Series editors' preface - Wood Tools

time of exposure, moisture content of the
wood, atmospheric composition and presence
of light-absorbing substances. The surface
changes include formation of free radicals,
chain scission, dehydrogenation and dehydroxymethylation
of cellulose and splitting
of double bonds, formation of phenoxy
radicals and quinone structures and polymerization
of lignin (Zavarin, 1984). Radiant light
energy may lead to heating of the surface
resulting in further damage to the surface
associated with restrained dimensional change.
Much old furniture may already have
succumbed to the effects of light since it has
not traditionally been regarded as a particularly
light sensitive class of material and may therefore
have been exposed to relatively high light
levels for considerable periods of time.
However, light sources should still be filtered to
remove UV and visible light should be kept at,
or below 150 lux for wood furniture. Furniture
decorated with distinctively coloured or stained
woods, particularly those still showing good
colour may benefit from a reduction of the
intensity of exposure to about 50 lux. Reduction
in the duration of exposure to light will also
benefit such furniture. Since the most highly
coloured parts of furniture are frequently those
on inside surfaces it will help to preserve them
if items such as doors, drawers and flaps are
kept closed as much as possible.
Heat
The strength of wood is inversely proportional
to temperature. A nearly linear two- to threefold
reduction in strength occurs as temperature
rises from –200 °C to +160 °C. The direct
effects of heat on wood are of two types.
Those effects that are maintained only as long
as the change in temperature is maintained,
and those that are permanent. The initial effect
of heating wood without adequate compensation
in RH is to reduce its moisture content.
The nature of any further effects depends on
such factors as species, moisture content, the
heat source and the level and duration of
heating. If a temperature of 55–65 °C is
maintained for long periods (2–3 months)
depolymerization of cellulose and hemicelluloses
begins. At about 250 °C pyrolysis and
volatilization of cell wall components occurs,
followed by combustion in the presence of air
or charring in its absence. Winandy and
Deterioration of wood and wooden structures 291
Rowell (1984) reported that heating Douglasfir
in an oven at 102 °C for 335 days reduced
modulus of elasticity by 17% modulus of
rupture by 45% and fibre stress at proportional
limit by 33%. The same losses might be
observed in one week at 160 °C. Heating
softwood at 210 °C for 10 minutes in the
absence of air reduced modulus of rupture by
2%, hardness by 5% and toughness also by
5%. Under the same conditions at 280 °C
modulus of rupture is reduced by 17%,
hardness is reduced by 21% and toughness is
reduced by 40%. Most of these effects are
outside the range that would normally be
experienced by furniture in most collections.
Although the lower temperature ranges quoted
could possibly apply to radiant heating of
wood surfaces these effects would probably
normally arise only as a result of a fire.
Like other materials, wood expands when
heated and contracts when cooled. The unit
amount by which a material expands (per unit
of original length per degree rise in temperature)
is called its coefficient of linear thermal
expansion. For wood in the direction of the
grain in the range –50 °C to +50 °C this
averages 3.39 10 –6 per degree centigrade
irrespective of wood species and specific
gravity. This is small in comparison with other
common solids. For example, the coefficient
of linear thermal expansion per degree centigrade
for flint glass is 7.9 10 –6 , for steel is
10 10 –6 and for aluminium is 24 10 –6 .
However, across the grain the coefficient of
linear thermal expansion for wood (for an
average specific gravity of 0.46) in the radial
direction is 25.7 10 –6 while in the tangential
direction it is 34.8 10 –6 . Values of thermal
coefficients for wood in the radial and tangential
direction vary directly in a straight line
relationship with the specific gravity of the
wood. The reasons that thermal changes are
not more commonly recognized in wood are
first that wood is usually used within a narrow
range of temperatures and secondly that
changes due to temperature are normally
masked by changes brought about through
fluctuations in RH and moisture content which
are usually much larger and in the opposite
direction. Heating a wood surface will tend to
reduce the moisture content and the resultant
shrinkage will outweigh expansion as an effect
of heating.