Series editors' preface - Wood Tools

88 Conservation of Furniture
using a pressure bar to restrain the veneer as
it is cut, it is possible to substantially reduce
this problem and in some cases to eliminate it.
The extent to which this is possible is determined
by the species, by the thickness of cut,
by the setting up of the machine and by the
temperature of the wood. Diffuse porous hardwoods
with well-distributed rays, such as
birch, are more likely to yield tight veneers
than ring porous hardwoods such as oak.
Holding a piece of veneer with one edge in
each hand and flexing it will usually reveal
tight and loose sides; veneers feel stiffer when
flexed to close the checks and more limp when
they are flexed open. If the veneer feels the
same in both directions it is probably tight.
Knife checks often show up months or years
after the veneer has been laid, as parallel-tograin
cracks through the finish. Whenever possible
therefore the tight side (also called the
face side) should be laid face up and great care
should be taken not to sand through it during
finishing. A comprehensive review of all
aspects of the preparation, selection and use of
veneers is given by Lincoln (1984).
2.6.2 Plywood and related materials
Plywood consists of layers of thin veneers or
plies glued together under huge pressure with
the grain directions at right-angles in each successive
layer. This allows the production of
large sheets of dimensionally stable material in
which most properties are approximately equalized
across the surface with the strength of
wood parallel to the grain predominating.
Shrinkage and swelling are very small, being
about 0.18% along the grain and 0.27% across it
over the range of 7–20% moisture content
(Joyce, 1970). Normally an uneven number of
plies is used to give a balanced construction but
even-numbered plywoods, in which the grain of
the central pair of veneers is parallel, are made.
Plywood is highly resistant to splitting parallel to
the face grain but splits relatively easily in its
thickness. Although cross-ply laminations of
wood are known from antiquity, plywood made
from thin veneers and used as a substitute for
solid wood is essentially a modern material that
owes much to the development of economical,
moisture-resistant adhesives.
Where one outermost ply is of better quality
than the other, the better-quality veneer is
referred to as the face and the other as the
back. Where they are of equal quality, both are
referred to as faces. The central ply is referred
to as the core and in plywoods with more than
three plies those lying immediately beneath the
face and back are referred to as cross bands or
cross banding. Manufactured boards in which
the central plies are replaced by particle board
or wood core strips glued together side by side
typically retain only the face veneer and cross
banding, or less commonly the face veneer
only. Examples illustrated in Figure 2.22
include blockboard, in which the core strips
may not exceed 25 mm, and laminboard, in
which the strips must not exceed 7 mm wide.
Plywood is graded according to the strength
and stiffness of the face veneers, the quality of
the face veneers including the size and num-
(a)
(b)
(c)
(d)
(e)
Figure 2.22 Examples of manufactured board
construction: (a) three ply (thicker veneer core); (b)
multiply (seven-ply); (c) laminboard (veneered outer
layers, laminated core); (d) blockboard (veneered outer
layers, blocked core); (e) five ply particle board core