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev. ed.), p. 75.]

Under ordinary conditions wood contains more or less moisture, so that the application of heat has a drying effect which is accompanied by shrinkage. This shrinkage completely obscures the expansion due to the heating.

Experiments made at the Yale Forest School revealed the effect of temperature on the crushing strength of wet wood. In the case of wet chestnut wood the strength decreases 0.42 per cent for each degree the water is heated above 60° F.; in the case of spruce the decrease is 0.32 per cent.

The effects of high temperature on wet wood are very marked. Boiling produces a condition of great pliability, especially in the case of hardwoods. If wood in this condition is bent and allowed to dry, it rigidly retains the shape of the bend, though its strength may be somewhat reduced. Except in the case of very dry wood the effect of cold is to increase the strength and stiffness of wood. The freezing of any free water in the pores of the wood will augment these conditions.

The effect of steaming upon the strength of cross-ties was investigated by the U.S. Forest Service in 1904. The conclusions were summarized as follows:

"(1) The steam at pressure up to 40 pounds applied for 4 hours, or at a pressure of 20 pounds up to 20 hours, increases the weight of ties. At 40 pounds' pressure applied for 4 hours and at 20 pounds for 5 hours the wood began to be scorched.

"(2) The steamed and saturated wood, when tested immediately after treatment, exhibited weaknesses in proportion to the pressure and duration of steaming. (See Table XVI.) If allowed to air-dry subsequently the specimens regained the greater part of their strength, provided the pressure and duration had not exceeded those cited under (1). Subsequent immersion in water of the steamed wood and dried specimens showed that they were weaker than natural wood similarly dried and resoaked."[51]

[Footnote 51: Cir. 39. Experiments on the strength of treated timber, p. 18.]

|------------------------------------------------------------------------------------------| | TABLE XVI | |------------------------------------------------------------------------------------------| | EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE | | (Forest Service, Cir. 39) | |------------------------------------------------------------------------------------------| | | Cylinder conditions | Strength | | |---------------------------------+--------------------------------------------| | | Steaming | Static | Impact | | | |---------------------------------+---------------------+----------| Average | | Treatment | | | | Bending | Compres- | Height | of the | | | | | | modulus | sion | of drop | three | | | Period | Pressure | Temperature | of | parallel | causing | strengths | | | | | | rupture | to grain | complete | | | | | | | | | failure | | |-----------+--------+----------+-------------+----------+----------+----------+-----------| | | | Lbs. per | | Per cent | Per cent | Per cent | Per cent | | | Hrs. | sq. inch | °F. | | | | | | | | | | Untreated wood = 100% | | | | | | | | | | | Steam, | 4 | | 230[a] | 91.3 | 79.1 | 96.4 | 88.9 | | at | 4 | 10 | 238 | 78.2 | 93.7 | 93.3 | 88.4 | | various | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 80.8 | | pressures | 4 | 30 | 269 | 80.4 | 78.4 | 89.8 | 82.9 | | | 4 | 40 | 283 | 78.1 | 74.4 | 74.0 | 75.5 | | | 4 | 50 | 292 | 75.8 | 71.5 | 63.9 | 70.4 | | | 4 | 100 | 337 | 41.4 | 65.0 | 55.2 | 53.9 | |-----------+--------+----------+-------------+----------+----------+----------+-----------| | Steam, | 1 | 20 | 257 | 100.6 | 98.6 | 86.7 | 95.3 | | for | 2 | 20 | 267 | 88.4 | 93.0 | 107.0 | 96.1 | | various | 3 | 20 | 260 | 90.0 | 93.6 | 84.1 | 89.2 | | periods | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 86.3 | | | 5 | 20 | 253 | 85.0 | 78.1 | 84.2 | 82.4 | | | 6 | 20 | 242 | 95.2 | 89.8 | 76.0 | 87.0 | | | 10 | 20 | 255 | 73.7 | 82.0 | 76.0 | 77.2 | | | 20 | 20 | 258 | 67.5 | 65.0 | 99.0 | 77.2 | |------------------------------------------------------------------------------------------| | [Footnote a: It will be noted that the temperature was 230°. This is the maximum | | temperature by the maximum-temperature recording thermometer, and is due to the handling | | of the exhaust valve. The average temperature was that of exhaust steam.] | |------------------------------------------------------------------------------------------|

"(3) A high degree of steaming is injurious to wood in strength and spike-holding power. The degree of steaming at which pronounced harm results will depend upon the quality of the wood and its degree of seasoning, and upon the pressure (temperature) of steam and the duration of its application. For loblolly pine the limit of safety is certainly 30 pounds for 4 hours, or 20 pounds for 6 hours."[52]

[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table 5.]

Experiments made at the Yale Forest School showed that steaming above 30 pounds' gauge pressure reduces the strength of wood permanently while wet from 25 to 75 per cent.

PRESERVATIVES

The exact effects of chemical impregnation upon the mechanical properties of wood have not been fully determined, though they have been the subject of considerable investigation.[53] More depends upon the method of treatment than upon the preservatives used. Thus preliminary steaming at too high pressure or for too long a period will materially weaken the wood, (See TEMPERATURE, above.)

[Footnote 53: Hatt, W. K.: Experiments on the strength of treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]

The presence of zinc chloride does not weaken wood under static loading, although the indications are that the wood becomes brittle under impact. If the solution is too strong it will decompose the wood.

Soaking in creosote oil causes wood to swell, and accordingly decreases the strength to some extent, but not nearly so much so as soaking in water.[54]

[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p. 7.]

Soaking in kerosene seems to have no significant weakening effect.[55]

[Footnote 55: Tiemann, H.D.: Effect of moisture upon the strength and stiffness of wood. Bul. 70, U. S. Forest Service, 1907, pp. 122-123, tables 43-44.]