Understanding Wood - 2nd Edition
A Craftsman's Guide to Wood Technology

The first edition of Understanding Wood was published in 1980 and has since sold more than 130,000 copies. It is widely held as a definitive reference work and the cornerstone of every woodworker's library. Now, Bruce Hoadley's comprehensive guide to wood technology has been revised and updated in this 20th-anniversary edition of a classic.

New information on composite materials, adhesives, and finishes brings this book into the 21st century, while more than 300 photographs bring important visual information to life. This edition covers the nature of wood and its properties, the basics of wood technology, and the woodworker's raw materials. Understanding Wood was written for woodworkers by a scientist with a love of woodworking. It will be sought after by craftsmen and collectors alike.

"The truly serious woodworker needs a knowledge of his material and the best place to get it is in Bruce Hoadley's Understanding Wood."
-- UPI, WIRE, Chicago Sun-Times

"Not one craftsman in 100 could know a quarter of the information Hoadley has collected in this handsomely illustrated volume."
-- American Forests

Introduction:

First, I am flattered and humbled that the first edition has been so well received. Apparently it has found shelf space with woodworkers of all sorts from carvers and turners to cabinetmakers and builders, both the novice and the expert. Even those whose primary concern is not woodworking or even wood itself, such as architects, engineers, museum professionals, and forensic scientists, have found something useful in it. It is, after all, the wood itself, not what we each do with it, that is the mystique.

Another great impression is the wonderful people I have met who share the fascination of wood, and the breathtaking accomplishment of their work. I have long marveled at the achievement of some of the European carvers and furniture builders of the 17th and 18th century and once believed that such work would never see an equal. But I now look with astonishment at the works displayed at trade shows and association annual meetings, in the pages of Fine Woodworking and current books, demonstrating an extraordinary level of skill. Certainly advances in woodworking tools, equipment, and materials have contributed, but the skills and perseverance of modern craftsmen cannot be denied. I have always enjoyed woodworking myself, but I have succeeded only to the "jack-of-all-trades" status and can only dream of the work I see rolling off the benches of others. I like to think that Understanding Wood may have helped here and there. I am proud just to be in their company.

A third thought was ever present as I considered the second edition: The properties of wood are still the same with the corollary that the same things that caused problems 20 years ago are still the main troublemakers today. Wood-moisture relationships are still the woodworker's arch-enemy. I have therefore tried to maintain a focus on moisture-related properties and problems.

What, then, is new and different in this second edition? First, converting black and white illustrations to color wherever possible should improve both understanding and enjoyment of the subject matter. In the chapter on wood identification, for example, the gallery of color macrographs should make the comparison to pieces of wood much more meaningful. Most important is the camera magic of Randy O'Rourke, whose color photography will please the reader from cover to cover.

Although wood itself has not changed, the availability and forms have. The second edition has new sections and commentary on modern products, particularly composites that were just evolving as the first edition was written. As predicted, engineered wood products have become a routine fiber-based commodity. With care, woodworkers and builders can appropriately incorporate these new materials into their work with functional and aesthetic success as well as economy, for we have now passed through the era where "solid wood" was the only mark of integrity. The changing availability of wood, together with the development of newer chain saws and portable mills, has developed a new incentive for going directly to the tree for material, opening a new approach to enjoy woodworking. "Shopping" for wood has also taken on some new angles, and I have added some discussion of the new markets for wood and wood products from the old familiar lumber yard to the internet.

As the bottom line, there is one thing that I think will never change -- the unlimited enjoyment and satisfaction that await those of us addicted to wood.
-- R. Bruce Hoadley, April 2000

Preface to the First Edition
The properties and characteristic qualities of the timber available are so numerous and important, and yet so little understood generally, that I am induced by solicitations of many friends to give, in these pages, information respecting them.

A handy-book on timber is, in the opinion of many, much required. The botanical treatises which are accessible are too strictly scientific in their form and treatment to interest the general reader, and they lack that practical application of knowledge to the wants of the shipwright and carpenter, which is one of the aims of this book to give.

I wish I had written those words, for they summarize perfectly the reasons why I have written this book. But they were, in fact, written in London in the year 1875 by Thomas Laslett, timber inspector to the Admiralty of the British Empire, in his book, Timber and Timber Trees.

Like Laslett, I have written my book for woodworkers, but this is not a book about woodworking. Rather, it is about wood itself, surely mankind's first workable material, and an ever-present part of our ever-changing world. We are not likely to run out of wood, because unlike most other materials, we can always grow more. And the more we learn about it, the more there is to know. A look at what else Laslett wrote in his book will make this clear.

Laslett believed, for example, that sap collected between the bark and the wood eventually congealed to form a new growth ring. He could not know about cellular reproduction and the additive formation of new wood cell by cell. Laslett deduced that trees grew taller because the bark squeezed the sap, forcing it upward. Today, with a microscope, we can see cells building sideways in the cambium layer, and twigs growing longer by cellular division. Lest we be tempted to smugness, however, we should imagine how primitive our scientific knowledge might look a hundred years from now.

The main reason I have attempted this book is the realization that a wealth of knowledge about wood has been accumulated by scientists, but almost none of it has been translated and interpreted for the individual craftsman. Working from scientific principles, technology is routinely developed by commercial and academic agencies but mainly shared among themselves in highly technical textbooks and obscure journals. The technology of industrial-scale woodworking has been well developed and widely published. But the same scientific principles have yet to be applied to the small-scale woodworking shop, whether that of the serious amateur, the independent cabinetmaker, or the artist/craftsman. For example, volumes have been written on how to dry lumber in carload quantities, yet it is almost impossible to find guidance when drying the boards cut from a single tree, or when drying a single board.

Much of what has been written about the craft of woodworking is reduced to sets of instructions of directions relative to tools and procedures, without any supporting information about the material itself. These how-to books commonly assume that all conditions relative to the wood are under control, or else they dismiss the point with an airy instruction such as, "Get some suitable hardwood of appropriate quality and dryness." Yet, for a person to pursue a craft with success, knowledge and understanding of the material must develop along with manual skill.

I also feel compelled to attack the mountains of misinformation available and commonly accepted by woodworkers. Most of it comes innocently from the misinterpretation of observations. For example, one book on sculpture states that bright light makes wood check, a conclusion reached when cracks appeared in wood brought up from a dark (and probably damp) cellar into daylight (and drier air). Another says that wood cups because its annual rings try to straighten out as it dries -- a correct observation of the direction of cupping, but pure guesswork as to its cause. Tradition carries along such misleading terms as "dry rot" and such misconceptions as "wood has to breathe." Dry wood will not decay, nor does wood breathe in the animal sense. Wood doesn't eat either, and it doesn't require feeding with furniture polish.

My strategy has been to begin with the tree, to examine the wood as the cellular product of the tree's growth. I have given special attention to wood-moisture relationships and dimensional change before going on to physical properties, strength in particular. Then I have tried to analyze such everyday woodworking operations as machining, bending, joining, and finishing in terms of the wood's physical and biological nature. It is my hope that the examples I have given can serve as models for readers in analyzing problems that arise in their own woodworking endeavors.

I also hope that this book will encourage craftsmen to delve further into the literature of wood science and technology, and to help them do so I have included an annotated list of references. Frequently, what makes technical literature opaque to the layman is its terminology. Therefore, against the wishes of some who would have me avoid scientific terms and "say it in simple words," I have tried to present and explain the standard terminology throughout the book, and I have included a detailed glossary. The serious woodworker will find that it is important to know that rake angle and hook angle have nothing to do with gardening or fishing, and that terminal parenchyma is not a horrible disease.

I suppose every author wonders when a book actually began -- for me, it goes back more than 50 years. I grew up in the Connecticut countryside where the surrounding woodlands were both playground and the source of material for "making stuff." The cellar of our house had an old workbench and chests of grandfather's tools, worn from years of use but begging for the chance once again to work miracles in wood. My earliest memories include climbing a wobbly stool to get on the workbench to turn on the light, the screech of the huge square-threaded screw of the vise, and lifting the heavy lid of the toolchest to stare at the mysteries within. I recall more trouble than triumph from my early years of woodworking experiments. I remember nails that bent over when driven into oak, saws that bound up tight in green wood, screws twisted off when driven without pilot holes, and planed surfaces ridged by nicks in the iron. But I still "made stuff" -- my frustrations were nothing compared to my fascination with wood.

I'll never forget the first time I saw a chain saw in operation. It was at a late-summer farm-equipment demonstration, when I was in my early teens. The farmers and loggers all watched in amazement as the saw bar melted through a 12-in. oak log in a matter of seconds, effortlessly taking slice after slice. It was quite a machine.

But I was not watching the raucous machine. I stood transfixed by those marvelous disks of wood, a dozen or more, that lay in the grass and sawdust. The demonstration over, the entire crowd followed the saw operator back to his table to learn more about the machine.

Except me. I was excitedly stacking up as many of the wooden disks as I could carry. They were red oak, creamy sapwood and medium brown heartwood, just tinged with peach. As I staggered through the goldenrod toward my father's car, they were unbelievably heavy. I still recall the vivid pattern of the rings, their pie-crust of bark, the cool dampness of the top disks under my chin, the pungent odor of the wood. The aching in my arms was a tiny price to pay for the unending array of things I would make from such a magnificent product of nature: lamp bases, clock faces, desk sets, picture frames. I could not believe that woodworkers had not already put this beautiful natural log to better use.

My next recollection is of having the disks safely home and proudly lined up along the wall shelf in our cellar workshop. I probably realized that some sort of drying would be necessary, but that could wait. I was content just to admire my treasure.

I am sure you already know the sad ending. By the following morning, the brilliant end grain had faded to a lifeless sandy color. In the days that followed, my castle of hope crumbled as the first few hairline cracks in the sapwood grew and reached toward the pith. Soon each disk had a gaping radial crack. In final mockery, even the bark fell off.

But why?

It is easy for me to believe that this one incident was the turning point that eventually led me to pursue my career as a wood technologist, for ever since I have wanted to know why the wood does what it does. I have been lucky enough to make some progress in my quest, and I have been able to share in the classroom some of what I have learned. Now I hope my book helps craftsmen understand wood, too.
-- R. Bruce Hoadley, April 1980

Table of Contents:

Part One: The Nature of Wood and Its Properties

1. The Nature of Wood
Growth rings
Grain
Sapwood and heartwood
Structural arrangement of growth rings and rays
Density and specific gravity
Systematic classification
Cellular structure
Softwoods
Hardwoods

2. Figure in Wood
Knots
Abnormal wood
Fungi
Insect damage

3. Wood Identification
What to look for
Physical properties
Identification techniques
Macrophotographs

4. Strength of Wood
Compression parallel to the grain
Compression perpendicular to the grain
Tension perpendicular to the grain
Tension parallel to the grain
Shear perpendicular to the grain
Shear parallel to the grain
Bending theory
The carrying capacity and stiffness of beams
Factors affecting strength properties
Compression failures and brashness
Structural grades

5. Other Properties of Wood
Thermal conductivity
Effect of temperature on wood
Burning of wood
Fluorescence
Psychological properties

6. Water and Wood
Free water and bound water
Equilibrium moisture content
Green vs. air-dried vs. kiln-dried
Dimensional change in wood
Estimating shrinkage and swelling
Uneven shrinkage and swelling

Part two: Basics of Wood Technology

7. Coping with Dimensional Change in Wood
Preshrinking
Control of moisture sorption
Mechanical restraint
Chemical stabilization
Design
Monitoring moisture
The moisture "widget"

8. Drying Wood
How wood dries
The dry kiln
Drying your own wood
Storing lumber

9. Machining and Bending Wood
Machining wood
Bending solid wood

10. Joining Wood
The elements of joints
Basic types of joints
Worked joints
Fastened joints

11. Adhesives and Gluing
Adhesive joints
Gluing fundamentals

12. Finishing and Protecting Wood
Surface condition
No treatment
Coating treatments
Penetrating finishes
Combinations and compromise
Slowing moisture exchange
Evaluation of finished surfaces
Preservative treatment of wood

Part three: The Woodworkers Raw Materials

13. Lumber
Lumber measure
Lumber classification and grading

14. Veneer and Plywood
Plywood
Classes of plywood

15. Composite Panels
Particleboard
Wafer- and strand-based panels
Fiber-based panels

16. Engineered Wood
Finger-jointed lumber
Glulam
Structural composite lumber
I-joists

17. Finding Wood
Trees
Recycling used wood
Local sawmills
Lumberyards
The woodworkers retail outlets
Industrial arts teachers
Magazines
Specialty woods
The Yellow Pages and other listings
Internet
Lumbermen

Afterword: Forests past and future

Appendix 1: Commercial names for lumber

Appendix 2: Finding the specific gravity of wood

Glossary

Bibliography

Hard-cover, 8-1/2 x 10-7/8 in., 288 pages, with color photos and drawings
Published 2005

Book ISBN: 978-1-56158-358-4