Series editors' preface - Wood Tools

RH changes lead to changes in moisture
content which result in dimensional change.
Eventually all parts come to equilibrium but
parts, even of a single piece of wood, equilibrate
at different rates due to differential
moisture access in a single piece of wood. Buck
(1961) demonstrated that yellow poplar
(Liriodendron tulipifera) responded more
rapidly to changes in RH than chestnut
(Castanea dentata). He observed that at 24 °C
the EMC will be reached about twice as fast as
at 12 °C and that the greater the change in RH
the faster the wood reacts. Because of the thickness
of a piece of wood it may be some time
before the inside reaches equilibrium whereas
changes in the outer layers begin almost
immediately. Similarly, access via the end grain
of a piece of wood is usually very much
quicker than side grain access. In a large piece
of timber, say a life-sized sculpture, the changes
in response to a fixed change in RH may take
months to complete whereas a thin veneer
would do so in hours and a sheet of paper
begins to react in minutes. In fact, using very
sensitive linear displacement transducers, tiny
movement of surface layers of wood can be
detected almost immediately (if the surface is
wetted for example). Areas of defective wood
may also respond differently and coated areas
will respond differently to uncoated areas. The
situation in wooden structure is correspondingly
more complicated. Differences in response
times and grain directions between components
of wooden structures leads to movement in
different directions and at different rates. These
differences may result in warping, dislocation of
parts, splitting and breakage of fibres. For
example, the stress on a large panel, even
when coated, will be greater at the ends than
in the middle, possibly leading to propagation
of cracks/splits. This is because moisture
exchange is faster from exposed end grain.
When seasoning timber, end grain is often
sealed with wax to reduce this problem. If
movement has not been allowed for in
construction, damage will result. If wood is
prevented from moving in response to changes
in relative humidity, plastic compression (see
section 7.3.4) can take place leading to permanent
reduction in the maximum dimensions of
the piece. This is typically seen in large panels,
floor boards, hammer heads, and mortise and
tenon joints but can also occur in a single piece
Deterioration of wood and wooden structures 293
of wood in which one part of a board is
restrained by another part of the same board.
Frequent changes in moisture content are a
principal cause of weathering. Stresses are set
up in the wood as it swells and shrinks due to
moisture gradients between the surface and the
interior. Stresses are greater the steeper the
moisture gradient and are usually largest near
the surface of the wood. Unbalanced stresses
may result in warping and surface checks along
the grain.
With time, the dimensional response of wood
may lessen, in part because hygroscopicity of
the wood may decrease, or because of mechanical
effects of repeated shrinkage/swelling
cycles, or stress-setting of the wood. However,
experiments with wood taken from artefacts
thousands of years old have shown the wood
to have retained its hygroscopicity and its
capacity to respond dimensionally to changes
in moisture content. Klein and Bröker (1990)
demonstrated that dimensional changes were
almost equal between a seventeenth-century
oak panel and a newly made oak panel, after
exposing them to fluctuating RH cycles. The
assumption should therefore prevail that
wooden objects, regardless of age, can demonstrate
dimensional movement when subjected
to variable relative humidity conditions.
Moisture content of wood can also affect
mechanical, chemical and biological aspects of
deterioration. As wood dries below the fibre
saturation point the strength increases with the
loss of bound water. For example, the
compressive strength parallel to the grain is
doubled when wood is dried from green to
12% and tripled when oven dried. Production
of acetic acid by hydrolysis of acetyl groups on
acetylated hemicelluloses increases in damp
conditions. Excessively high moisture levels are
associated with staining and damage to wood
surfaces and finishes. High moisture contents
also support the growth of fungi and moulds
and favour the establishment of insect pests.
For internal woodwork, a theoretical ideal is
for RH to be kept constant at a level of 50%
with no more than ±5% variation. In practice
the set point depends on the previous history
of the piece. Objects which have spent most
of their lives in unheated churches at RH
50–70% or more are likely to crack and split
if suddenly introduced into a much drier
environment. Objects may be able to tolerate