The Structural Conservation of Panel Paintings

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Wood, as the biological product of higher-order plants, has a chemical

composition that is understandably complex, and a thorough discussion of

wood chemistry is quite beyond the scope of this article. However, even a

brief summary of the more important fundamentals of cell-wall chemistry

provides a basis of understanding of the anisotropic physical and mechani-

cal properties of wood—especially its hygroscopic nature and dimensional

behavior—and of chemical reactions involved in such practical conservation

procedures as finishing, gluing, stabilization, and preservative treatment.

Chemical composition

The bulk of cell-wall substance is a composite of three major types of

organic molecules: cellulose, hemicelluloses, and lignin. These con-

stituents can be thought of as skeletal, matrix, and encrusting substances,

respectively. Only minor amounts of inorganic (ash) content are present

in wood. In various amounts, depending on species, additional substances

called extractives, or extraneous materials, may also be present, mainly as

additives to the heartwood.

The major chemical constituents of wood are typically present in

the following approximate percentages:

Cellulose 40–50%

Hemicelluloses 20–30%

Lignin 25–30%

Ash 0.1–0.5%

Extractives 1–5%

Of the major constituents, cellulose is the most easily described

and is in many respects the most important. Wood cellulose is chemically

defined as (C 6 H 10 O 5 )n, the basic monomer ofwhich is called glucose anhy-

dride. As shown in Figure 5, glucose anhydride units are alternately linked

in pairs to form bimers (cellobiose), which in turn are repetitively end

linked to form the long-chain linear polymer cellulose. The average degree

ofpolymerization (DP) of cellulose is in the range of 10,000. The hemicel-

luloses found in wood are polysaccharides of moderate size (DP averaging

150–200 or greater) of the types that are invariably associated with cellu-

lose and lignin in plant-cell walls. Predominant types include xylan (the

principal hemicellulose in hardwoods), glucomannan, and galactogluco-

mannan (the major hemicellulose of softwoods). Many other forms of

Chemical Properties

of Wood

Figure 5
Representative portion ofa molecule ofcellu-
lose, the major chemical constituent of wood. H

CH 2 OH

H

OH H

H

H OH

Cellobiose

H

H

OH

H

H

Glucose

anhydride

H

H

H

H

H OH

O H

H

OH

H

H

CH 2 OH

OH

H

OH

CH 2 OH

C O C C C O C C

O

CH 2 OH

C

C C C

O

O

C

C C

C

C

H C

OH

C

O

C C

O

C O