Timber species vary in their structure and chemical ingredients. The structure of wood determines the natural protection from decay an the ability of chemical preservatives to penetrate the timber.
Sapwood has low natural durability, low density, a high moisture/starch content compared to heartwood. This make-up is conducive to fungal or insect degradation. Wood kept dry is able to resist invasion by fungi or insects.
Heartwood has chemical deposits within the cells with little to no carbohydrates and lower moisture content. Heartwood in some species, like sneezewood, can be highly durable. Radiata Pine or Eucalyptus Saligna species, the durability is low.
Durability may be increased by the addition of chemicals that are toxic to fungi or insects or reduce the moisture content of the wood. The addition of these chemicals is the basis of the timber preservation industry.
The ability to penetrate wood with chemicals toxic to insects or fungi is largely dependent on the preservative liquid to protect it. Most preservatives will not pass through cell wall membranes and require pressure to push the chemicals deep into the wood. The depth to which penetration is achieved is dependent on density, chemical inclusions within cells, moisture content, cell types and techniques used.
Softwoods such as radiata pine are more easily penetrated than compared to hardwoods.
Deterioration of Wood
Depending on the conditions of service, timber may be attacked by one or more agents to cause degrade. Proper design and preservation practice can eliminate or minimise such attack.
The main causes of attack:
The fungus develops from spores and sends out filaments called hyphae. These penetrate the wood structure and break down the wood tissues into simple chemical compounds on which they feed.
Timber attacked by fungi is sometimes covered by hyphae resembling cotton wool, collectively called mycelium. When the fungi mature it produces mushrooms. The mushrooms form a fleshy plate standing out on edge of the decayed wood or a thick flat skin covering part of the wood.
Fungal spores are produced by the fruiting body in vast numbers and may be carried by air currents, animals or birds for considerable distances to other wood where they will germinate.
Timber preservation is largely built around making the wood nutrient toxic, unpalatable or uninhabitable. The science of wood preservation is the treatment of wood to increase durability and give an extended service life.