Damage to wood and wood products may be caused by:
- poor handling, inadequate packing for transport or exposure to changing relative humidity (physical degradation);
- heat (thermal degradation);
- light and UV radiation (photochemical degradation);
- biological agents such as fungi, bacteria or insects (biological degradation); and
- acidic or alkaline chemicals (chemical degradation).
Poor handling and packing, exposure to fluctuating relative humidity levels and insects usually cause physical damage to wood whereas heat, light, fungi, bacteria and corrosive chemicals cause chemical changes to the wood substance. These latter changes can lead to the complete destruction of the wood.
Wood may be physically damaged by either poor handling or as a result of stresses induced by changing moisture gradients. Its relative softness means that wood can be easily damaged by contact with harder and sharper objects.
Excessively hot and dry conditions cause wood to shrink and crack. Conversely, cold and damp conditions cause wood to swell and warp. Substantial damage may be caused by large, rapid fluctuations in relative humidity levels. This is particularly so if wooden parts are closely joined and if their respective grains run contrary to one another. The restriction of free movement often leads to warping and cracking.
The rate of change of relative humidity levels and the way in which the wood has been sawn (along or across the grain, radially) determine whether bowed, cupped, twisted, cracked or split wood will result from exposure to inappropriate or changing relative humidity conditions.
The expansion and contraction of wood may damage either the wood itself or materials attached to it (Figure 3). Paint layers, for example, will tend to craze and flake if the underlying wood is subjected to alternating periods of expansion and contraction.
Physical damage may occur if no precautions are taken when wooden objects are transported from one climatic region to another. For example, wooden sculptures transported from tropical regions to drier areas commonly develop serious cracks as moisture is lost from the wood. Similar damage may occur if items composed of wood or other organic substances are exposed to hot sunlight during the day and cold, moist conditions at night.
The most severe form of thermal degradation is obviously the complete destruction of wood by burning. The individual components of wood (hemicellulose, cellulose and lignin) are affected by heat before the state of combustion is reached.
Heat weakens the bonds between fibres, leading to increased brittleness and fragility. For example the embrittlement of newspapers with age can be attributed largely to thermal degradation of hemicelluloses present in the ground wood pulps. The extent of degradation is determined by the type of wood concerned, the temperature and the duration of exposure. As chemical breakdown usually occurs more rapidly in an aqueous environment, the presence of moisture increases the effects of thermal degradation.
Physical damage will occur if a wooden object is placed close to a heat source. The resultant loss of moisture often results in shrinking and cracking of the wood. Thermal degradation is considered to be the most serious threat to wood kept indoors.
Chemical, mechanical and light energy factors combine to contribute to the deterioration of wood that is exposed outdoors (weathering). The general appearance and surface finishes of historic structures are often affected.
When wood is kept reasonably dry and exposed to sunlight or UV radiation, the surface tends to turn brown. A grey finish is observed when the effects of light and moisture are combined. The reactions are chiefly due to changes in the lignin caused by exposure to UV radiation. Colour changes are usually restricted to the surface of the wood.
In addition to the problems associated with photodegradation of wood itself, excessive light exposure will cause bleaching of certain dyes or pigments and fading or discolouration of surface finishes.
Given the right conditions, a variety of different organisms will attack wood. These include:
- fungi (such as the brown, white and soft rots); and
- insects (such as termites, furniture beetles and powder post beetles).
The type and quantity of extractives present in the heartwood are important in determining the biological resistance of wood.
The sapwood of most species and the heartwood of many others are prone to attack by a number of different wood-rotting and wood-staining fungi. Wood-staining fungi (mould) flourish in damp, dirty and unventilated conditions. Although moulds merely stain the surface of wooden items, wood-rotting fungi, such as dry rot, severely damage structural timbers. Wood-rotting fungi require moisture, some air, moderate temperatures and the absence of toxic influences to develop.
Termites (white ants), black ants and other insects are also responsible for serious damage to wooden materials. The most commonly occurring wood-boring insects found in museums are the furniture beetle and to a lesser extent, the powder-post beetle.
The furniture beetle usually attacks softwoods such as pine and fir and is commonly found in damp floorboards. It also will attack mature hardwoods such as oak and walnut and is responsible for considerable damage to old furniture.
The powder-post beetle only attacks hardwoods and occurs most frequently in damp timber or in structural timbers of recent buildings.
Wood is generally resistant to a large number of chemicals, but it is still vulnerable to chemical attack, particularly under conditions of high acidity (pH less than 2) or high alkalinity (pH greater than 11). Under these conditions the long cellulose chains and the shorter hemicellulose chains are broken producing a softer, more friable object.
Chemical degradation is enhanced by high temperatures and the presence of iron salts, sulphur dioxide and oxidising agents such as chlorine and nitric acid.