Series editors' preface - Wood Tools

The strength of wood in compression perpendicular
to the grain – in particular, the elastic
strain limit – is important to the performance of
joints such as mortise and tenon and dowel
joints. Racking loads may concentrate excessive
compression loads on mating parts of joints, but
in addition, self-induced compression set resulting
from restrained swelling under variable
moisture conditions is a major cause of joint
failure. In evaluating the failure of furniture, it
is rare that components break in two; it is more
common that objects seem to simply fall apart,
indicating that the limiting strength was related
to joints. In turn, the root of the failure might
well be attributed to the manner in which the
object or its joints were designed or fabricated,
rather than to the strength properties of the
wood per se. The nature and properties of various
forms of furniture construction are discussed
below and their deterioration and failure
are discussed in Chapter 7.
2.6 Manufactured timber products
In addition to traditional forms of solid wood,
mention must also be made of the many forms
of manufactured wood products. These are
based on veneers, on veneers combined with
solid wood and on reconstituted wood particles,
fibres, flakes and chips. A concise guide to these
materials is given by Schniewind (1989). Their
production and properties are discussed by
Hoadley (1980) and more comprehensively
reviewed in the United States Department of
Agriculture Handbook of Wood and Wood-based
Materials (Forest Products Laboratory, 1989).
The use of these materials in the production of
furniture is reviewed by Hanks (1981) and the
chemistry of wood polymer composites is discussed
by Meyer (1984). Deterioration of these
materials in relation to their production and use
in furniture is discussed by Klim (1990), who
also reviews conservation treatment.
2.6.1 Veneers
Veneers, thin sheets of wood cut with the grain
parallel to the surface, can vary in thickness,
before use, from 0.25 mm up to about 6.4 mm.
All decorative veneers produced in Europe are
cut into 0.6–0.9 mm for face quality, average
thickness 0.7 mm (1/40 in). In the United States
Wood and wooden structures 87
and Australia most veneers produced are cut
0.9 mm (1/28 in). They are produced by three
basic methods, sawing, rotary peeling and slicing.
Those of high value or striking appearance
are used for surface decoration but veneers are
also used structurally to make plywood boards
and to make bent laminated and moulded
shapes that could not otherwise be achieved in
a single thick piece of wood without chemical
modification. The original method of production
by sawing has been replaced in all but a
very few cases by knife cutting for commercial
production. Logs intended for slicing are cut
open and examined for figure, grain, texture
and defects before cutting into suitable sections
for veneer production. The halved, quartered,
or flitched log sections are then usually
steamed or soaked in hot water for one to several
days before cutting. Logs intended for
rotary peeling are treated in the round.
In rotary cutting, the log is held at either end
in a large lathe and rotated against the knife,
which is moved forward at each rotation to
preserve the thickness of the cut. This method
of cutting is specially suited for high volume
production of large sheets of veneer used to
make plywood. However, it is also used in one
or other of its five basic variations to produce
many freak figured decorative veneers, such as
bird’s eye maple, figured betula and masur
birch, that cannot be extracted by any of the
other cutting methods.
In slicing, a flitch of wood held in a frame is
moved against a knife. Four different methods
of cutting and mounting the flitch are used to
extract the best figure. Veneer slices are kept
in sequence during subsequent drying operations
so that the sequence of a figure from one
leaf to another is preserved. They are usually
sold in this form as a flitch that may subsequently
be broken up and sold in books or
individual leaves or sheets.
Whether rotary peeling or slicing is used, the
cutting action is essentially similar in both. As
the knife separates the veneer from the flitch,
the separated layer of wood is severely bent
and stresses build up in the region near the
knife edge. When these stresses exceed the
strength of the wood, failure occurs resulting in
a series of knife checks across the side of the
veneer that was against the knife. This side of
the veneer is called the loose side and the side
away from the knife is called the tight side. By