WOOD'S APPEARANCE
by William Cumpiano
Previously published in American Luthierie # 2
Fall arrives in New England. You can almost sense the trees preparing for the long, cold winter to come. They signal a season at end by painting their leaves bright, fiery colors; soon, they will drift to the ground, forming crisp autumn blankets. But less dramatic changes are happening inside the trees, just under the bark. If we had X-ray vision, we could peer through that tough, corrugated overcoat and see a thin, oozing, vital underlayer. Over time, we could see the living hydraulic process winding down as the tree slowly drifts into deep slumber.
The tree's bark forms a cylindrical coat of armor which protects an exceedingly thin tissue of growth activity, called thecambium. The living tissue of the cambium can become alternatively active and inactive during several well-defined times of the year. During active periods, the birth of new wood occurs here by the additive accumulation of cylinders of new cells around the previous year's growth. During inactivity, the tree stands frozen in time, with only a biological clock ticking the cold hours away.
In temperate zones (for example, my back yard in Northampton, Massachusetts), spring marks the beginning and late fall the end of a yearly growth cycle that matches up to the starting and stopping of growth activity of the cambium. The cycle leaves its permanent imprint in the tree by these cylindrical growth layers. Much later, the evidence persists as a finely organized patterns of lines on the surface of the sawn lumber.
Not so on tropical woods: constant climate and several growth periods during a single year (and, as a result, a far more gradual ebbing and flowing of life activity) result in no such obvious demarcations. This is why no well-defined growth rings can be seen on Central American mahogany or South American rosewood (the stunning colored lines on rosewood are not growth rings, as we shall later see).
As fall approaches, and as activity just beneath the bark starts to slow down, cells created at the cambium will be born smaller, darker, denser, more tightly packed than those born during the summer. The pattern of lines and spaces we can see on the lumber surface is simply that: light, large, square cells alternating with tougher, darker, flatter ones.
Until next April or May, no more wood cells will be born. But spring brings on an awakening, and a spurt of activity once again commences. A cylinder of new cells appears all around the tree, just under the bark. These earliest cells first appear with soft, gelatinous walls and with no strong bonds holding them together. If you were to perversely plunge a knife into the bark and pry, the bark would easily become detached: the pulled-off scab would reveal a moist, scummy surface underneath. This is the time when farmers find removing bark from freshly cut trees the easiest - when the cambium is producing brand new, immature cells which are liquid and slimy. Thus, with gentle prying, the bark just sloughs off. This is certainly not so later in the year, when these same cells have matured and lignified; the bark must then be laboriously carved off the tree. May is also a time of year when naughty kids can wound a tree by pounding its trunk with toy bats, and crush those young forming cells under the bark. Scars, forever enshrined, will years later be deep inside the wood.
The cambium is not so much a clearly defined region as it is a "line of activity" where cells divide. As they divide, some grow outward towards the bark and others grow inward, adding to those already inside the tree. The outwardly-growing cells enter and become part of a region just under the bark known as the phloem. As the phloem accumulates cells it actually moves, causing the trunk's slow increase in girth. The swelling of this outer cylinder forces the bark to split and crack, like dried mud on an expanding balloon.
The forming cells oriented towards the interior will form part of a stationary cylinder of liquid-conductive and nutrient-storing vascular tissue called the xylem, where they are deposited as growth layers. The first cell born into the xylem in May will become as firmly attached to the last one in November as all the other cells are attached to each other, leaving no consistent plane of weakness in the wood - the wood tissue remains cohesive across the start/stop region. That this is so is an impressive mystery of nature.
The aggregate of large, low density cells that form early in the year are termed springwood, or earlywood. The stuff of the growth rings, the cells which form late in the year are termed summerwood, or latewood. These cells form the hard, stiff soundboard "reeds", so carefully counted by some luthiers.
The very stiffest samples, however, are those displaying a predominating cross-grain structure of specialized cells which grow oriented towards the center of the tree, called ray cells. These can be barely seen on some samples but are unmistakable on others. They can be seen on the surface as ghostly cross-hatches, imparting both a furry-looking and luminescent quality to the soundboard. They appear most dramatically when a soundboard board is accurately quarter-sawn. However, even well-sawn samples can be deficient in these strength-inducing cells or possess them in a poorly developed form.
We have seen how cells originating at the cambium get deposited in layers Onto the xylem. Here they perform their function of conduction and storage so critical to the living organism. But after these cells mature, they eventually stagnate and enter a state of perpetual dormancy.
While living and vital to the tree, they exist in a zone within the tree near the cambium forming an irregularly cylindrical subdivision of the stem which is the sapwood. As they decline, their appearance usually changes and they become part of the heartwood, the central core of the tree. These forever-to-remain-dormant cells become dead parts of a living system - akin to hair and fingernails in people. And, as such, they are still useful, for they become the tough, fibrous mass that holds the tree upright. Both woods exist in the same tree in relative proportions which can very between trees of the same species, of different species, or within the tree itself.
So sapwood cells "die" and become part of the heartwood. They undergo dramatic changes as they die. The cause of death is uncertain; reduced water requirement from the tree crown may cause the "shutting off" of sapwood cells. Perhaps the tree "poisons" itself by choking its cells with the waste products of its own breathing. Related to this death process is the progressive accumulation in these cells of a complex of diverse substances called infiltratives. Inflitratives are of primary importance to all woodworkers. They determine most of the visual, tactile and olfactory qualities of the heartwood and, as a result, the eventual appearance, usefulness, and value of the lumber.
INFLITRATIVES
For example, pigmented infiltratives will impart the characteristic chocolate brown color to walnut, the reddish hue to mahogany, the rainbow of colors seen in rosewood, and the black color of ebony. These pigments can also swirl through the tissue in varying concentrations, causing a dramatic marbling of color through the lumber ~d the creation of a "pigment figure" which is distinct from the figure imparted by the arrangement of the vessels and fibers (known as "grain figure"). But more on that later.
Some infiltratives are commercially extracted and become--you're right: extractives. Some extractives are oily and aromatic. Extractives in redredar gives it its wonderful fragrance and insect - repellent properties. Extractives from other trees can be processed into camphor, anise, wintergreen, sandalwood incense and perfume bases (as are the volatile extractives found in Brazilian rosewood). Still other extractives are acidic, such as tannins; they can cause corrosion in contacting metals, and can as well interfere with the proper setting of paint and glue. The tannins in oak darken the cut surface when exposed to moisture.
Thus, the many recognizable and commercially desirable (and undesirable) qualities of the heartwood are a result of the presence of these infiltrated substances. Some researchers believe that the infiltration itself may be the cause for the tissue to become heartwood in the first place.
Trees can be classified on the basis of the distinctiveness of the heartwood. Some tree species have hardly any heartwood at all: these are called sapwood trees or trees with retarded heartwood. In some species the pigmented infiltratives are light colored (maple, spruce) and the heartwood remains light-colored: these are called light heartwood trees or ripewood trees. When the pigmented heartwood infiltratives are dark-colored, the heartwood is said to be obligatory colored and the trees are described as regular heartwood trees. Finally, facultatively colored heartwood, or irregular heartwood trees are trees in which the pigmented substances appear in some areas of the heartwood and not in others. Stunning visual effects are a result.
Enzymes infiltrating the heartwood can change the color of the wood after it has been cut into lumber. These enzymes will oxidize on exposure to air causing the darkening of the freshly cut lumber surface. African Padouk is blood red when freshly cut, and weeks later, the surface metamorphoses into a dull brown. Enzymes will also darken wood touched by sweaty fingers.
Fungal attack and other forms of decay will cause random patches of contrasting colors to appear on the surface of some of the tropical hardwoods used by luthiers. In some cases, the fungus itself is colored and in others the fungus will cause chemical changes in the pigmented materials in the wood cells. Most pigmenting fungi will not significantly affect the strength and other mechanical properties of the wood although, in some cases, the impact resistance of the material may be somewhat reduced. Insect attack may also cause localized color changes, usually when boring insects carry these fungi into the bore holes. All these changes can be commonly seen in rosewood, which can lie in the jungle for months after felling, a time during which considerable biological attack can occur. The wood then develops blotches of inky-black stains around every bore hole.
GRAIN / FIGURE
The terms "grain" and "figure" are used interchangeably by most people. Wood technologists, however, make a distinction between the two. To them, grain usually describes the pattern of orientation of the fibers within the sample, while figure describes all the plainly visible surface characteristics of the sawn piece.
In the trade, however, figure implies something special, a quality found in some pieces which imparts a higher-than-average economic value. In actuality what we can usually perceive of the wood is a result of the normal growth of the wood and the particular way in which it has been cut from the log ~ abnormal grain structure and uneven pigmentation. Although normal grain structure is part of the visible "grain figure" of the wood, the grain is not generally considered part of the "figure" unless it is in some way notable or dramatic. Usually the grain becomes striking when genetic or environmental circumstances result in distorted or abnormal grain.
Often, pigment figure and grain figure coincide. But in many cases the two figures ignore each other in interesting and dramatic ways. A particularly striking visual effect occurs when the heartwood grows to one side of the stem rather than straight through the middle. This sometimes occurs in the jungle, where the tree's leafy crowns are crowded by adjacent trees. The crown thus thrives off to one side of the tree, and the heartwood in the stem correspondingly becomes situated to one side of the trunk. The dramatic opposition of pigment figure to grain figure in Brazilian rosewood may be a result of this type of heartwood growth.
Pigment figures are obvious in such native hardwoods as black walnut and sweetgum, and in imported timbers such as Circassian walnut, zebrano, zebrawood and Brazilian rosewood. Pigment figure can exist as irregular blotches in the log, such as in sweetgum and Brazilian rosewood, and thus will appear as swirls no matter how the plank is cut from the log. But pigment figure can also appear in regular formation, as it does so dramatically in zebrawood; in this case its appearance on the lumber surface precisely depends on how the plank is cut from the log.
Grain figure can also cause an impressive wood surface appearance. Ribbon or stripe figure is the result of the spiral growth of wood fiber cells around the central axis of the trunk periodically reversing; i.e., the cells are oriented in a left-handed helix and then reverse to a right-handed helix. These reversals continue for the life of the tree. The wood is then said to have "interlocked grain". A plank which is quarter sawn from such a log will display a ribbon or striped figure. This display of light and dark bands arises from light being reflected differently from each separate zone of grain orientation. If the ends of a board possessing ribbon figure are reversed, the light and dark bands pop out in the reverse manner.
Wood with pronounced interlocked grain can be seen on such domestic woods as sycamore and elm. Interlocked grain is the common condition in tropical woods, and luthiers come across it most commonly in mahogany.
Curly grain figure occurs when fiber cells grow in waves. Curly growth usually occurs at right angles to the long axis of the tree. Best available information has it that it is caused by a genetic quirk which is passed on through several generations. It occurs most frequently in maple and birch, but can appear sporadically in many other woods as well, including walnut, rosewood, mahogany and ebony.
A combination of interlocked and wavy grain results in an interrupted ribbon figure. When these grain stripes measure about a foot or more in length and appear twisted the patterns are called "broken stripe"; when the stripes are interrupted by irregular curly wrinkles the figure is termed "mottled". When the mottles are very fine, the pattern is termed "bee's-wing": it occurs with great rarity in mahogany.
Another dramatic configuration present in a small number of flat-sawn maple boards is "blister" figure. The surface appears marked with alternating hollows and mounds, separated by narrow ridges. This is also a genetic anomaly and such a grain reflects light in the same reversing manner as ribbon figure. When the areas enclosed by the ridges are longer across the grain than parallel to the grain the figure is "quilted".
We finally arrive at a familiar grain figure that is a source of much puzzlement and misunderstanding. "Birds-eye" figure results in localized, swirling distortions in the fiber alignment, caused by conical indentations which extend from the surface of the bark towards the center of the tree. Once started, these fiber disturbances continue in successive growth layers for the life of the tree. The disturbances appear as "bird's-eyes" on flat sawn lumber surfaces and as sausage-like configurations on quarter-sawn faces. Their actual origin remains hypothetical. Wood technologist Bruce Hoadley theorizes that their origin is both fungal and genetic. He suggests that early in the tree's life, fungal attack disturbs the genetic integrity of the cell division processes at the cambium. This sets the stage for permanent changes of the cambium and the subsequent production of "birds-eyes".
When we behold a piece of wood, any piece, how much of what is happening on its surfaces and within its surfaces can we perceive? Take a closer look. Now look even more closely...
..from my class notes, in Wood Anatomy 1010, Prof. Bruce Hoadley, University of Masssachusetts at Amherst, Wood Technology Department