Series editors' preface - Wood Tools

294 Conservation of Furniture
larger variations if these are achieved slowly.
Many objects seem to be able to accommodate
±10% RH on a seasonal basis with little readily
apparent damage. Control of heating represents
an important step in RH control.
Pollution
Pollution is not a major source of structural
deterioration in wood but is more likely to
affect its surface qualities, particularly colour,
and other non-wood materials present in furniture,
for example textiles and coatings. Both
sulphur dioxide and ozone are implicated in
the fading of dyes present in many decorative
timbers. Particulate pollution is more likely to
cause concern than to cause damage though
repeated careless removal of particulates can
damage high gloss surfaces. Wood itself is a
source of organic acid pollution, particularly
acetic acid produced by hydrolysis of acetylated
hemicelluloses that can lead to corrosion
of lead and its alloys and damage to materials
that contain calcium carbonate, such as
marine animal shells for example (Farmer,
1962). This type of damage to objects can
occur where wood is used in showcase
construction (Padfield et al., 1982). Other
wood and metal combinations may cause
degradation and staining of wood (Forest
Products Research Laboratory, 1970; Pinion,
1973). Interaction of oak, and other timbers
with a significant content of tannins, with iron
causes a blue black stain to be produced.
Fungi
Although fungal decay occurs in wood under
appropriate conditions, in fact accounting for
about 90% of all timber losses in the world, it
is rare in furniture. The comparative durability
and resistance of wood to decay by microorganisms
is explained by its sub-cellular
organization. Highly complex, insoluble
polymers which make up the cell wall require
enzymatic breakdown before they can be
assimilated by micro-organisms. However,
enzyme action is mainly restricted to noncrystalline
regions of the cellulose and since
wood is more crystalline than most other plant
tissues it provides greater resistance to fungal
and bacterial degradation. Lignification also
creates a physical barrier to enzymatic attack
on the polysaccharides so that only those
organisms that possess enzymes capable of
altering the protective association of lignin are
capable of destroying wood. The low nitrogen
content of wood (only about 0.03–0.1%
compared to the 1–5% in herbaceous tissues)
further reduces susceptibility to decay. Also,
for reasons which are not fully understood, a
higher moisture content is required for the
initiation of deterioration in wood than in
other plant tissues. Finally, substances toxic to
such organisms are present in the heartwood
of many timbers.
Fungi are broadly classified into the sap
stain fungi which eat only the cell contents
and the wood-destroying fungi and soft rots
which cause decay in wood by disintegrating
cell walls to obtain food for growth and reproduction.
Based on physical and chemical
changes produced in wood and the resulting
alterations in colour of decaying wood, wood
destroying fungi are classified as brown rots,
white rots and soft rots (Panshin and de
Zeeuw, 1980). Brown rots break down cellulose
components leaving brown powdery
residues of lignin which make the wood
appear darker in colour. The wood surface
displays small cracks across the grain and in
an advanced state of decay rectangular
pockets of degraded wood may fall out.
Brown rots are more commonly associated
with softwoods. They are the commonest form
of fungal decay in the UK accounting for the
great majority of damage to wood in buildings.
White rots break down both the cellulose
and lignin content of wood causing the
wood to become lighter in colour, softer and
somewhat fibrous and result in up to 90%
weight loss in affected timber (Otjen and
Blanchette, 1987). White rots are more
commonly associated with hardwoods. Soft
rots are further classified as cubical, spongy,
pocket, or stringy according to the pattern of
deterioration they cause. Soft-rots occur under
special circumstances where extremely high
moisture levels are found, or when no other
rots can prevail (Blanchette and Simpson,
1992; Caneva et al., 1991). The terms dry rot
and wet rot are somewhat misleading, as all
fungi require a minimum moisture content in
wood of 20% and will not grow below this
level whilst a level of 35–50% or more is
preferred by many. However, once growth has
started certain fungi are able to transport the
necessary moisture considerable distances via