6 Wood Discoloration

3.3 Wood Moisture Content 61
survived. Water is taken up from the substrate wood, the soil, and from masonry
etc. Altogether the moisture content of wood is the most important factor
for wood degradation by fungi and thus also for wood protection. Moisture
in wood exists in two different forms: Bound or hygroscopic water occurs
within the cell wall by means of hydrogen bounds at the hydroxyl groups
mainly in the cellulose and hemicelluloses and to smaller extent in the lignin.
Freeorcapillarywaterinliquidformislocatedinthecelllumenaswell
as in other holes and cavities of the wood tissue (e.g., Siau 1984; Smith and
Shortle 1991).
There are several methods of measuring wood moisture content (Vermaas
1996): oven-drying method, microwave drying Danko (1994), distillation, Karl
Fischer-titration, moisture meters based on electrical and dielectrical properties,
continuous moisture meters, capacity admittance moisture meters, and
hygrometric methods. Determination of the moisture content without destruction
is done electrically by means of resistance measurement (Skaar 1988; Du
et al. 1991a, 1991b; Böhner et al. 1993; Chap. 8.2.4). With increasing moisture
content of wood from the oven-dry phase to the fiber saturation range (about
30% u) the electrical resistance decreases approximately by the factor 1:10 6 .
Moisture can be rapidly determined in practice using an indelible pencil that
is the pencil line runs if the fiber saturation point is exceeded.
The proportional wood moisture (% u) is determined gravimetrically by
the wood mass before and after drying a wood sample at 103 ± 2 ◦ C: u (%) =
[(MW − MD) : MD] × 100 (MW = mass of wet wood, MD = mass of dry wood).
If heat-implied changes in the wood samples shall be excluded to take
care of wood extractives and cell wall components for subsequent microbial/enzymatic
degradation experiments or chemical analyses, drying of the
wood specimens can be performed in an evacuated desiccator over silicagel
or P2O5. Wood samples may be also conditioned to specific relative humidity
conditions prior to and after decay, e.g., at 20 ± 2 ◦ Cand65±5%relativeair
humidity. With the latter method, the theoretical dry weight (MDt) of a sample
results from: MDt = (100 × MC): (100 + u) (MC = mass after conditioning,
u = % wood moisture after air conditioning). However, weight loss methods
using moisture-conditioned wood samples instead of oven-dry blocks are
influenced by changes in hygroscopicity: For brown-rot, mass loss is slightly
overestimated, for white rot, no difference occurs, while for soft rot, mass loss
is slightly underestimated using the moisture-condition method (Anagnost
and Smith 1997).
To quantify the moisture content of fungal nutrient substrates, including
wood, only the proportional water content of the substrate was considered in
previous investigations. At the disposal to microorganisms, however, not the
whole water content of the substrate is available, but only that part of the total
water, which is not bound by solved substances (salts, sugars, etc.). The relative
vapor pressure of a substrate (water activity aw, 0–1) results from the quotient
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