Water within wood structure

Moisture and Temperature. Water exists in wood as liquid and vapor. Within the wood structure, water fills cell lumens and chemically bonds with hydroxyl groups in the cell walls. To obtain a measure of how... 

Temperature. An interesting interplay between temperature and RH has been seen in closed display cases housing moisture-containing material (22). Normally, as the temperature in the display case rises, the local RH would correspondingly decrease. With moisture-containing material, however, the RH actually increases with increasing case temperature. At elevated temperatures, water evaporates from the wood structure. This liberated water vapor enters warmed air, which has a greater potential to retain moisture. Thus, RH within the case increases. 

The wood structure after long immersion—which, in the case of archaeological wood, may be hundreds or thousands of years—is swollen. The lignin and ash content of the wood increases relative to the cellulosic portions as immersion time increases. The skeleton of waterlogged, swollen lignin and any remaining cellulosics has free water within the cells. This water is strengthening and prevents collapse. 

The take up of water or other liquids within the cell walls of wood involve the take up of a molecule at a time and its movement from one adsorption site to another (molecular jump phenomenon) under a concentration gradient. This is distinct from flow of bulk liquids into the coarse capillary structure under a capillary force or pressure gradient. 

If a force is applied to wood within the proportionality limits, the wood will bend and if the force is released, the wood returns to its original form with an elastic recovery. In contrast, if the wood is dried under stress, a substantial superposition of stresses occurs in conjunction with the drying and shrinking process. Since the ordering of macromolecules or larger structural elements under tension is different from those under compression, as the water molecules are removed, new hydrogen bonds can form between different subunits of the structure to support the distorted structure in its new form. 

Lignin is present in plants for which water conduction is important. Of greatest interest is its presence in trees. The lignin content depends on the type of tree about 28% for softwoods and 20% for hardwoods. The cellulose content is approximately 45% in the wood of both types, while the hemicellulose content is roughly 17% in softwoods and 25% in hardwoods. Lignin structure can vary within the same plant,... 

The deposition of extraneous materials within the fine structure of wood fibers has been shown to reduce the accessibility of the cellulose in the fiber wall to the hydrolyzing action of strong acids and bases and also to water as shown by the reduced moisture regain capacity of certain woods high in extractives content, such as redwood. Although it has not been demonstrated experimentally, it is logical to suspect that these substances also would reduce the accessibility of cellulose to cellulases. 

Curve 2 applies to objects having a low heat conductivity coefficient, e.g., wood. In this instance, equilibrium temperatures are reached within a shorter time as compared with metal objects. Dehydration of wood takes place at 500°F, decomposition at 700°F and ignition probably at 800°F, corresponding to 1,300, 3,000 and 4,000 Btu/hr. sq. ft., respectively. This means that wooden structures and vegetation in an area with heat intensities of 3,000-4,000 Btu/hr sq. ft. and higher may catch fire and burn. Paint on equipment may also be damaged. Therefore, it is recommended that equipment located in this area be protected by heat shielding or water sprays if the installation of a stack of sufficient he ht to reduce heat radiation is impracticable. 

Within the anatomical structure of wood, any combination in series or parallel of vapor diffusion (in lumen and pits) and bound-water diffusion (in the cell walls) is a possible pathway to drive water from high to low moisture content regions (Figure 40.16). Because Equations 40.12 and 40.15 use the same driving force, the expressions for the macroscopic bound-water diffusivity in the radial and... 

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