Series editors' preface - Wood Tools

308 Conservation of Furniture
problematic since the primary wood has
reduced dimension in comparison to the
unchanged inlay.
7.3.3 Loose and broken joints
Loose and broken joints can occur when joints
are overloaded or are forced to take loads for
which they were not intended, for example
tension or racking, especially in chairs. Mortise
and tenon, and dowel joints in chairs are
among the most common joint failures while
joint failure in carcases is much less common.
Moisture cycling of joints, especially those of
the mortise and tenon type, is a well-recognized
form of failure that both contributes to
and is exacerbated by glue failure (Hoadley,
1980). Although glue failure can cause joints
to become loose it should be remembered that
not all joints will have been glued originally.
Glue failure may be the result of old or
overcooked glue in the original assembly or
the result of glue being allowed to gel on the
surface of the wood before the joint was
closed. Glue also can be broken down by
biological degradation of the proteins, or by
extreme dry conditions causing the glue to
become brittle. The nitrogen in animal
adhesives provides a source of food for
woodworm and attack by this insect is
frequently concentrated in the region of joints.
The initial quality of fabrication of the joint
will contribute to the speed at which it deteriorates.
A joint that was poorly fitted in the first
place, and therefore required a thicker glue
layer, will lose its strength faster than a tightly
cut joint with a thin glue line.
7.3.4 Shrinkage splitting and warping
Factors that contribute to splits and checks
include rapid changes of RH, compression set
and subsequent shrinkage of wood, faulty
construction and removal of an item from its
established environment to a radically different
environment. Modern heating systems
producing excessively dry air are a prime
cause of damage. Air outside in northern
temperate climates in winter is typically at a
temperature where it is able to hold only small
amounts of moisture (absolute humidity).
Although the relative humidity of this outside
air may still be quite high, when the air enters
a building its temperature may be raised
15–20 °C or more. The absolute humidity
remains the same but the amount of water that
air at the new temperature is capable of
holding is now greatly increased so the
relative humidity drops drastically. Figures of
30% RH are common and RH as low as 20%
may be encountered. This is graphically illustrated
in Figure 7.15. The general relationship
between temperature and relative humidity is
illustrated in Figure 6.3b. Splits and checks
will occur in most pieces of furniture under
extreme dry conditions. Especially vulnerable
are those pieces in which the construction
prevents expansion or contraction of the furniture
materials and pieces which are moved too
quickly from a cool damp environment such
as an unheated church into a hot dry environment
such as a normal domestic interior or
museum exhibition hall. In sound wood, splits
and checks always occur parallel to the grain
of the wood because wood is weakest in
tension across the grain. Movement of water
is greater through end-grain than through sidegrain
and splits commonly occur at the ends
of boards, especially where these are
unsealed. When two or more layers of wood
are joined together, such as on veneered
surfaces or with plywood, the dimensional
forces can be complex. The shrinkage or
expansion causes stress in any of the wood
layers, decreasing from the outer layers
inward. Substrate wood and solid wood panels
frequently develop splits, while checks are
more common in veneer. Dimensional forces
may also cause checks in plywood. Minor
(1993) demonstrated that lathe-checks are
introduced in the individual veneer layers
when they are rotary cut and that these checks
open further when the outer veneer layers
shrink relative to the core of the plywood.
Frame and panel construction is designed to
provide strength and rigidity while allowing
for movement of the panel. However, under
some circumstances splits may still be
observed in the panel due to differences in
speed of reaction of bulky frames and thin
panels. Under conditions of high relative
humidity, both the frame and the panel will
pick up moisture and swell. This can result in
the panel being pinched around its edges so
that it is no longer able to move freely. In a
subsequent period of low RH, the thinner