The Structural Conservation of Panel Paintings

heartwood-degrading microorganisms can result in substantial degradation

(Blanchette et al. 1990; Blanchette 1992).

Anatomical characteristics of sound wood reveal great variation

among tree species (Fig. 1a, b). Wood from coniferous trees (commonly

referred to as softwood) is composed primarily of tracheids (90–95%).

These cells have tapering ends that are closed. Transport of water and

minerals is facilitated from one tracheid to another via pit apertures.

Other cells include parenchyma cells and, in some species, resin canals.

Wood from angiosperms (called hardwood) contains vessel elements,

fibers, and parenchyma cells. Vessel elements have large lumina and cell-

wall layers that differ from fibers. The middle lamella region of woody

cells, found between cells, is highly lignified. The secondary wall layers are

cellulose-rich regions, but they do contain some lignin. In general, soft-

woods have more lignin and less cellulose than do hardwoods. Additional

and more detailed information on wood anatomy and chemistry can be

found in writings by Fengel and Wegener (1984), Hoadley (1990), Miles

(1978), Panshin and de Zeeuw (1980), and Shigo (1994).

Fungi

Wood deterioration by fungi may occur from several sources. These

include the following: surface molds that cause localized discoloration;

stain fungi that penetrate deep into the sapwood causing blue, gray,

green, red, or other dark coloration; and wood-destroying fungi that

decompose cell-wall polymers (Table 1). In all situations, moisture is an

important factor for spore (or other fungal propagulum) germination and

for successful colonization of the substrate by the fungus. If the moisture

content of the wood is below the fiber saturation point of approximately

28% (based on the oven-dry weight of the wood), there will not be

Microbial Degradation

ofWood

56 Blanchette

a b

Figure 1a, b
Transverse sections of sound wood:
(a) Spruce (Picea) showing earlywood and
latewood (small arrows) tracheids, ray
parenchyma cells (arrowheads), and resin
canal (large arrow); (b) aspen (Populus) with
large vessel elements distributed throughout
earlywood and latewood regions (small
arrows) surrounded by fibers and ray
parenchyma cells (arrowheads). Scanning
electron micrographs; bar 5 500 μm.