sufficient free water available for fungal growth and development. The
ideal environment for protecting wood from attack is often considered to
be a relative humidity (RH) of less than 60%. In some modern museums,
humidity can be well regulated; however, over past decades or centuries,
many painted wooden objects have been subjected to environments con-
ducive to fungal growth. The duration of this exposure and amount of
moisture accumulation govern the type of fungus that may be established,
as well as the extent of attack.
With current knowledge of wood-destroying fungi and their pat-
terns ofdeterioration, it is possible to examine wooden cultural proper-
ties, determine the type of fungus that caused the damage, and identify
typical characteristics for these forms of decay, such as microstructural
damage to cells and loss of strength properties.
Three categories of wood decay are most commonly associated
with wood that has been buried, entombed, or exposed to decay-promoting
environments for a considerable length of time: brown rot, soft rot, and
white rot (Table 1). For example, Fayum portrait paintings may have seri-
ous decay problems in parts of the wood, depending on the tomb envi-
ronment and exposure to moisture (Martin and Reisman 1978). Panel
paintings may show decay even if they have not been exposed to burial
environments or are not thousands of years old. Painted wooden cultural
objects from more recent times may be affected by poor storage condi-
tions in damp cellars, churches, castles, country houses, or other highly
humid environments. Since conservators may encounter a wide range of
materials from different environments, all major forms of degradation by
wood-decay fungi are presented below.
Distinct forms of decay are found in wooden materials because
the enzymes and degradative mechanisms of different groups of fungiA G W D C M I 57Table 1 Changes in wood due to degradation by fungi
Decay Wood characteristics Strength loss Cell-wall components Morphology
Brown rot (dry rot)^1 Brown. Cracks and checks Large losses of strength in Cellulose depolymerization Porous and shrunken cell
when dry, producing cubical early stages of decay. and loss. walls, skeleton of altered
fragments. lignified wall material.
Soft rot Brown. Often localized to Loss of strength in late Cellulose degraded. Cavities present in
wood surfaces. Cracks and stages of decay. secondary walls, or
checks when dry. secondary walls eroded,
leaving only the middle
lamellae.
White rot Bleached appearance. Major strength losses in Lignin, cellulose, and All secondary cell-wall layers
Retains shape and intermediate to late stages hemicellulose degraded. and middle lamellae are
composition until decay of decay. eroded.
is advanced.
Fungal stain Various discolorations in No strength losses. Free sugars, nutrients, and Preferential colonization of
sapwood. wood extractives utilized; ray parenchyma cells; no
increase in melanin-like cell-wall degradation.
compounds and pigmented
substances.
Surface molds Discolorations on wood No strength losses. Readily assimilated substances Preferential colonization of
surfaces only. are removed. parenchyma cells; no cell-
wall degradation.
(^1) Dry rotis acommon term used to describe brown rot in some wood products.