Timbers soft-wood

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

Botanical Origin. Wood is a natural material familiar in at least some way to everyone. Wood is obtained from two broad categories of plants known commercially as softwoods and hardwoods. These general names cannot be used universally to refer to the actual physical hardness or density of all woods because some softwoods are quite hard (e.g., Douglas-fir and southern yellow pines) and some hardwoods are soft (e.g., yellow buckeye, aspen, and cottonwood). Nevertheless, the names do accurately apply to many woods within these two categories and thus can be used as practical designations for the two general classes of commercial timbers. 

Bacteria and soft rot fungi are more tolerant of the low oxygen conditions in saturated wood, and in wood that is buried in sediments bacterial decay tends to predominate. Early reports identified unicellular bacteria in foundation piling and shipwreck timbers, but more recently three bacterial decay types in wood cell walls are now recognised - erosion, tunnelling and cavitation bacterial decay. 

Despite the large number of wood-decay fungi growing on the surface of stored timbers, few signs of recent attack were observed. Soft rot cavities were observed but were not numerous. Such decay patterns were only observed... 

Abrasive action by water and sand in aquatic environments may remove the outer softened layers of soft-rotted timber, and thus expose the underlying layers for further attack (76). This exposure will result in a more extensive attack at the water level, giving a characteristic appearance of the smooth rounded shapes common to driftwood. Wood-degrading bacteria may often also contribute to the surface softening in aquatic environments. 

Soft-rot fungi are fully capable of attacking wood without any additional nutrients. However, the addition of nitrogen greatly increased the rate of attack (54). This finding may explain why soft rot is often observed in timber exposed in contact with soil from which nitrogen can be extracted. Several observations suggest that the rate of decay is faster in more fertile soils. 

Savory (82) reported that soft-rot attack could occur at quite low moisture contents, as well as under waterlogged conditions. He stated that the mi-croftingi can attack wood which is too wet or too dry for Basidiomycete decay. He also reported severe soft rot in beech strips exposed at a constant relative humidity of 90%, corresponding to an equilibrium moisture content of 20-21%. Apparently it is a misconception that soft rot only occurs in very wet wood. Kirk and Cowling (9) state, for example, that only water-soaked timber is attacked by soft rot. In fact, soft rot may occur over a wide range of moisture conditions, from quite dry to fully waterlogged wood. 

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