Series editors' preface - Wood Tools

86 Conservation of Furniture
2.5.3 Factors affecting the strength of
wood
Besides the normal variability of strength
among and within species, many other factors
may affect the strength of wood. These factors
may be broadly grouped into natural defects
and irregularities, factors related to the environment
and the effects of biological agents.
When the grain direction is not parallel to the
long axis of a wooden component, it is said to
be cross-grained (sometimes referred to as
short-grained). Cross grain may occur from spiral
grain in the tree or by the manner in which
the timber is sawn. In linear furniture parts such
as legs and spindles, whose performance
depends on longitudinal properties such as
bending resistance, cross grain may result in
serious strength loss. A slope-of-grain of one in
five, for example, may result in 50–60% reduction
in the modulus of rupture. Knots in wood
are another major weakening defect. Loss in
strength results not only from the abnormal tissue
and grain direction of the knot itself, but
from the cross grain of wood distorted around
the knot. Compression wood, the reaction wood
formed in conifers as a result of crooked or
leaning stems, is usually higher in density and
compression strength than normal wood, but
the wood is weaker in tensile strength and in
both modulus of rupture and modulus of elasticity
in bending. In hardwoods, tension wood
is exceptionally weak in compression parallel to
the grain though it may be stronger in tension
and tougher than normal wood of the same
density. It exhibits abnormally high longitudinal
shrinkage and slightly increased tangential, but
normal radial, shrinkage. The lignin content of
the cell wall is deficient compared with normal
wood and gelatinous fibres may be present.
As wood dries below the fibre saturation
point, strength increases with the loss of bound
water. The greatest increases are in compression
along the grain: strength is approximately
doubled when wood is dried to 12% moisture
content, tripled when oven-dried. Modulus of
rupture is increased much less, and modulus of
elasticity is increased least upon drying.
Strength of wood is also affected by temperature,
increased as temperature is lowered,
decreased as temperature is increased. Over the
range and duration of naturally occurring temperature
changes, strength changes are tempo-
rary. However, if exposed to higher than natural
temperatures, or for prolonged periods,
permanent loss of strength may result. Effects of
heat in reducing strength are least in dry air,
greatest in moist air or steam. The use of steaming
to temporarily plasticize wood for permanent
bending of furniture parts is well known.
Strength of wood is also related to duration of
loading. Time-related creep in wood reduces
strength over long-term load periods. For example,
a beam might carry a short-term (5 minutes)
load three times as great as it could carry
for a long term (a hundred years or longer).
The destructive effects of wood-inhabiting
insects such as termites, carpenter ants and beetles
need little elaboration, as the physical loss
of wood will result in proportional loss of
strength. Fungi are a major cause of deterioration
in wood. In order for the threadlike
hyphae of fungi to develop in wood, four major
requirements are necessary: favourable temperature
(70–85 °F is ideal), oxygen (20% or more
air volume in the wood), moisture (fibre saturation
point or above is ideal), and food. Woodstaining
fungi utilize the residues of stored
materials in parenchyma cells of sapwood but
they do not attack cell walls. Therefore,
although the staining fungi discolour the wood,
they do not reduce its strength. However, the
wood-destroying fungi utilize enzymes to break
down and assimilate the cell wall substance,
producing various forms of decay or rot. Initial
stages of fungal invasion, termed incipient
decay, may at first have insignificant effect on
strength. Impact strength is the first strength
property to be affected. If allowed to continue,
total loss of strength may result. It should be
noted that Chlorociboria both deposits a green
stain in the wood and causes losses of strength.
Controlling moisture is the principal approach
to preventing decay. If wood is maintained
below 20% moisture content, decay fungi cannot
develop.
2.5.4 Role of wood strength in furniture
In furniture, strength of wood plays a critical
role in various ways. Bending strength may
determine the integrity of legs and stretchers in
chairs and tables, the rails or posts of beds, or
the planks of benches and leaves of tables.
Hardness usually predicts how well surfaces
resist indentation under practical use or abuse.