The Structural Conservation of Panel Paintings

provide the principal active sites for bonding of adhesives and finishes and

for other chemical reactions with wood. Because the average length of cel-

lulose 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 arrange-

ment at various points along its total length. Therefore, within the fibrillar

network, the random endwise 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 ofwater

molecules (or other chemicals capable ofentering the fibrils), dimensional

response would occur perpendicular to the fibril direction.

Insummary, knowledge of the linear organization of cellulose

within the fibrils, the dominance of the S 2 layer, and the near-axial orienta-

tion offibrils within the S 2 layer provides a foundation for understanding

the greater strength and dimensional stability of the cell in its longitudi-

nal, as compared to transverse, direction. It follows that wood itself—as

the composite of its countless cells—has oriented properties.

Virtually every property or response of wood, from its strength to its

decay susceptibility, is related to its moisture condition—but probably no

property is of greater concern than its dimensional behavior in response

to moisture. We recognize that such problems as warping and checking of

panels and flaking of paint are among the most challenging conservation

issues. Ifthere is to be a hope of preventing or correcting such problems,

the fundamental relationships involving wood, moisture, and the atmos-

phere must be recognized.

Before exploring interrelated details, we can easily summarize

underlying principles. First, the wood in trees is wet, containing large

amounts of moisture in the form of sap, which is mostly water. It is appro-

priate to think of wood at this stage as being fully swollen. Second, when

wood is taken from trees and dried to a condition appropriate for common

Wood-Moisture

Relationships

C  P P  W 11

Structural diagram of longitudinal wood cell

(Micro) fibril Crystallite

Amorphous

region

Cellulose

Cell

lumen

Cross section

of cell wall

Wood cell

or “fiber”

Se

co

nd

ar

yw

all

Pr

im

ary

wa

ll

Orientation of

cellulose in cell

wall layers

S 3

S 2

S 1

S 3

S 2

S 1

Figure 6
Diagrammatic model of a longitudinal wood
cell, showing the orientation offibrils within
layers of the cell wall and the arrangement of
cellulose within fibrils.