BOOK REVIEW:
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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