Wood Basics

Basics
of
Wood

Finishing a resaw in chinaberry.

Charlie Self

Wood is that lovely stuff that goes "tonk" when you rap it with a knuckle. Wood is many things, so many it is hard to
imagine, but for the woodworker, wood is the absolute and ultimate construction material for projects ranging from
the size of a pea, on up past the size of a house. Wood is the natural resource that mankind has used longer than
any other, except probably stone. And life was a lot more comfortable after wood and stone were combined to work
as fuel, tools, housing, what have you.

Most of us who work with wood prefer it to all other construction materials, whether those materials are for building
construction or smaller projects including such items as boxes, barrels, furniture of many kinds and types, toys,
shelves...well, you name it, and at one time or another, it has been built of wood. And probably still is, somewhere,
somehow.

Can you get excited over a steel chair frame? A steel desk? A steel file cabinet? What would you think of a plastic
dressing table—whoops, that one is now common in restrooms where babies need changing.

Wood is very close to an ideal project construction material: tools readily shape it, dampness (especially with heat)
helps it bend, and abrasion, as well as other stresses, grind it down only slowly. No synthetic material equals wood
for working qualities, and finished appearance, especially if we keep in mind the needs of the wood when making
projects. The primary feature to remember: wood moves.

For larger projects needing wide, flat expanses of wood, plywood is useful, because it tends to equalize expansion
and contraction rates in different directions, reducing splits and warping and cupping. Solid woods are ideal for
areas such as cabinet or storage project door frames where narrower stock is not as subject to extremes of
movement.

Wood Growth
Photosynthesis grows trees, and all other green plants. Energy from sunlight in a complex chemical reaction joins
with water from the ground to form sugars, all taking place with the help of chlorophyll, the substance that gives
green leaves their color.

There’s no need to detail the entire journey of carbon dioxide passing into leaves through the stomata to join water
rising from root hairs--where the water enters by osmosis. Water--or sap--flows through the sapwood to the tree’s
crown. The sapwood is the living wood in a tree, and serves two purposes, aided in some cases by the heartwood
that no longer transfers sap: Wood provides strength to keep a tree standing; it also stores the food made by
leaves. The inner bark is the transfer agent for the food created in the leaves, which is the reason girdling a tree--
removing the bark layers down to the sapwood--inevitably kills a tree. New wood is produced in the cambium, which
encloses the living parts of the tree trunk, or bole. Wood shows growth rings in those parts of the world where
seasonal growth stops for part of the year. In tropical areas where seasonal changes don’t stop growth, there are
no growth rings in the trees. When sapwood is too far removed from the cambrium to serve as a sap flow agent,
chemical changes occur, changing its make-up, and, often, color. Heartwood is thus formed.

This article isn’t a text on wood growth through all conditions and in all areas so let’s just say that once heartwood
starts to form, a tree is reaching for maturity. It may have decades, even centuries, to go, and there are all sorts of
failures along the way, but tree growth continues much as above.

Tree Cell Structure
Different cell structures in the tree create different handling needs for particular parts of a tree, and for particular
trees. This is an incredibly complex subject, and I hope to simplify it, without losing too much meaning, because the
different characteristics of hardwoods and softwoods, as well as the different characteristics of different kinds of
hardwoods, depend on these cell structure types.

Let's define softwoods and hardwoods. The definition is simpler than we might expect, but has nothing to do with
hardness or softness. Cone bearing trees, or conifers, that do not drop their needles (leaves) are generally classed
as softwoods. A few are deciduous--leaf dropping--but by far the greatest number are not. These are the
gymnosperms, a plant grouping that has naked seeds. Hardwoods come from broad-leafed trees, usually
deciduous, but not always (the live oak is an example of an evergreen hardwood--and its wood is very hard and
durable: it is live oak that Old Ironsides, U.S.S. Constitution, is said to be made of), with protected seeds, or
angiosperms. Note that the definitions really have nothing to do with softness or hardness. As a rule, softwoods are
less strong than hardwoods, less dense, and lighter. But balsa is a hardwood, as is tulip poplar in the domestic
varieties, and both are softer than the trees classed as southern pine, and are much less strong, and lighter. Balsa
is the lightest commercially available wood.

Figure versus Grain
For woodworking purposes, the different figures--not grain, but figure is
what you usually see when viewing a board--come from cell arrangements.
Those cell arrangements are determined by type of wood and species of tree,
and always serve the purposes of food movement and mechanical support.
The scan is of flame figured cherry.

Softwood Structure
Softwood structure is simpler than hardwood structure, so it goes first. The primary softwood cell is a tracheid, a
long, hair-like structure, usually about 100 times as long as it is wide. The various species have different length
tracheids; an average cubic inch of softwood can contain some four million of these cells. Texture in softwood is
based on the diameter of the tracheids, with redwood having the largest, and, thus, the coarsest texture. Fine
textured woods have smaller diameter tracheids, often as small as 1/3 the diameter of the redwood (the range is
from 20 to 60 microns). There is a difference, in the same tree, in diameter of tracheids in earlywood and latewood.
The earlywood may be larger in diameter, and the cell walls may also be thinner. The variation from earlywood to
latewood determines the evenness of the grain: in the southern pines, the difference is extreme, often as much as
triple. In other woods, there is little variation. Earlywood erodes at a faster rate (during sanding, after the project is
finished and in use).
Many softwood, or coniferous, species also offer us resin canals, lined with living cells that ooze resin into the
canals. Normally, these canals are found, in North America, in pines, spruces, larches, and Douglas firs. Our
hardwoods don’t have them--but some exotic species do. Resin canals in pines are often large enough to be seen
with the naked eye.

The resins in these canals can create problems. Sapwood resin canals
contain liquid resin, which may eventually work it way to the surface if not
set adequately during drying. Setting needs at least 175 degrees in the kiln,
so air drying can’t do much good with such woods, a point to keep in mind
when home drying softwoods. You must at least finish softwood in a kiln
with the correct temperature. If you’re working up wood for projects of
importance, air dry to 15-18% moisture content, and then let a commercial kiln
finish the wood. For cheaper results avoid softwoods in the four species with
resin canals, all in the Pinaceae family. In other words, don’t use Pinus,
Picea, Larix or Pseudotsuga for projects where bleed-through of resins might create a problem.
Resin, or pitch, is a useful by-product of many trees, and most especially my local southern pines, providing
turpentine and pine tar oil (you’ll find much pine tar oil in dandruff shampoos) among other helpful items.
Softwood rays are usually a single cell wide, but may be 40 times as long, and are just about invisible without a
microscope. Thus, there is no ray figure, or pattern, on quarter-sawn softwood lumber. The above scan shows red
cedar, eastern style.

Hardwood Structure
Now that I’ve over-simplified softwood structure to a fare-thee-well, it’s time to do the same for hardwoods. Here, we
face a real problem; hardwood structures are far more varied than softwood structures. There are several
immediate differences: North American hardwoods lack resin canals--though you’ll find gum ducts in some tropical
exotics. Rays in hardwoods are the most obvious difference in many cases, especially in oaks where quartersawing
produces a pronounced, and lovely, ray pattern. As noted, softwood rays are invisible until one slaps a magnifier on
the wood.To keep this explanation as short as possible, we’ll look at the sap conducting cells for hardwood: these
are called vessel elements, are of large diameter, and have thin cell walls, and no end walls. They’re arranged in
the tree in an end-to-end pattern, thus forming a canal for sap. The smallest thickness cells in a hardwood tree are
the fibers, which have closed ends and thick walls. These provide strength.

The left hand scan is bird's eye maple. The right hand scan is osage orange.

When vessels are cut across their ends, they form pores, so that hardwoods are also classed as porous woods
(softwoods are non-porous). Some hardwoods are listed as diffuse porous, and others as ring porous. Both sound
more complex than they really are: ring porous woods have the pores arranged in concentration in the earlywood;
diffuse porous woods have pores arranged fairly evenly throughout the wood, which makes for a smoother wood.
Oaks are ring porous woods. Cherry is a diffuse porous wood, as are maple, birch, basswood and tulip poplar).
And there are groups of woods that fall between the two extremes, forming semi-ring porous (or semi-diffuse porous,
depending on the writer, the mood and the phase of the moon) woods. The Juglans family, including black walnut
and butternut, inhabits this category.

Pore size is also a gauge of texture, with oak being a rough-textured and cherry, maple and other smaller pored
woods smooth-textured.

Tyloses are a structure that forms in some hardwoods. Tyloses are sac-like parts that appear in the cavities
between vessel elements. Some species have them thickly, as in white oak, and others, such as red oak, and
hickory, have very few. Tyloses obstruct the flow of air or liquids through wood, making species packed with them
much better for making barrels, boats and on, where liquids must be kept in—or out.

Add Complexity
To further complicate hardwood structure, there are other longitudinal cells to go with fibers, though none can be
seen individually. Masses are easily told from surrounding cells, with fibers usually showing up darker, while two
other types, tracheid and parenchyma cells, show up lighter. Of real value to the woodworker only when
microscopic identification of wood is needed, these are covered no further here as individual entities, but these
three types of cell make up hardwood rays.

Small rays may be only a single cell wide, but others may be 40 cells wide and very long (white oak rays to four
inches length are uncommon, but present). Rays are planes of structural weakness, and checks may form along
their planes. They also provide a natural plane for splitting woods, as in shingle making with a froe and mallet.
Getting a smooth surface with rays can be work--though it’s possible, and the result is lovely. The larger the rays,
the larger the problem--chip tear-out relates to ray size. When planing wood with rays, work a bit across the grain,
instead of dead-on the grain, and you’ll get less tear-out. It is also advisable to make sure all planer knives are
freshly sharpened, and cleanly honed.

Figure
Not as in “Go figure” but as in “Sheest, that wood has a lovely grain.” No, it doesn’t. It has a lovely figure.
Confusion that often results from comparing grain and figure, then explaining the way each is formed: figure is the
term used to refer to longitudinal markings of wood. That is, the appearance of a board, viewed flat. There are far
more complex explanations of what it is, and how it got that way, but figure is a flat look at the wood as it was cut--
and it may well be cut in a number of ways.

Flat sawing, or sawing around the board, sees the log flipped after several passes (the log may first be squared by
sawing off a slab on each of four sides), to cut another side, and then flipped again and so on. Through sawing is
also flat sawing, and produces similar patterns, in flat-figure boards for the most part, though there’s also a
combination of patterns in some of the boards.

Quartersawing is most desirable from a figure, and wood stability, standpoint most of the time, but you must give up
something. It is the most wasteful of both time and materials, though you’d never know that from many illustrations. It
takes longer, and there are a large number of small sawmill operators who don’t know how to quartersaw (though
few who will admit not knowing), and even more who don’t wish to be bothered unless heavily reimbursed. Most non-
custom milled wood today is flat sawn. Flat-sawn boards have the characteristic U or V shapes at the board ends.

Hardwood Plywood Slicing
When wood is cut for plywood, the choices expand even more. Most plywood today is not from sawn logs, but from
peeled or sliced logs. Rotary cut, or peeled, logs are mounted on a huge lathe and turned against a very, very
sharp blade. This is the usual type of veneer cutting, and is used, according to Georgia-Pacific in 80 to 90 percent
of all veneers. It produces a very bold figure because it follows the log’s growth rings.
Plain, or flat sliced, veneers start with half a log mounted with the heartwood side flat to the carriage. The knife then
slices off wood on a line parallel to the guide plate, and gives a figure very much like that of flatsawn lumber.
Half-round slicing sees the log mounted off-center in the lathe, which gives a cut slightly across the growth rings, to
combine the figure of rotary and flat sliced veneers. This cut is often used on both red and white oaks.
Rift cutting is used primarily on oaks, and mostly on white oaks at that. The rays mentioned earlier radiate from the
center of the log something like curved spokes in a wheel, and the rift cut goes perpendicular to these rays to give
what is called a comb grain effect.

When we combine the effects of different kinds of cutting with the immense number of variations in wood figures
themselves, we see an incredible diversity. Within the characteristic appearances of each species, or family, of
woods, there are is infinite number of variations on the original theme.

There are bird’s-eye and quilted figures in maple, and flame figures in birch and cherry, and striped figures in
woods with interlocked grain (sweet gums, some elms, among other North American hardwoods), fiddleback figures
in mahoganies and other woods, and onward and ever different figures in burls and crotches.

Knotty Wood Problems
Irregularities in wood, other than figure, create problems. Knots are probably the primary problem we face in finding
wood that does what we want it to do, in both appearance and strength, in a variety of projects, but there are other
things to drive you crazy--sap stain and its blue mess; warp and wane; checking; among others.
There are times when you will be able to use defects of most types to either reduce the cost of wood, or to enhance
the appearance of a project. That includes some knots, though not nearly all. Mostly, we work around knots and
other defects.

Commercial hardwood is graded as if every knot is a defect, which, for small, tight knots, pin knots, and some spike
knots, is to your advantage. A knot is nothing more than a part of the limb that got embedded in the tree when the
base of a branch was enclosed in the growing trunk. Eventually, surrounded material forms a tight knot, and, if
there are years of growth added to the first stub, the knot is encased. This may become a loose knot, a defect even
less desirable. Knotholes crop up when loose knots drop out of the board. Spike knots are formed by the way a
board is cut---knots that extend across the face of a board become spike knots, usually only found in boards that
made by radial sawing. Flat sawing produces round knots from the same defect. Pin knots crop up in many places,
are smaller than a quarter inch in diameter, and seem to me to always come in groups of three or more.

The left hand scan shows pin knots in white oak. The right hand scan shows a spike knot in sycamore.