The Structural Conservation of Panel Paintings

attack cell-wall components in different ways. As decay progresses, gross

differences in color and physical characteristics are readily observed.

Microscopic observations are required, however, to identify correctly the

decay patterns in incipient to moderate stages of decay.

Brown rot

Brown-rot fungi cause a diffuse depolymerization of cellulose early in the

decay process, resulting in significant losses in wood strength properties

(Blanchette et al. 1990; Eriksson, Blanchette, and Ander 1990). In more

advanced stages, wood polysaccharides are removed, leaving lignin chemi-

cally modified but undegraded. The resulting wood is a brown, lignin-rich

substrate that cracks and checks into cubical fragments. Hyphae of the

fungus colonize cell lumina and produce extracellular enzymes that diffuse

throughout adjacent cell walls. Morphological characteristics show wood-

cell walls consisting of a fragile network of residual lignin (Fig. 2a–d).

These cells have little integrity and easily shatter into minute particles.

Optimum wood-moisture content for brown-rot fungi ranges from 40%

to 80% based on the oven-dry weight of the wood (Scheffer 1973; Zabel

and Morrell 1992).

58 Blanchette

cd

ab

Figure 2a–d
Transverse sections of brown rot:
(a)Collapsed and distorted tracheids are
evident in spruce with advanced decay. The
removal of cellulose has left a degraded cell
wall that consists ofresidual lignin (scanning
electron micrograph); (b) brown-rotted tra-
cheids appear porous and have little strength
and structural integrity left (scanning electron
micrograph); (c) and (d) brown-rotted wood
from the Statue of the Scribe of Mitry,
V Dynasty (2340 B.C.E.), from Saqqara
(Metropolitan Museum of Art, New York,
MMA 26.2.4). Brown rot has caused the
cells to disrupt into a fine mass of degraded
cell-wall material. The residual lignin may
fragment into dustlike brown particles.
Transmission electron micrographs;
bar 5 15 μm.