Appendix 8: The newbie woodworker’s guide to the anatomy and physiology of wood

A work in progress 6-22-07

 Details! Details! Details! So much to learn about Wood, Its Anatomy, Its Characteristics, Its Folklore.

Hardwoods. Softwoods.

Native. Introduced. Species. Plantation. 

Straight Grain. Curly Grain. Vertical Grain. "Bird's Eye"

Spring Wood. Summer Wood. Heart Wood.

Seasoning. Flat Sawn. Rift Sawn. Quarter Sawn.

End Check. Wane (bark). Warp. Wind.

Rip Cut. Cross-cut. Miter cut.

Knot, Burl. Branch. Crotch.

Veneer. Plank

The simple truth: If you wish to succeed as a woodworker, you need to understand something about the characteristics of wood.  Fortunately, however, start where you are now, with little or no understanding, and -- through the experience of woodworking yourself -- build up an understanding of wood.

Sources: Mario Dal Fabbro, How to Build Modern Furniture. New York: F W Dodge Corp., 1951; Willis Wagner, Modern Woodworking: Tools, Materials, Procedures, South Holland, Il: Goodheart-Willcox, 1974; Elbert L. Little, The Audubon Society Field Guide to North American Trees: Western Region New York: Knopf, 1979; Ernest Joyce,  Encyclopedia of Furniture Making New York: Sterling, 1979, chapter 9;  R. Bruce Hoadley, Identifying Wood: Accurate Results With Simple Tools Newtown, Ct: Taunton, 1990; R. Bruce Hoadley,  Understanding Wood: A Craftsman's Guide to Wood Technology Newtown, Ct: Taunton, 2000. 

(Note: Think of this page as a sort of catchall location, the repository for the technical terms about woodworking that "tend to fall through the cracks", so to speak. Much more can be added; for example, woodworker's manuals by Charles Wheeler (1904) and Paul Hasluck (1905) discuss these same topics, which tells us that the anatomy and physiology of wood has known for numerous generations of woodworkers. While I personally am not determined to trace these "scientific" facts back to their discovery in the laboratory, I do seek to find out when these details about wood's properties were injected into woodworker's manuals.  

Finally, for this discussion, before the “wood” is harvested, ie, the wood is a living tree, I will refer to the material as a “tree”. In discussing the tree after harvest, I use the term “wood”. And since I live on the West Coast, my descriptions of trees show a Western bias.)

Rhetorical Question: What is Wood? Simplistic Answer: The hard, fibrous inner tissue of the trunk and branches of a tree or shrub.

A Tree’s Structure and Growth

Structurally a tree consists of a Trunk, Branches, Roots and Leaves. Its Trunk is the major woody "stem", the tree's "output". Structurally, a tree also consists of long narrow tubes or cells (called fibers or tracheids), each about the size a human hair. The length of these cells vary from about 1/25” in hardwoods to approximately 1/8” in softwoods. Tiny strands of cellulose make up the walls of the cells, held together with a natural cement called lignin. (See the Wood Magazine image below.)

(What is a "hardwood"? What is a "softwood"?  Strangely, whether a wood is classified a hardwood or a softwood has nothing to do with its actual hardness or softness. Instead, this classification scheme is derived from the tree's "leaves"; i.e., is the tree deciduous, meaning does it grow and shed a set of leaves annually -- meaning that the tree is "leafless" for part of the season. Or,  is the tree an "evergreen", meaning does it retain its "leaves" season after season.

Softwoods: Trees that have needle or scale-like leaves and are ever-green for the most part, with Cypress, Larch, and Tamarack being exceptions. The term has no reference to the actual hardness of the wood. Softwoods -- often referred to as conifers -- botanically they are gymnosperms.

Hardwoods: Timber obtained from broad-leaved, flower-bearing trees. Hardwood trees are deciduous trees; the exceptions prevail in the warm regions. As a  classification term, hardwood designated such hard European woods as Beech and Oak but also includes some of the softest of woods. Included in the category are Ebony, various Mahogany, Maple, Teak, and American Black Walnut.) 

A tree's growth takes place at the tips of the roots, the leaves, and the cambium layer of cells just under the tree's bark. Water is absorbed by the roots, travels through the sapwood to the leaves, and combines with carbon dioxide from the air.

(Bark: The outer covering of a tree's trunk and branches, and described as "corky", as in oak, "papery", as in birch, or "leathery", as in Douglas Fir or Western Red Cedar.) 

(Knot: Branch or limb embedded in the tree and cut through during lumber manufacture.)  

Through photosynthesis, sunlight changes these elements to food (carbohydrates) which is then carried back to the various parts of the tree.

Lignin: Technically, a complex oxygen-containing organic compound, a mixture of polymers of poorly known structure. "After cellulose, it is the most abundant organic material on earth, making up a quarter to a third of the dry weight of wood". Removed from wood pulp in the manufacture of paper, its wide use includes  as a binder in manufactured  building material such as Particleboard and Medium Density Fiberboard; also as a soil  conditioner, "filler" in the chemistry of some plastics, "adhesive" for linoleum. Lignin is (1) a substance unaffected by water, common chemical solvents, or heat,  (2) a cellular structure that makes it possible to drive nails and screws into the wood, and (3) accounts for the lightweight, low heat transmission factors and sound absorption qualities.

New cells form in the cambium layer image ?  The inside area of the layer (xylem) develops new wood cells while the outside area (phloem) develops cells that form the bark. 

The growth in the cambium layer takes place in the spring and summer and forms separate layers each year. These layers are called annual rings, image ? . 

In most woods the annual ring is composed of two layers; springwood and summerwood.

In the spring, trees grow rapidly and the cells produced are large and thin walled. As the growth slows down during the summer months the cells produced are smaller, thicker walled and appear darker in color. image ? . 

The change from springwood to summerwood may be either abrupt or gradual depending on the kind of wood and growing condition. In such woods as maple, basswood and poplar there is little difference in the cells formed, while in oak, ash, and southern pine the difference is pronounced.

(HEARTWOOD: The wood extending from the pith or center of the tree to the sapwood, the cells of which no longer participate in the life processes of the tree.) 

These annual growth rings are largely responsible for the grain patterns that are seen in the surface of boards cut from a log. In tropical climates, generally the growth rings of woods grown in these areas are not as easily defined. The growth of the tree is controlled more by wet or dry seasons than temperature changes.

Sapwood contains living cells and may be several inches or more in thickness. Fast growing trees usually have a thicker layer. The heartwood of the tree is formed as the sapwood becomes inactive and usually turns darker in color because of the presence of gums and resins. In some woods such as hemlock, spruce and bass-wood there is little or no difference in the appearance. Sapwood is as strong and heavy as heart-wood but not as durable when exposed to weather. 

Wood Cells
 

The wood cells or fibers -- that make up the structural elements of the wood -- are of various sizes and shapes and grow firmly together.The strength of the wood depends on the thickness and structure of the cell walls rather than the cell size.

When magnified, a section of wood looks something like a honeycomb.(The image on the left is from WOOD Magazine's special interested publication, "Projects and Shop Tips ", 2006, page 149. Image is used with permission from WOOD magazine.)

 

Most of the cells run along the length of the wood; however, some run at right angles or radially to the center of the tree. These cell groups conduct sap across the grain and are called wood rays or medullary rays. In most kinds of wood they are small however, in oak and sycamore they are quite large and noticeable, especially when the wood is quartersawn.

In addition to the regular cells and those in the wood rays, hardwoods contain some relatively large cells called vessels that provide main arteries in the movement of sap. Still other cells called Parenchyma, are found in both hard and softwoods, and function mainly for the storage of food.

These various cells, which differ in size, shape, and arrangement, along with deposits of resin and other coloring matter, all add together to provide interesting and attractive grain patterns and textures.  

Seasoning of Wood: [in Glossary]
 

Moisture Content and Shrinkage -- See also Moisture Meters and Seasoning of Wood.
 

Before wood can be used commercially, a large part of the moisture (sap) must be removed. When a living tree is cut, more than half of its weight can be "moisture". The heartwood of a "green" birch tree has a moisture content of about 75 percent. Most cabinet and furniture woods are dried to a moisture content of 7 to 10 percent.

The amount of moisture or moisture content in wood is expressed as a percent of the oven-dry weight. To determine the moisture content a sample is first weighed and then placed in an oven and dried at a temperature of about 212 to 220 deg. F. The drying is continued until it no longer loses weight. The sample is weighed again and this oven-dry weight is subtracted from the initial weight. The difference is then divided by the oven-dry weight.

Moisture is contained in the cell cavities (free water) and in the cell walls (bound water). As the wood is dried, moisture first leaves the cell cavities. When the cells are empty but the cell walls are still full of moisture, the wood has reached a condition called the Fiber Saturation Point. This is about 30 percent for nearly all kinds of wood.

Fiber Saturation Point

The stage in the drying or wetting of wood -- where the wood's cell walls are saturated and the cell cavities are free from water -- is known as the fiber saturation point.  This point is at the 30% moisture content, based on oven dry weight; below this point is where shrinkage occurs. In other words, the fiber saturation point is important because wood does shrink until this 30% point is reached. As the moisture content is reduced below 30%, and moisture removed from the cell walls, the wood's cells become smaller. For example, with a 1% moisture loss below the fiber saturation point, the wood will shrink about 130th in size. When dried to 15% moisture content the wood is reduced by about one-half the shrinkage that is possible. What this means is that a plain-sawn birch board that was 12" wide at 30% moisture content will mea­sure only about 11" wide at 0%. In general hardwoods shrink more than softwoods.

 

That wood shrinks most along the direction of its annual rings (i.e., tangentially or crosswise) and about one-half as much across these rings, but shrinks very little in its length has always intrigued me, that is, until learning about a wood's cellular shapes. How this shrinkage affects wood cut from a log is shown in the image above and in the image on the left. The image on the left comes from  Charles Gardner Wheeler, Woodworking for beginners : a manual for amateurs New York : G. P. Putnam's Sons, 1904, and -- as crude as it looks -- shows more clearly than my prose how wood "shrinks".
 

Types of Cuts and Sections of Tree From Which Various Figure Patterns of Wood Come From [not entirely happy about the quality of this image of a tree trunk; will, however, be adding better quality images of "crotch", "rotary-cut veneer", and so on. from here to the end is definitely under construction 2-18-07]  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Types of Trees 

Species: Following Elbert Little in the Audubon Society's Guide, in its broader, meaning, "species" connotes "a population of plants or animals that interbreed with one another, but are not capable of interbreeding; with members of other populations".