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Understanding Wood , by R.Bruce Hoadley

by J.Norman Reid
Delaplane, VA

This is a book I've owned for several years, during which I've very much wanted to read it. But it was not until after I dug into it that I realized how much I actually needed to read it. This is not merely a useful book for woodworkers; it's an essential book from an authoritative source. Hoadley has degrees in forestry and wood technology, teaches at the university level, and is a woodworker himself. His scientific background has led him to compile this comprehensive reference work on the properties of wood and how they relate to wood's uses in woodworking. His efforts have resulted in a book of enormous benefit to woodworkers.

Chapter 1 discusses the nature of wood, beginning with how trees grow and why that matters for woodworkers. Included are useful reviews of how wood is sawn into lumber and how the cell structure of different species affects their characteristics. Here, and throughout the book, Hoadley clarifies often confusing terminology about wood; in this chapter he focuses on the parts of trees and features of wood. I was surprised, for instance, to see how many definitions of the common term "grain" there are and the importance of clear distinctions to understanding just what is meant by the term.

Chapter 2 concentrates on the many types of figure in wood, how the way wood is cut affects figure, the various types of knots and when they are acceptable in use, abnormal wood—juvenile, reaction and compression, and the effects of insect damage and fungi on figure. A helpful table on page 44 compares species in terms of their resistance to decay.

Chapter 3 considers the identification of wood species. Hoadley rejects the idea that "looks like" is sufficient for identifying wood. Instead, he argues that a study of wood's anatomical features, especially using a microscope, is necessary for definitive identification of species. He describes a simple kit that can be used and what to look for, including—appropriately—a tree diagram with branches that spell out decision points in the identification process. The chapter concludes with macro-photos of a number of common species of domestic and imported woods.

Chapter 4, on the strength of wood, is possibly the most important chapter in the book, but also the most difficult to absorb. This complex subject deserves, and gets, detailed treatment. He compares stress—the load on an area of wood—and strain—the amount of deformation that occurs from the original length of the wood—and then defines strength as the ability of wood to resist stress. A valuable table on page 79 compares the strength properties of commonly available species. The chapter continues by considering bending wood, the beam capacity of wood, and factors that affect wood's strength, including moisture, time under load, temperature, cross grain, grain variability and localized defects. The chapter concludes with discussions of compression failures and the strength of structural grades of wood.

Chapter 5 reviews other properties of wood, including thermal conductivity and why wood feels warm to the touch, expansion due to temperature changes, burning and fluorescence under ultraviolet light—a property I did not know existed in wood. A table on page 106 compares the fluorescent properties and colors of different species. The chapter concludes with a brief consideration of the psychological attractiveness of wood.

Chapter 6 considers the hugely important subject of wood and water. As we should all know already, moisture content affects the dimensions of the lumber we use. Hoadley compares free water vs. bound water in the wood's cell walls, reviews the concept of equilibrium moisture content (EMC), compares the moisture content of green wood in heart and sapwood in different species, and the shrinkage of various species in a table on page 117, along with an important discussion of how to estimate shrinkage. The chapter concludes with a review of different types of uneven shrinkage, or warping.

Chapter 7 addresses alternatives for coping with dimensional changes in wood. These include preshrinking, using mechanical restraints such as banding and chemical treatments. The most important consideration, though, is design. Hoadley goes on to the subject of monitoring relative humidity in the shop and home and the use of moisture meters. A useful addition shows how the woodworker can easily make a "moisture widget" to monitor the dimensional changes in a particular species that is being used in a current project.

How wood is dried affects its quality; that is the subject of Chapter 8. A helpful figure shows how wood dries from the outside in and how poor drying can lead to defects. He discusses commercial kilns and how they are constructed and operate and how the woodworker can dry his or her own wood. A table on page 156 compares estimated drying times for different species.

Chapter 9 addresses machining and bending wood. It reviews cutting edges and their properties, planing wood and milling it by machine, grain direction, a comparison of types of wood chips, the importance and effects of blade sharpness, and a brief discussion of sharpening. The chapter concludes by discussing bending green wood, steam bending and plasticizing wood with ammonia.

Joining wood is the subject of Chapter 10. The key concerns in joinery are the stress system involved, the direction of the grain, anticipated dimensional changes and the condition of the wood's surface. Useful discussions of both worked joints, such as mortises and tenons and dovetails, and fastened joints using screws and nails, follow.

The next chapter reviews adhesives and gluing and considers the effects of clamping time, pressure equalization on the joint and glue shelf life on the stability of the resulting joint.

Chapter 12 gives a brief treatment on finishing and protecting wood. The options presented are giving no treatment to the wood—often appropriate for carved figures such as Hoadley creates—and coatings of various kinds. He concentrates on penetrating finishes which have the advantages of easy application and avoidance of dust effects but at the same time highlight any imperfections in surface preparation. He briefly discusses finishing to slow moisture exchange and provide preservative effects.

Chapter 13 considers wood for lumber purposes and offers useful clarification of terminology as well as tables of standard lumber sizes, classifications and grading standards. Chapter 14 addresses veneer and plywood, how it's cut, knife checking and how to identify it, the classes of plywood and their relative strengths. Chapter 15 describes composite panels, including the types of particle board and their strengths.

The next chapter describes engineered wood, such as finger jointed wood and glulam—glued-laminated lumber. I was interested to learn that laminated veneer lumber (LVL) has more consistent quality than standard lumber. The final chapter, on finding wood, assesses cutting and processing your own—which he cautions against—recycling used wood, local mills and lumber yards. He offers advice on sources of information about finding wood supplies. Helpful appendices give the commercial and scientific names for common species and rules for finding the specific gravity of wood. A glossary of terms, bibliography and index complete the volume.

Understanding Wood is an essential resource for woodworkers and should be in the libraries of all who intend to do serious work. It is a comprehensive reference tool on the properties of wood and the tables and charts are indispensable for understanding how the species we use are likely to behave. Make no mistake; some sections of the book are not an easy read, especially if you choose to work through the formulas for calculating wood's properties. But you can also skim those sections and still come away with Hoadley's principal conclusions. And like me, you may learn some surprising things about this medium in which we work. I highly recommend this book.

CLICK HERE to order your copy of Understanding Wood

The author is a woodworker, writer, and woodworking instructor living in the Blue Ridge Mountains with his wife, a woodshop full of power and hand tools and four cats who think they are cabinetmaker's assistants. He is the author of the forthcoming book Choosing and Using Handplanes . He can be reached by email at nreid@fcc.net .

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