Texture, Grain, and Figure

Figure Caused by Pigmentation

       There are a few woods in which a well defined pattern, or figure, is developed because of uneven pigmentation (63). Color patterns appear as uneven streaks in figured red gum (21). Strongly contrasting colors may be concentrated in growth increments of a few tropical species such as zebra wood. Several other tropical woods belonging to the rosewood group exhibit variegated pigmentation, ranging in color from reddish brown to black, which produces figure patterns. Figure types caused by pigmentation lack the luster of figures caused by grain variations (23).

Figure Types Caused by Variations in Grain Orientation

       The normal orientation of longitudinal cellular elements in wood is parallel to the longitudinal axis of the woody cylinder. Various deviations from the parallel develop a number of well-defined figure patterns.
       Certain portions of trees also play a major role in development of figure patterns in wood (Figure 3). These are defined as follows:

       Longwood The bole or stem (from stump to first limb or fork) produces the majority of wood. It normally is available up to 16 feet (4.88m) in length and may or may not exhibit patterns different from typical patterns found in flat or quarter-sawn lumber. This type of wood is used primarily for architectural installations and wall paneling. It may exhibit any or all common figure types in some areas, but generally shows little figure development (Figure 4).

       Crotch Crotch figures are characterized by feather-like patterns that are obtained by cutting through forks where main boles branch. They normally are obtained from walnut or mahogany and were used extensively in furniture during the 18th and 19th centuries. They seldom are seen today except in high grade gun stocks. Excessive end grain exposure and random orientations frequently cause crotch-figured wood to check severely. This type wood must be reinforced with backing to prevent such checking (Figure 5).

       Stumpwood As the name implies, this particular figure type is obtained from stumps. It seldom is seen except in walnut and is characterized by wrinkles and blotches of color variations. It is a very attractive figure that may be observed occasionally in furniture and gunstocks (Figure 6).

       Burls Burls are irregular, spherical growths that occasionally occur on trees near ground level (24,53). They occur primarily on elm, walnut, maple, and redwood. Rarely do they occur commercially in other trees. Many of the most prized possessions of early emperors and kings were made from burlwood (53). Utilization of elm burls for furniture in the U.S. during the 1920's probably led to introduction of Dutch elm disease into this country. Ceratocystis ulmi (causal fungus of Dutch elm disease) probably was introduced into this country within elm-burl material imported from France for use as veneer and subsequently in furniture manufacture. Burls are usually small and characterized by eye-like markings surrounded by swirls and distorted tissues. Burl tissue is soft and “velvet-like” to the touch (Figure 7).

Figure Caused by Ray Structure

       Figure occasionally results from cutting wood so that rays are parallel to the wide face where a series of shiny, light-reflecting surfaces are formed (63). These are known as ray flecks. In such woods as the oaks (Quercus) or sycamore (Platanus), ray flecks are very pronounced and exhibit typical patterns (Figures 8 and 9).
       Ray flecks are present in all woods and reach their greatest development in sections that are quarter sawn or sliced. They normally do not contribute materially to figure other than to produce a lustrous surface. However, some woods may exhibit conspicuous mottled figure caused by extra large rays (20).

Figure Related to Spiral Growth

       Spiral Grain As mentioned before, normal orientation of longitudinal cellular elements is parallel to the longitudinal axis (Figure 10). However, slight undetectable spiraling is a rule rather than an exception in most trees. In certain trees, elements are sloped or spiraled circumferentially to an extent sufficient to be detected. When wood with this characteristic is split, exposed surfaces will not be parallel to the longitudinal axis as in straight-grained material (Figure 11). Spiral grain occurs quite commonly in certain species and is considered a defect in many cases (60, 85). Spiral grain in simple form does not cause figure since the surface reflects light rays at approximately the same angle at which the rays strike. However, figure initiated by other causes may be enhanced by spiral grain.

       Interlocked Grain Interlocked grain is a variation of spiral grain that is exhibited almost universally in tropical woods and in a few native hardwoods such as elms, gums, and sycamore (6). Interlocked grain is the cause of a particular figure type commonly called ribbon stripe or stripe figure.
       Interlocked grain is spiral grain that is produced in one direction for a given period of time, then for some unknown cause is reversed with the same angle and is produced in the opposite direction for a similar length of time (Figure 12). This pattern is repeated many times throughout the tree life and results in a grain that is interlocked. Wood exhibiting interlocked grain is almost impossible to split.
       When interlocked grain is produced uniformly and regularly in a given tree and the log is quarter sawn or sliced into veneer, a series of longitudinal parallel stripes is exhibited by the wood. These stripes alternate in color shades (dark, light, dark, light, etc.). When such wood is shifted slightly or turned end for end, the stripes will be reversed (light stripes become dark and dark stripes become light). An example of this phenomenon is shown in Figure 13.
       Grain direction in such wood does not correspond to growth increments but is independent of them. Stripes are produced by the direction of fibrous elements being viewed either toward or away from the observer (Figure 14). In unilateral lighting, stripes that appear bright are those where grain is oriented away from a viewer and light is reflected back. In dark stripes, grain is oriented toward the observer and end grain is exposed that then absorbs light causing the stripe to appear darker. Turning the specimen end for end reverses the angle of elements in relation to the observer and they appear reversed. Any slight movement of light source, specimen, or angle of observation will change relative brightness of pattern.
       A variation of stripe figure that sometimes occurs is that of the broken or interrupted stripe (Figure 15). This is caused by an indistinct interlocked grain in combination with poorly developed wavy grain.

Figure Related to Undulating Growth

       In contrast to previous examples of spiral and interlocked grain, there are a number of figure types that are formed by grain which is straight in a sense that it is parallel to the longitudinal axis in whole perspective but undulating or wavy when viewed closely. Some of these are wavy in the radial plane while others are wavy in the tangential plane (22). Together they collectively are called wavy or curly grain, irrespective of the plane in which they occur.
        In a few cases, very sharp indentations occur where locally restricted growth produces circular or lenticular indentations in the tangential surface. Various names such as birdseye, dimple, bear scratches, etc. have been given to such figures based on size, shape, or form of the indentations (4, 63, 82).
        Cause of undulating growth can be traced to the method of cell division and enlargement described later. On a larger scale, undulating grain can be observed to produce an interference pattern or "moire" (wavy) figure that appears to the viewer as an optical illusion of movable stripes or contrasting patches of light and dark areas resulting from the fibrous structure (43).

Radial Surface—Curly and Fiddleback Figure

        When the radial surface of wood exhibits wavy growth or grain, the tangential face is smooth and the wavy or undulating grain can be observed, but is not evident to the touch (Figure 16). When a wood section with curly grain is split, it separates along the rays and along the wavy grain and is exhibited as a corrugated surface that can be seen and felt on the radial surface (figures 17 and 18).
        Well-developed wavy grain is rare in trees of most species, but the figure is observed frequently in maple (Acer), ash (Fraxinus), birch (Betula), and walnut (Juglans). Wavy grain is quite common near limbs and roots, where it is localized in small areas. Wavy grain frequently is called “curly” when waves can be measured in terms of numbers per foot and “fiddleback” when waves are measured in numbers per inch (19, 77). True fiddleback is restricted to uniform, straight-grained wood of high quality; curly woods frequently exhibit sloping grain and other defects (22). Trees are tested for evidence of curly/fiddleback grain by chipping away the bark and examining the inner wood (Figure 1), as described previously. Because curly/fiddleback figure is restricted primarily to the radial plane, trees with this type of figure do not exhibit any visible external evidence such as bulges, flutes, or bark irregularities (figures 16 and 17).
        Wood with curly or fiddleback figure commonly is cut in a manner to display a radial surface, which would be corrugated if the wood had been split (Figure 18). Appearing on smooth surfaces in place of waves are series of alternately bright and dark stripes that shade into one another and produce an illusion of actual waves. Changes in brightness result from differential light reflection. Relatively high absorption by exposed fiber ends produces dark bands; reflection and diffraction from fiber walls cause bright bands (Figure 19). Because the fiber walls are curved sharply and act as concave or convex reflecting surfaces, any change in angle of view or incident light makes the apparent waves seem to shift. Tangential surfaces present much the same appearance (Figure 20). The illusion of undulations, however, results from regular and repeated, parallel, wavy lines that produce an interference pattern on the exposed plane (Figure 16).
        When cut on the true quarter, fiddleback figure will show as a series of straight, evenly spaced, horizontal or slightly canted stripes. When cut slightly off true quarter, these stripes may appear wavy or branched. The latter effect is a result of interference patterns rather than differential light reflection and diffraction. Reversing the direction of viewing will reverse the bright and dark pattern.
        The name "fiddleback" is derived from extensive use of this type of wood for backs of string and fretted instruments since the mid-sixteenth century. However, not all instruments were made with fiddleback figured wood. Even the great luthier, Stradivari, made some instruments with plain wood (figures 21 and 22). Stripes are sometimes simulated through various techniques to give a reasonable facsimile of true fiddleback figure (Figure 23). Simulated figure may be detected easily because it does not extend completely through the piece (Figure 24).
        The preferred wood for musical instrument manufacture is European sycamore (Acer pseudoplatanus L.) instead of the native sugar maple because of the greater density of the latter.

Tangential Surface—Blister and Quilted Figure

        Curly or wavy grain occasionally affects the tangential surface instead of the radial surface (8). When this occurs, the effect rarely may be indicated by a series of irregular annual bark bulges. External evidence of figure, however ,usually is not visible through the bark. Birch (Betula) exhibits this figure occasionally, and debarked logs may resemble a corrugated drainage pipe (58). A similar figure in some species of pine (29) is observed but rarely (figures 25 and 26). Tangential surfaces (circumference) exhibit a series of bumps and flat areas occurring in an irregular pattern. Bumps may enlarge and coalesce; then the junction between adjacent bulges is reduced to a line (figures 27 and 28). Rounded figures are designated “blister”(Figure 27); rectangular figures, which usually result from coalescense, are designated “quilted” (Figure 28).
        To develop blister and quilted figure, lumber must be flat sawn, and veneer rotary cut, because the figure occurs on the tangential face. Both lumber and veneer with blister or quilted figure differ from other figure types in that the front, or outside face, shows an apparent series of mounds (convex areas) where the back face shows a similar series of concave or depressed areas. The results of “book matching”² the pattern are therefore unsatisfactory (Figure 29).
        Well-developed blister and curly figures seldom occur in the same tree. However, several other imperfectly formed figure types (some commonly called “ghost”figures) occur more frequently together and this results in confusion of terms.

Figure Related to Indented Growth Rings  

        A third type of figure that is recognized originates from indented growth rings. In this class fall such figures as dimple, birdseye, and indented ring or “bear scratches.” For unknown causes certain specimens of Douglas fir, lodgepole pine, sugar maple, Sitka spruce, birch, and mahogany exhibit reduced growth in localized circular or lenticular areas that, continued year after year in the same location, results in circular or lenticular depressions in the wood (figures 30 and 31).
        Indented ring figures affect only the tangential surface but differ from other figure types affecting this surface (blister figures) because they are formed primarily by sharp, conical, or lenticular depressions indented toward the pith. This contrasts with blister figure, where major affected areas bulge toward the bark.

        Dimple Indentations in growth rings that create numerous localized, conical depressions that are small and shallow (usually less than one growth ring), are termed dimples (17). Tangential surfaces of wood from lodgepole pine (Pinus contorta) characteristically exhibit dimples (Figure 32), which are useful in identification.

        Birdseye Indentations that create less numerous localized, conical depressions that are larger and deeper (usually one or more growth rings), are termed birdseyes. Birdseye of sugar maple (Acer saccharum) is the best known of indented ring figures (Figure 33). Birdseye figure has been reported in a number of other species including ash, walnut, and birch; in Cuban mahogany (Sweitenia mahogani), it takes the form of a giant birdseye (59).
        Birdseye figure in sugar maple apparently occurs rather commonly but is well developed only in localized areas of the stem. Veneer manufacturers report that this figure may disappear completely as peeling progresses and reappear in different stem areas (11).
        Split sections of birdseye-figured sugar maple show sharp, conical indentations in the tangential surface. These indentations seldom exceed one-sixteenth inch in depth, usually occur in a random pattern, and may vary considerably in size and distribution among trees. Birdseye frequently occurs in combination with wavy or curly grain, which enhances the appearance. A characteristic of birdseye figure is its continuation along the same radius for a number of years. Birdseye figure is formed as a result of local suppressions in divisions of cambial tissues that cause indentations to develop. This retardation effect continues for several growing seasons and produces an indented conical depression.

        Bear Scratches Indented growth rings in the form of elongated or lenticular depressions are termed “bear scratches.” Bear scratches occur most commonly in Sitka spruce (Picea sitchensis) and Douglas fir (Pseudotsuga menziesii). The term is derived from the appearance of lighter colored tissues that form longitudinal streaks on the tangential face. For many years, such streaks were thought to be caused by animal claw injury.

²Two opposing cut surfaces turned as pages of a book and used side-by-side to yield bilaterally symmetrical patterns.

        Since the development and anatomy of unfigured wood are outside the scope of this publication, interested readers are referred to the “Textbook of Wood Technology, Vol. 1”(63). Phases of figure formation that differ from normal development are described.

        Spiral Growth Spiral growth apparently originates from a modified anticlinal division of cambial initials that develops an organized intrusive growth, which results in a helical orientation (either right or left) of the cambial initials and in subsequent woody tissues. Helical or spiral orientation of elements in mature woody stems causes a twisted appearance. Certain tree species characteristically have elements that spiral to the right; others, to the left. Still other species exhibit regular alternation of spiral direction in successive growth increments to form interlocked grain. The variations appear to be largely hereditary within species. Many tropical woods exhibit interlocked grain so regularly that straight grain is considered unusual.

        Undulating Growth After initiation by cambial division, cells enlarge first by elongation and reach final size after differentiation. Elongation of individual cells initiated at the same time is sometimes delayed, resulting in some that are longer than others. Cells that are somewhat slower in elongation are restricted in available space, particularly when the ends are held rigidly in place. As cell elongation continues, central portions are pushed to either side resulting in a sinuous grain orientation. A certain amount of intrusive growth occurs where enlongating tips of adjacent cells force their way between cells lying above and below, particularly between ray cells (figures 34 and 35). This penetration evidently takes place while the intruding cell is relatively undifferentiated. After intrusive growth has occurred, the cell may become a vessel, fiber, or other element and continues to enlarge normally (figures 36 and 37). Such enlargement may result in lateral displacement of tissues, which appears to be the primary method by which the radial plane is affected. Lateral displacement also may affect the tangential plane (figures 38 and 39).
        Indented growth rings–Indentations in growth rings have been traced to localized suppression of growth, where individual cell elements are bent sharply to conform with resulting depressions. In a few cases, however, the indentations appear to be localized areas of normal growth surrounded by areas of exceptionally rapid growth. Relatively smaller areas of stimulated growth result in bulges characteristic of blister figures. Apparent lenticular depressions characterizing “masur” of European birch and “bear scratches” in other species exhibit parenchymateous tissues.

Possible Causes

        The ultimate cause of figure in living trees is largely unknown. It has been suggested that stresses of various types ranging from lean forces (forces produced in leaning trees) to unfavorable climatic conditions, disease, suppression, and several other factors contribute to figure formation (31).
        Dormant buds acting over a considerable period of time have been suggested as causes of various birdseye and burl figures. Contrary to popular belief, birdseye figure has no connection with dormant buds. Splitting a block of wood containing birdseye serves to illustrate that indentations responsible for this figure point inward toward the pith and not outward toward the bark as would be the case with dormant buds (66).
        During the early 1930s, considerable interest was stimulated concerning possible causes of birdseye figure in sugar maple and expressed through a number of publications in various journals that suggested suppression (34), heredity (35), or other factors (1,69) as the cause. No formal studies were initiated, and interest apparently expired by the middle 1940s. An unpublished report concerning anatomy of birdseye figure in sugar maple contains indications of pathogenic activity involved in formation of this figure (11).
        The common dimple figure in lodgepole pine (Pinus contorta) and several other coniferous species apparently has been traced to localized resin blisters in phloem (4, 17).
        Drape figure in mahogany, after many years of speculation concerning cause, finally was shown to be a result of complex actions of the strangler figure (39). The specific role of disease organisms in figure initiation is considered later.

Abnormal Tissues Associated with Figure

        As a general rule, abnormal tissues such as compression wood and tension wood are not associated with figure types that affect the radial plane. It often has been observed, however, that types of figure that occur near branches or tree bases and on undersides of leaning trees may be accompanied by reaction wood. In well-formed figured wood where the pattern is regular, little reaction wood normally is encountered.
        Tension wood often is associated with figure types that affect the tangential plane. In some cases, the entire blister surface is composed of tension wood. Export logs possessing blister or quilted figures, therefore, often require bands to reduce splits and checks. Loggers have reported that blister-figured logs sometimes seem to explode when cut (7). Enormous stresses may develop on the underside of leaning trees, where this figure characteristically is found. Tension wood adversely affects the finishing qualities of blister-figured wood but greatly enhances its luster.

External Expression

        The most easily recognized external expression of figure in trees is an overgrowth commonly referred to as a burl. Burls are relatively large, abnormal bulges that sometimes form on trunks and limbs of almost any tree species (Figure 40). Burl surfaces may be corrugated or smooth, but usually are somewhat gnarled. Conifer burls generally are corrugated. Fiber alignment within burls, which is extremely irregular, produces a striking figure that is highly prized for veneer and turned articles. Burls for commercial use are obtained from Oregon myrtle (Umbellularia californica), elm, maple, walnut, and redwood (Sequoia sempervirens).
        Dormant buds are frequently associated with burls (15, 24) and some burls apparently originate from single, well-defined points or series of points. Small burls develop outward, forming fan-shaped tissue masses, until several coalesce to fashion the typical burl figure (Figure 41).
        Figures other than burl types rarely give external expressions that can be interpreted reliably as to quantity or quality of figure within the tree. Bark characteristics, while indicative of figure in birch, are not reliable guides (51, 55, 58). European birch with extremely rough bark is likely to have disturbed grain; smooth bark usually indicates straight grain (56).

Internal Expression

        Figured wood occurs in almost every species that produces wood of commercial importance and in many non-commercial species. Most current information concerning figure in wood has been derived from species that supply the majority of figured woods of commerce.
        In certain species (and possible varieties), individual figure types appear to be characteristic. For example, birdseye figure appears to be characteristic of sugar maple but also occurs in various specimens of ash, birch, and Cuban mahogany. Similar, but not identical, figure occurs in several softwood species.
        Since figured trees occur sporadically in all species to a greater or lesser extent (72), it appears safe to conclude that species is not the deciding factor in figure formation.

Effect of Climate, Soil, and Location

        No particular climate, soil, or geographical location appears to be essential to formation of figure in trees. Certain climates, soils, and geographical locations, however, can be cited as being conducive to formation of certain figure types in some species. For example, figured wood of several species currently is produced in the Pacific Northwest to a far greater extent than in other areas. One explanation for this fact may be that exploitation of hardwood species is relatively recent in the Pacific Northwest.
        While it is true that several types of figure often are found in diseased or deformed trees, most of the best figured wood develops in well-formed, straight, healthy trees. Open-grown trees with short boles and wide branching appear to be influenced by factors conducive to figure formation. Considerable numbers of figured logs are harvested from grafted trees grown in orchards, particularly several varieties of English walnut propagated on various rootstocks (figures 50 and 51).
        Certain areas appear to produce more figured trees than other areas. For example, birdseye figured sugar maple is common in northern portions of the natural range of this species but rarely occurs elsewhere. Curly figures are common in yellow birch in the northern U.S. and Canada and also in various species of birch in Europe. Fiddleback figure occurs commonly in sycamore maple in certain parts of Europe and in bigleaf maple in Oregon and Washington. Fiddleback occurs, but is less common, in sugar maple and various varieties of walnut grown on the West Coast. In contrast, interlocked grain woods are relatively rare in the U.S. but are so common in other areas of the world that this grain type is considered to be normal.
        All figured wood types occur in almost every part of the world where figure is recognized. Based upon present knowledge, figure does not appear to be unique to any soil type, climate, hemisphere, or any other common factor. All types of figure occur in almost every species. Figure exists in only a few species to a commercially important extent.

Effect of Growth Rate

        Rate of growth apparently has some effect on the type or strength of figure produced. It often has been stated that growth retardation is a definite factor in figure formation and that the best development occurs only in slowly grown trees. Equal strength of figure, however, has been observed in rapidly grown specimens as in those grown slowly. Fast-grown wood is considered undesirable for certain products, which may prejudice the selection of figure in fast-grown wood.
        The only figure type that has been studied in detail with regard to growth rate is birdseye figure in maple. During the early 1930s, it was concluded that growth rate is probably a minor factor (34, 35, 69).

Effect of Tree Size

        Tree size apparently has little effect on figure; small and large trees have been observed to exhibit equally well developed curly and blister figures. However, there is some effect of tree size on the strength of figure because figure normally increases in intensity toward the bark. This results in the best developed figure occurring in the outer areas of tree stems. There is usually a sterile, juvenile zone surrounding the pith that is devoid of figure and frequently contains reaction wood.
        Commercial figured wood rarely occurs in trees less than 10 inches in diameter. Many small trees that may contain figure are overlooked because of size restrictions for veneer logs. These restrictions reduce the meaning of available size and figure correlations since much recorded information concerning this aspect is based on export-import data (10).

Effect of Disease

        Some tree disease organisms characteristically cause their hosts or host parts to grow abnormally. This abnormal growth can be expressed either as undergrowth (hypotrophy) or overgrowth (hypertrophy). Either type might result in abnormal woody tissues. However, there also could be specific situations where the abnormality could be expressed only as a misalignment of cells and/or tissues.
        Hypotrophic reactions of trees or tree parts can result from either subnormal cell division (hypoplasia) or degeneration of cells. Various disease agencies, both biotic and abiotic, can cause hypotrophic reactions. Unfavorable weather conditions (temperature extremes, etc.), unfavorable soil conditions (extremes of nutrition, water, etc.), or pathogenic organisms (viruses, bacteria, fungi, etc.) frequently cause hypotrophic reactions within trees.
        Hypertrophic reactions of trees or tree parts result from an abnormal increase in cell numbers (hyperplasia), an abnormal increase in cell size (true hypertrophy), or both. These reactions may be expressed visually on merchantable portions of trees by various malformations and excrescences described as tumors, galls, or burls. There is no great difference in definition of these three terms except that the term, “burl,” usually connotes a prominent overgrowth that is relatively common and has a commercial value. Most hypertrophic reactions of trees apparently are caused by various injuries, pathogenic organisms (especially bacteria), insects, or specific growth disorders. A brief summary concerning various aspects of tree galls/burls (both non-infectious and infectious) has been published (15).
        Little evidence exists to support the theory that specific commercial figures are caused by disease organisms, but the literature cites many speculations to this effect. Burl portions of drape figure in mahogany are induced by actions of parasitic rootlets of the strangler fig; swirl portions are induced mechanically where bulges form around constrictions caused by horizontal strands of vine (39).
        Birdseye of sugar maple (30) and masur³ figure in birch (41) formerly were thought to be induced by disease organisms. Present information, however, rejects these speculations except that a burl on birch associated with mazur is possibly induced by Taphrina betulinum (70).
        Stem pitting of apple (48) and quick decline of citrus (14) produce wood similar to mazur figure in birch (33). It is generally believed that these conditions of apple and citrus are virus-induced, but there is some evidence to the contrary (16).
        Most burls are initiated from single points (Figure 41), which may or may not indicate pathogenic activity. Microscopic examination of sections from various commercial types of figured wood usually shows no evidence of recognized pathogenic activity. Perhaps future work with virus diseases of woody plants will give more insight to some additional specific figures that are caused by disease.

³ Masur also is spelled mazur, mazer, and “wisa” depending upon the specific language.