Movement And Shrinkage

Sound jointing techniques upon which the stiffness, appearance and general usefulness of furniture depend must take into account the natural movement of wood, and its propensity to shrink, swell and warp under fluctuating

Oven Determine Moisture Content
12 Drying-oven for testing moisture content
Timbers Dead And Live Knots

13 Measuring moisture content atmospheric conditions. It is, therefore, vitally important that the woodworker should understand exactly what kind of structural material it is which ceaselessly moves in sympathy with its surroundings, and what precautions he can usefully adopt to minimize that movement. Surface protection with paint, polish or even metal coatings may conceivably delay the natural process but can never entirely arrest it; and it is not generally realized that while age itself may exercise a certain restraining influence, no matter how old the wood may be, the innate tendencies will continue to persist throughout its effective life.

Washboard Defect Woodtimberlumber

Shrinkage

A full understanding of the reasons for the shrinkage of wood, the probable extent of the movement and the direction it is likely to take (wood is not a homogeneous solid and the movement is not equal in all directions) is essential if furniture is to be soundly constructed, for it is impossible to lock wood fibre permanently against its natural inclinations, and its strength is such that it will eventually overcome every effort to confine it. Furthermore, the increasing use of artificial heating means drier atmospheres and greatly increased shrinkage values, and this equally applies no matter how old the wood is, for even antique furniture will rapidly disintegrate in over-heated surroundings. It should be pointed out, however, that heat alone is not inimical to wood: it is the degree of dryness occasioned by the heat which is the deciding factor.

The probable extent and direction of the shrinkage is best understood by visualizing the tree as a compact cylinder composed of innumerable smaller cylinders or annual growth rings fitted tightly within each other (Figure 15).

As the tree dries out so the shrinkage will take place in every direction except in the length, the circumference of the outer cylinder will grow shorter and the cylinders within will each grow progressively smaller and more tightly packed (1,2). This circumferential shrinkage along the length of the annual rings is always greater than the shrinkage between the rings, and while a precise ratio is not possible (for the figures depend upon the density of the wood species and the oil and resin content of the fibres), it is generally assumed that the amount of shrinkage between the rings will only be a little over half that along the length of the rings.

The effect of these shrinkage values is illustrated in 3 and 4. In 3 the greater amount of shrinkage in a tangentially cut plank will be across the width, as shown by the heavy arrows, with about half that amount between the rings as indicated by the smaller arrows; whereas in radial or quarter cut wood the main shrinkage will be in the thickness, which is relatively unimportant (the amount shown in the drawings is exaggerated for illustration purposes). Thus, the radial or quarter cut plank (4, 5 A) is always

Tortrix Viridana Effects Wood

TANGENTIAL

TANGENTIAL

Timbers Dead And Live Knots

15 Shrinkage of wood more stable, for not only is there less shrinkage across the width, but the pull is fairly equal from one annual ring to another, and therefore there is no tendency towards distortion. In 5B the annual rings in the tangentially cut plank are of different lengths, and the pull on the underside away from the heart will be much greater than on the side nearer the heart. A tangentially cut plank will, therefore, pull in, rounding as it dries, as shown by the arrows, while circular sections (6) will go oval, and square sections (7)

diamond shaped. Where the grain direction is irregular as in 8, then the plank may twist in several directions across the width, while a further complication in tangential timber is that when it is cut from the log the saw cut is parallel to the long axis of the tree and, therefore, diagonal to the growth rings. It must be accepted, therefore, that wood is always somewhat unpredictable in its behaviour, and only very generalized rules are possible.

Shrinkage factors

Shrinkage factors along the length can usually be ignored for they are only fractional, but both tangential and radial shrinkages can be considerable, according to the particular species. Unfortunately, the greatest degree of shrinkage takes place between the critical moisture contents of air dried wood (20 per cent) and fully conditioned wood (9 to 10 per cent). For example, English oak, which is usually classed as a medium stable timber, will shrink approximately 5/16 in (8 mm) for every 12 in (304 mm) of width across the face of a tangentially cut board, and 3/16 in (4.5 mm) for a radially cut board, if dried down from 20 to 12 per cent moisture content; while beech, which has a large shrinkage value. will shrink 3/8 in (9.5 mm) and 7/32 in (5.5 mm) respectively. There is, however, no conformity between species, and other woods will exhibit less tangential and more radial shrinkage.

Dimensional changes for some of the more commonly used types are given below. As the moisture content of wood at any given humidity rate varies with the species, the values are based on conditioning from an outdoor humidity rate of 90 per cent down to an environmental humidity of 60 per cent.

Approximate comparative movement

Moisture content

Tangential

Radial

Timber

range

shrinkage

shrinkage

class

90%-60% humidity

in

mm

in

mm

%

perft

perm

perft

perm

Afzelia

14-9.5

1/8

10.4

1/16

5.2

Very small

Abura

18-12.5

13/64

16.9

1/8

10.4

Small

Afrormosia

15-11

5/32

13.0

5/64

6.5

Small

Agba

-17-12

7/32

18.2

3/32

7.8

Small

Beech

20-12

3/8

31.2

13/64

16.9

Large

Chestnut

17.5-12.5

5/32

13.0

5/64

6.5

Small

Elm

20.5-12

9/32

23.4

3/16

15.6

Medium

Idigbo

15-11

1/8

10.4

1/16

5.2

Very small

African mahogany

20-13.5

5/32

13.0

7/64

9.1

Small

C. Amer. mahogany

19-12.5

5/32

13.0

1/8

10.4

Small

Muninga

13-10

5/64

6.5

1/16

5.2

Very small

English oak

20-12

5/l6

26.0

3/l6

15.6

Medium

Yellow pine

17-11

3/l6

15.6

7/64

9.1

Small

Ramin

20-12

3/8

31.2

3/l6

15.6

Large

Sitka spruce

19-12.5

5/32

13.0

7/64

9.1

Small

Teak

15-10

5/32

13.0

3/32

7.8

Small

Utile

22-14

3/16

15.6

11/64

14.3

Small

Distortion, i.e. warping, twisting, etc., is not dependent on shrinkage values, but generally speaking timbers whose tangential and radial shrinkages are near to each other distort very little.

CHARACTERISTICS AND DEFECTS Grain, texture and figure

While the general term grain is normally used to cover many different characteristics of wood, e.g. straight grain, coarse grain, curly grain, raised grain, etc., strictly speaking it should only denote the direction or arrangement of the wood fibres in relation to the longitudinal axis of the tree or of the converted plank, with the term texture as descriptive of the relative size and arrangement of the constituent cells, and figure denoting the ornamental markings brought about by structural characteristics.

Grain

Where the wood fibres follow or are parallel to the long axis of the tree or plank then the term Weight grain is used. Any slight deviation from the parallel is known as oblique or diagonal grain, and pronounced deviation cross grain. If the arrangement of the fibres twists about the long axis then the twist is known as spiral; while regular waves or ripples create wavy grain, and irregular curves curly grain. The term interlocked grain refers to a condition in which, for some unknown reason, the direction of the fibres regularly changes or reverses in successive growth layers, and is often known as ribbon or stripy grain (sapele, etc.). All these grain arrangements occur naturally in the tree, although diagonal grain can be caused by poor saw-milling, and spiral grain cut through and through in the normal way will show as simple diagonal grain on the face. Additionally, the term 'grain' is used in connection with methods of milling, etc.; thus a straight cut across the face of the plank will show end grain, while cuts parallel to the long axis (ripping cuts) produce long-grain, and oblique cuts short-grain. Quarter sawn timber/lumber is sometimes known as edge-grain timber, and plain or flat sawn as flat sawn timber. The terms rough grain, raised grain, smooth grain, etc. are not descriptive of the innate characteristics of the wood, but only of the finished surface.

Texture

Texture is concerned with the relative size and arrangement of the cells. Thus a wood with large open pores usually referred to as coarse grain is more correctly coarse textured; with small pores set close together which can be brought to a good finish as fine or close textured; with uniform pores showing little difference between springwood and summerwood even textured, with the reverse for uneven textured. Oak, ash and elm are coarse textured (coarse grained) woods, because there is an alternation of large open pores of the springwood with the densely packed fibres of the summerwood.

Figure

Structural characteristics, medullary rays, pronounced or irregular growth rings, variations in colour or texture, knots and abnormalities all produce the ornamental markings or 'figure' on the surface of the wood, and are of great importance to the furniture-maker. All these are innate, i.e. natural characteristics, and can be further developed or exaggerated by the methods of sawing adopted. For example, radial cuts in true quartered oak and chestnut follow the path of the medullary rays, and show the typical 'flash' or 'silver grain' to best advantage. These rays are also visible in quarter-cut beech, while sycamore will sometimes produce a magnificent flame figure, and plane a rich lacy pattern (lacewood). Again, quarter sawing of some timbers produces very straight regular grain patterns, instead of the usual contour markings of flat sawn timber where the saw cuts through the annual growth rings. If there are concentric bands of colour encircling the tree, as in Rio rosewood, Macassar ebony and other exotics, then quartering will produce boldly marked vertical stripings, and plain sawn wood large irregular blazes of predominant colour. Woods with a marked contrast between springwood and summerwood often show the annual growth rings very conspicuously if plain cut, and even more so if rotary cut into veneers, of which typical examples are the resinous softwoods, Douglas fir (Columbian pine), yellow pine, pitch-pine, etc., and the ring-porous hardwoods, ash, oak, elm, etc. Bold demarcations between sap and heartwood yield the prized laburnum oysters, the strong pattern of royal walnut, and the delicate feathering of some walnut crotches.

Some of the most beautiful figurations are obtained from irregular grain structure. Wavy and curly grain can yield quilted and swirl figure, and the very beautiful dapple and mottle in which alternate light and dark bands cross the grain direction instead of running with it. This transverse movement can occur in many woods, and has always been highly prized in sycamore for the backs of violins. A variation of the stripy figure is 'roe', in which local irregularities break up the stripe, while similar irregularities or dimples in the cambium layer produce the blister grain of Douglas fir, and smaller inward-growing dimples one type of burr or burl as in burr maple. Sometimes the crotch or fork of the

16 Grain and figure in wood

16 Grain and figure in wood

Brittleheart Wood
Australian silky oak Quilted maple
Timbers Dead And Live Knots
Satinwood African mahogany

tree produces the self-descriptive and highly prized fan, feather, swirl and curl figuring; while the buttress or root base of some trees, notably walnuts, the very bold-patterned stump figure.

Some abnormalities produce striking figure, notably burr formations caused either by fungal irritation of the cambium layer, severe pollarding or lopping, or in the case of elm, natural growth irregularities. Burrs or burls can also be caused by large numbers of small twigs which fail to develop, and yield the familiar bird's eye in maple, etc. Examples of some of these figure markings are shown in 16, but it would be impossible to include all the known varieties, for practically every species of tree is capable of yielding outstanding effects in grain. texture and colour, and there are many recorded instances of exceptional logs suitable for veneering bringing very large sums of money.

Coloration

The natural colouring agents in wood are water soluble, and will tend to leach out if repeatedly soaked or exposed to weathering for long periods. Typical colour examples are logwood, which yields a commercial water soluble fierce red dye, and fustic a khaki yellow. Generally speaking, wood is lighter in colour when freshly sawn and gradually darkens as the wood surfaces oxidize, but as the colours are not permanent they may ultimately fade. The sapwood is usually lighter than the heartwood, and this serves as the usual method of identification; but some species exhibit little if any difference, notably ash, sycamore, beech, holly, silver fir, etc.

The precise description of colour in any specimen is virtually impossible, as so many factors apply. No two trees are ever alike, and at best a wood can only be described as having general characteristics modified or accentuated by growth conditions, exposure to air, heat, sunlight, etc. In general, most woods, with the exception of jet-black ebonies and blackwood, and pure white holly, have a yellowish cast which can never be bleached out completely. It is this predominant yellow in conjunction with reds and blacks which give the innumerable shades of brown. Greens do exist, but are usually the result of abnormalities, while clear greys are the result of weathering or treatment by chemical means.

Woods which change colour under normal indoor conditions, with only occasional exposure to direct sunlight, include the following:

Those which fade - rosewood, walnut, mansonia. teak, agba, gaboon, etc.

Those which yellow - African mahogany, sapele. walnut (most species), agba, gaboon, maple, plane, sycamore, oak, etc.

Those which redden - cherry, yew, beech, kingwood. purpleheart, padouk, etc.

The lists are by no means complete, and individual specimens may not necessarily behave in the manner indicated, while all polishes tend to darken wood in time, and oil and wax pronouncedly so.

Odour

Most timbers have trace odours, some very pronounced, due to the presence of essential oils which often decide the use of the wood. Notable examples are cedar, camphorwood, etc. Some timbers are also known by their particular scent or odour, such as sandalwood, rosewood, jamwood, sneezewood, stinkwood, etc. Teak, Australian walnut and certain African woods have a highly objectionable smell when freshly worked; but most scents and odours, with the notable exception of camphor and cedar which are used as moth deterrents, rapidly fade on exposure to the air, due to the surface evaporation or drying out of the essential oils.

Durability

Hardness is no criterion of durability even though most, but not all, durable woods are essentially hard and dark in colour, due to the higher content of wood substance and the presence of antiseptic tannins and resins. A notable exception is western red cedar, which is not a true cedar but a very light softwood so impregnated with phenolic-type resins that it is almost impervious to decay.

The question of durability hardly arises in indoor domestic furniture, for all woods are sufficiently durable under controlled conditions. Where, however, continuous moist conditions occur in the presence of free oxygen then the wood is immediately subject to decay in various forms. Beech or elm, for instance, will last for centuries, either as furniture or totally immersed in water or deeply buried in the earth; but if either is laid on wet soil, or only shallowly buried in the upper layers which contain free oxygen, it will speedily rot.

Wood for all work exposed to weathering must, therefore, be chosen carefully for natural durability and resistance to decay. Unfortunately, it cannot be assumed that because a wood is highly resinous it is therefore extremely durable, although it will be more durable than wood with a low resin content. Woods of proven durability include oak, chestnut, yew, teak and greenheart; while if the work is to be painted or otherwise protected then obviously woods of only average durability can be used. Some woods are, however, inclined to repel paint, while others will not absorb sufficient preservative except under pressure, therefore reference should be made to the standard textbooks on wood preservation (see Bibliography).

Fire resistance of wood

Although wood might appear to be one of the most inflammable of materials, some species, notably crab wood, jarrah, iroko, padouk and teak, are very resistant, and all woods of large dimension char outwardly, cutting off the supply of oxygen necessary to support combustion. However, built-in fixtures in exhibition-work, public buildings, etc. are sometimes required to be fireproofed, or composed of fire-retardant materials. Plywoods and chipboards in fire-retardant quality can be obtained to special order; or the completed product can be coated with special paints or clear varnishes, or treated with various chemical preparations, the most widely used of which is ammonium phosphate. Alternatively, plywood panels or partitions can be interleaved with plasterboard or soft asbestos to give a 'one hour' standard resistance.

It is, or should be, the responsibility of the buyer or his agent to specify precisely the degree of resistance required, and the materials or treatment to be used; but the terms 'fireproof or 'fire-resistant' should not be accepted without qualification, otherwise they may be liable to serious misconstruction. It is usually more correct to claim that a combustible material suitably treated is 'fire-retardant' only.

Defects

Every tree is a prey to defects from the moment it emerges as a seedling to the last stages of seasoning, and these defects can be innate (inherent vice), such as the characteristic natural shrinkage of wood; acquired defects occasioned by seasonal checks, insect and fungal attack, etc.; and artificial defects caused by incorrect sawing and seasoning. As, however, any one defect may arise from several causes it is more convenient to classify them as natural or artificial.

Natural defects

Knots These are in effect the basal stumps of incipient or cast-off branches in the living tree. Where the tree itself naturally prunes its branches owing to lack of light caused by overcrowding, or where such branches are artificially pruned in controlled forestry and cleanly sawn, then the cambium layers will heal over the wound and the knot is then live or embedded (17:1 A). Where, however, a mature branch is broken off, leaving a long ragged stump, then the cambium layer cannot heal the wound and the stump dies, forming a deal or loose knot, often rot affected (17:2A).

All knots whether live or dead affect the mechanical strength of the timber, owing to the abrupt change in the direction of the fibres, and constitute blemishes which detract from the value. They are, therefore, graded as follows:

Pin knots Small knots 1/2 in (12.5 mm) or under, often caused by the shedding of early branches. Usually allowable in prime timber.

Spike or splay knots (17:3) Knots sliced through their length during sawing, and commonly known as 'slash' knots. They are difficult to plane up, especially in softwood, while large specimens are not permissible in hardwood unless allowed for in the measurement.

Encased knots Dead knots which are still sound and difficult to dislodge, and often ringed with resin in softwood.

Branched knots Two or three knots springing from a common centre.

Knots are classified as small, medium and large, the latter usually 11/2 in (38 mm) in diameter and over; but gradings are not precise and vary according to the country of origin.

Shakes Both the medullary ray and springwood cells of ring-porous hardwoods are weaker than the remainder, and built-in tensions are created which tend to level out. either in the growing tree under certain adverse conditions or in the felled log during seasoning. Thus extensive splitting may occur in the weakest links, i.e. radially along the medullary rays, and tangentially at the junction of springwood and summerwood. Various forms of shake are common, as follows:

Radial shakes The log splits from the pith or heart radially along the medullary rays, usually indicating that the tree has passed its prime. Sawing losses can be minimized by placing the cuts either side of the shake, always provided the growth of the tree does not twist upon its axis, in which case the shakes become spiral, rendering the log useless for long lengths. Where only one shake is present it is known as a 'simple heart shake', while two shakes in line compose a 'double heart shake' (17:4), and several a 'star shake' (17:5).

Frost shakes (17:7) project inwardly from a definite frost rib on the cambium and are. as their name implies, the result of severe weather.

Tangential shakes The soft springwood of the log splits away from the harder summerwood, either during seasoning or through shearing stresses in the growing tree caused by old age, excessive bending under strong winds, intense heat, etc. A frequent cause in oak is the depradations of the tortrix viridana moth, whose caterpillars strip the young leaves in early summer, with the result that growth is checked and the wood rings fail to cohere. Where such shakes run along part of the annual ring only, then they are known as "cup shakes' (17:6A); but where the log is completely encircled then they become 'ringshakes' (17:6B). Usually such shakes seriously detract from the value of the timber. English walnuts are particularly prone to cup and ring shakes, as the trees are rarely felled until they are long past maturity.

Cross shakes {thunder shakes) These failures are caused by compression and not by splitting or shearing, while the actual rupture is across the grain and not with it as with all other shakes. The probable cause is not thunder, as the name suggests, but either felling shatter (the sudden impact as the felled log hits either hard ground or another fallen log), or mechanical strain in the living tree. Chiefly confined to the softer varieties of tropical hardwoods, and appearing either as a definite fracture or an overriding of the fibres, showing only as a faint raised line across the width of the wood, which will snap like a carrot under strain. This particular type of shake often occurs with a soft condition in the heartwood, known as 'brittle heart', 'carrot heart", etc.. and agba is particularly liable to this defect. End splitting and sun checking (see below) are usually regarded as artificial defects due to errors, in seasoning, but a marked propensity to split and check may be inherent in some species and such defects may be part natural and part artificial.

Pitch veins, pitch pockets, etc. Sometimes known as resin pockets, they can appear either as thin veins or shallow cavities filled with resin. Usually caused by damage to the cambium layer in resinous woods, they may remain hidden and thus constitute a serious danger if the wood is used structurally.

Pith flecks Repeated damage to the cambium layer by small insects is often healed over with bark, and may show as small dots or patches of brown cork deeply buried in some woods, notably birch, alder and sycamore. They have no effect other than that of unsightliness.

Rind galls, etc. Patches of ingrowing bark, probably caused by exterior damage to the

Rind Galls TimberFrost Shake Damage Tree Images
18 Shakes and knots in English walnut
Frost Shake Damage Tree Images
Rind gall in cherry

growing tree. Other natural defects include 'callus', or tissue formed over a wound in a tree resulting in unnatural growth incorporated in the normal wood growth; 'canker', caused by fungoid disease; and 'cat face", a partially healed fire scar.

Internal sapwood Normally, the sapwood dies ring by ring, forming heartwood. but on occasions patches of sapwood survive within the heartwood, and show as lighter patches as sometimes seen in Rio rosewood. It is not known how the condition arises. Sapwood also can be prematurely killed by frost or other agents, while the cambium is repaired and continues to grow, forming new sapwood over the dead patches which appear in later years as a dark ring. The wood usually separates and breaks away along the ring during conversion.

Burrs/Burls These are not usually classed as defects as they may enhance the value of the timber considerably; they are, however, true defects. They can be caused either by fungal or insect attack irritating the cambium layers, and resulting in large rapid growths, usually at the base of the tree, or by numbers of small twigs which fail to develop owing to insufficient nourishment, forming a dense mass. The knobs in severely lopped or pollarded trees, and the witch's broom in birch trees, are typical examples.

Artificial defects

All woods shrink on drying, some pronouncedly so, thus creating internal strains and stresses. Normally, the natural elasticity of a healthy wood structure will distribute these stresses evenly, but if the structure is unequal or lacking in elasticity (innate defect), and if incorrect seasoning imposes too great a strain, then various forms of distortion, splitting, etc. will occur. Weighting down during seasoning helps to reduce distortion.

Cupping or rounding (17:13) The plank hollows across the width, forming a rounding on the underface, often due to incorrect piling.

Bowing (17:14) The plank is curved like a bow throughout its length. A succession of short bows is usually caused by sagging between too widely spaced stickers or by stickers which are not placed exactly over each other.

Springing (17:15) Sometimes known as 'edge bend', the wood remains flat but bends edgewise on its own plane.

Twisting (17:16) The plank twists on its longitudinal axis with the result that the long edges are straight, but the diagonals are curved. Usually known as 'in winding'.

Warping, casting Synonymous terms for distortion in one or more directions (see Twisting, above).

End splitting (17:9) The butt end of the plank splits open, usually caused by too rapid drying. but some species will always split.

Sun checking (17:10) The wood surface is covered with small splits along the grain caused by too rapid drying in hot sun. Not serious unless the splits penetrate deeply.

Flaking The surface of the wood lifts in innumerable small flakes or layers which spring under the cutting action, preventing a smooth surface. Sometimes due to structural weakness, but can also be caused by incorrect seasoning.

Diagonal grain The grain runs obliquely to the longitudinal axis, usually due to incorrect sawing, but some timbers exhibit marked deviations in grain direction which cannot be avoided. Although the condition may make surfacing more difficult it is not important, except in structural members where the impact strength loss is high, and in bending where a grain slope of 1 in 25 may mean a bending strength loss of 4 per cent, and a 1 in 5 slope a loss of 45 per cent.

Case hardening (17:11) If the wood is kiln dried too quickly then the surfaces dry out at a rate quicker than the rate of movement of moisture by capillary attraction from the centre of the plank, with the result that the dry outer layers are in tension, and the moist interior in compression. Cuts which close ahead of the saw are often due to case hardening. Provided the actual wood fibres are not ruptured the condition can be cured by steaming and redrying.

Honeycombing (17:12) If the kiln drying of case-hardened timber is continued to dryness then the natural shrinkage movement of the moist interior as it dries will be locked in by the rigid outer skin, resulting in severe internal stresses and subsequent checking or disruption of the wood fibres, not visible from the outside. There is no cure for the condition, which severely depreciates the value of the timber.

Collapse The too rapid kiln drying of green timber can result in a flattening of the wood cells, caused by vacuums created by the withdrawal of water to below fibre saturation point at a rate faster than it can be replaced by either air or live steam. This condition is known as 'collapse' and is characterized by extensive shrinking and warping, particularly in the springwood, giving a washboard effect. It can also be caused by too slow drying at too high a temperature, or too high a humidity rate, and can sometimes be remedied by steaming and reworking in the dry kiln.

How To Sell Furniture

How To Sell Furniture

Types Of Furniture To Sell. There are many types of products you can sell. You just need to determine who your target market is and what specific item they want. Or you could sell a couple different ones in a package deal.

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