Timber condition

End use, relative hazard Principal hazard Choice of timber Condition of timber Choice of preservative Quantity of preservative uptake Treatment process... 

Lead is relatively easily corroded where acetic acid fumes are present and under such conditions it either should not be used or should be efficiently protected. Generally, any contact between lead and organic material containing or developing acids will cause corrosion for instance, unseasoned wood may be detrimental. Trouble from this cause may be prevented by using well-seasoned timber, by maintaining dry conditions, or by separating the lead from the timber by bitumen felt or paint. Lead is also subject to attack by lime and particularly by Portland cement, mortar and concrete, but can be protected by a heavy coat of bitumen. A lead damp-proof course laid without protection in the mortar joint of a brick wall may become severely corroded, especially where the brickwork is in an exposed condition and is excessively damp. 

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). 

The availability of different timber species varies considerably with economic and political conditions, and the effects of past exploitation. 

Impermeable timbers have a good resistance to polluted atmospheres where acid fumes rapidly attack steel. Wood has given excellent service in the buildings of chemical works and railway stations. Permeable wood species and sapwood can suffer defibration problems caused by the sulphur dioxide of industrial atmospheres. Tile battens are particularly vulnerable. The heartwood of Douglas fir, pitch pine, larch, Scots pine/European redwood and many tropical hardwoods give good service in these conditions. 

Tekmaheat Oy manufacture three kiln systems a compact chamber of 15-25m per batch, having an annual production capability of 1600-3000 m a drive-through chamber having a volume of 40-60 m, with an annual capacity of 5000-10 000 m and a multi-stage production line, in which the timber is pre-heated, dried, heat-treated and conditioned in a continuous throughput process, which has a capacity of 30 000-50 000 m per annum. 

Railroad ties are not generally appropriate in an organic garden because of the tar that permeates them, which may be exuded in some conditions landscape timbers made of similarsized blocks of hardwood that have not been treated are available as an alternative. 

A most effective way to extend the Nation s timber supply is to use wood so that its service life is increased. The service life of wood in hazardous use conditions can be increased severalfold by the proper use of wood preservatives. It is estimated that the preservative treatment of railway ties results in an annual savings of 2.4 billion board feet of lumber and that, if utility poles were not treated, an additional 20 million mature trees of pole-stock quality softwoods would be needed each year simply as replacements for those destroyed by decay and termites. 

Another area of necessary research is development of treatments that will increase resistance of wood to fire penetration. The work done by Schaffer (31,33) and others in this field should be carried further. The slow rate of fire penetration in thick wood members is one of the basic assets of wood and has been accepted and utilized for many years in heavy timber construction. But thin wood members and paneling have a considerably higher fire penetration rate than thick wood members under severe fire conditions. A fire-retardant system that will give slower fire penetration means more available safety time for fire fighting personnel and for evacuation of occupants from a burning building. 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

In summary, in high-yield, short-rotation plantation forestry in rain forest sites such as Jari, and other locations with similar ecological conditions, nutrients are likely to become limiting after the second or third rotation therefore, yields will not be maintained without extra fertilizer inputs. Repeated fertilization can upset soil nutrient balances and alter microbial populations, and it would also pose problems for the economics of producing low-value commodity timbers (see also Jordan, this volume). 

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