Series editors' preface - Wood Tools

76 Conservation of Furniture
cules, the apparent groupings referred to as fibrils
(sub-groupings sometimes termed micro fibrils).
It is the orientation of fibrils which defines
the layering of the secondary wall. Within the
thinner S 1 and S 3 layers, the fibril orientation is
nearly perpendicular to the cell axis, whereas
fibrils within the dominant S 2 layer are oriented
more nearly parallel with the cell axis.
Experimental evidence provides a theoretical
explanation of the arrangement of cellulose
within fibrils. In random areas, called crystallites,
cellulose molecules (or more likely, portions
of cellulose molecules) are aligned into
compact crystalline arrangement. Adjacent
areas where cellulose is non-parallel are called
amorphous regions. The hemicelluloses and
lignin are also dispersed between crystallites
and through the amorphous regions.
Within the fibrils, water molecules cannot
penetrate or disarrange the crystallites. Water
molecules can, however, be adsorbed by
hydrogen bonding in one or more layers to the
exposed surfaces of crystallites and components
of amorphous regions, namely at the sites
of available hydroxyl groups. Such polar groups
of the polysaccharide fractions on exposed wall
surfaces provide the principal active sites for
bonding of adhesives and finishes and for other
chemical reactions with wood.
Because the average length of cellulose molecules
is far greater than the apparent length of
the crystallites, it is concluded that an individual
cellulose molecule may extend through
more than one crystalline region, being incorporated
in crystal arrangement at various
points along its total length. Therefore, within
the fibrillar network, the random end-wise
connection of crystallites would appear to offer
linear strength to the fibril. Since crystallites
would be more readily displaced laterally from
one another due to the intrusion or loss of
water molecules (or other chemicals capable of
entering the fibrils), dimensional response
would be expected perpendicular to the fibril
direction. In summary, the linear organization
of cellulose within the fibrils, the dominance of
the S 2 layer, and the near-axial orientation of
fibrils within the S 2 layer, together provides a
foundation of understanding of the greater
strength and dimensional stability of the cell in
its longitudinal direction. It follows that wood
itself – as the composite of its countless cells –
has oriented properties.
2.4 Wood–water relations and
movement
No other area of wood science and technology
is more important to object conservation than
wood–moisture relationships. The moisture
condition of wood is related to properties ranging
from thermal conductivity and strength to
adhesive bonding and fungal development.
However, the most telling influence of moisture
in wood is upon dimensional behaviour.
Solving and preventing the array of problems
related to dimensional movement in wooden
objects begins by recognizing the fundamental
relationships involving wood, moisture and the
atmosphere. It is customary to express the
amount of water in wood as moisture content.
The moisture content (MC) of wood is defined
as the ratio of the weight of water in a given
piece of wood to the weight of the wood when
it is completely dry. The water-free weight of
wood is also referred to as its oven-dry weight,
determined by drying a wood specimen at
100–105 °C until it ceases to lose weight.
Moisture content is expressed as a percent and
is calculated as follows:
MC = Wi – Wod 100/Wod where
MC = moisture content, expressed as a
percentage
Wi = initial weight
Wod = oven-dry weight.
Water exists in wood in two forms, as bound
water and as free water. Water adsorbed and
held within the cell wall by hydrogen bonding
is called bound water. Water in wood in excess
of the fibre saturation point exists as liquid
water in the cell cavities and is called free
water. The fibre saturation point (FSP) is the
moisture condition of wood wherein the cell
walls are completely saturated with bound
water but the cell cavities are devoid of free
water. It is usually expressed as a numerical
value of moisture content and is generally in
the range 25–30%.
The sap contained in living trees is primarily
water, with small amounts of dissolved minerals
and nutrients. Wood in living trees is always
above the fibre saturation point, although the
moisture content may vary from slightly above
the fibre saturation point to 200–300%,