Series editors' preface - Wood Tools

286 Conservation of Furniture
compression parallel to the grain, though it
may be stronger in tension and tougher than
normal wood of the same density. Compared
to normal wood it has abnormally high longitudinal
shrinkage and slightly increased
tangential shrinkage but normal radial shrinkage.
The lignin content is deficient and gelatinous
fibres may be present. Reaction wood is
a generic term used to refer to both compression
and tension wood. Heart rot, a form of
decay caused by fungal activity near the heart
of the tree, attacks the pith and heart wood
and may lead to the production of a
completely rotten core or pipe running down
the centre of the tree. This is not normally
active after seasoning.
Various wood-boring insects attack trees
and green timber. These include wood wasps
(Sirex wasps), Ambrosia beetles, pin hole
borers, various longhorn beetles and jewel
beetles – depending on country of origin.
Although the wood may contain holes, attack
by these insects usually stops when the timber
dries (kiln drying kills eggs and larvae).
Ambrosia beetles and pin hole borers,
belonging to the families Platypodidae and
Scolytidae, are most common in tropical
woods, but almost every species of wood is
liable to attack and both heartwood and
sapwood are affected. The larval stage of these
insects feeds not on the wood itself but on a
mould fungus (ambrosia fungus) which is
introduced into the wood by the adult beetles.
The fungus grows on the walls of the many
small circular tunnels, which are created by
the adult and which are 1–3 mm in diameter
depending on species. Infestation can occur in
the standing tree but is more likely to occur
between felling and seasoning. Circular holes
are evident on the outside of the wood corresponding
to the diameter of the tunnels but
the extent of damage is not fully apparent
until the log has been converted. The infestation
is terminated when the timber is dried
during seasoning and cannot then recur or
spread to other converted timber. The timber
is safe for structural use but may be unacceptably
marred by the holes and tunnels and by
the dark discoloration that often surrounds
them.
There are many species of longhorn beetles
(Cerambycidae) and jewel beetles (Buprestidae)
widely distributed in tropical and
temperate forests, most of which are major
forest and mill-yard pests but attack only trees
and logs, the infestation dying out during
seasoning to leave large oval shaped tunnels
(3–25 mm in diameter) packed with bore dust.
The damage is not normally significant from a
structural point of view and can usually be
eliminated during manufacture. An exception
to this rule, the house longhorn beetle
(Hylotrupes bajulus), is further discussed in the
sub-section on insects below.
Knots are formed by the inclusion of lateral
branches in the main stem or trunk and may
influence the strength properties of wood
depending on their size, position and type.
Irregular grain which occurs around knots may
constitute an area of weakness and for this
reason they are often regarded as a defect.
However, when imaginatively used they can
contribute to the decorative potential of wood
surfaces. Types include intergrown, spike
knots, tight knots and loose knots (Figure 7.1).
Burrs are abnormal outgrowths or protuberances
formed by growth around groups of
dormant buds or wounds. Burrs constitute
areas of instability and weakness in furniture
but have important decorative value when
used in veneer form. Shakes are partial or
complete separations between adjoining layers
of wood and may be due to causes other than
seasoning. They may be caused by relief of
growing stresses or by impact when a tree hits
the ground after felling. Shakes caused by
felling are also known as thunder shakes.
Occasionally shakes are caused by lightning.
Compression shakes are seen as fine crinkled
lines across the grain due to stresses set up
by weight of rain, snow winds, and unequal
growth. Heart shakes, splits that extend from
the pith may be present before conversion.
Other types include star shakes (several heart
shakes together in shape of a star), cup or ring
shakes (which follow the line of growth rings)
and radial shakes (along the rays). Pitch
pockets, resin canals, gum ducts and latex
tubes may be a normal feature of the wood
or a response to injury of the cambium. They
affect the finishing process and may bleed
through finishes, or they may be revealed at
inconvenient moments during the processing
of wood.
Within a species there is considerable variation
in density and strength and the strongest