Wood is naturally very durable--in moderate, sheltered environments it can last for thousands of years without substantial change. Biological deterioration is found principally in the form of insects and fungi. While this activity can result in the complete destruction of an object, it pales by comparison to use and the abuse by humans. And by contrast to human intervention, biological deterioration is an often preventable form of damage.
Many other environmental agents (for example, light and heat) are causes for decay of wood. Since a tree is naturally part of ecosystems, it must decompose. Natural decay contributes to the supply of nutrients for other plants and animals, including the offspring of a decomposing tree. However, when decay processes become evident in historic objects, there is justifiable cause for alarm.
Most of the biological damage is started in poor environmental conditions for storage and display. Continuous biological decay is dependant upon four conditions. If any one is removed, the decay is arrested. They are:
a supply of oxygen
a supply of moisture
a supply of food
a moderate temperature range.
Let's examine them in detail.
Supply of Oxygen
A continuous supply of oxygen is required by terrestrial life forms, including those which consume wood. When the supply is diminished, or removed entirely, the consumption ceases and the organism will be destroyed. This is in fact a new method for eradicating wood destroying organisms. These so-called "anoxic" methods require the use of sealed chambers, and are sometimes a good alternative to pesticides.
Supply of Moisture
Control of this condition can the most difficult. The moisture content of wood is dependent upon the amount of moisture in the immediate surroundings. The term "relative humidity" is derived from the relationship of moisture in the air and temperature of that air. The amount of moisture (in the form of water vapor) can be stable in a given volume of air, and as that air is heated and cooled, the relative humidity changes. Since the capacity of warm air to hold water vapor is greater, the relative humidity is lower than a cooler room with the same amount of water vapor. Conversely, as the air is cooled, the relative humidity "goes up." This explains why on a summer day a basement can be extremely humid, compared to the upper floors of a home. Relative humidity is usually reported as a percentage, comparing the amount of water vapor in the air at that temperature to the amount that is held if the air is saturated (100% relative humidity).
Wood, like many materials, will reach a balance of moisture content with the surrounding environment. Wood takes on moisture in high relative humidity conditions, and releases it when the humidity is lower. In extreme changes, the wood can swell (high humidity) and shrink (low humidity). Excessively high moisture conditions can cause swelling of wood which can result in crushed components, and finish and glue failure. Excessively low moisture conditions can cause damage to wood such as splitting. Between the extremes of moisture lies the ideal moisture content for good preservation, but also the range that supports biological decay processes. Extremely high moisture content (more than 60%) is the ideal for mold growth. But even moderate conditions can support decay: some insects, such as termites, can generate their own moisture as they consume wood.
Supply of Food
While this factor may seem terribly self-evident, in some cases, the actual food source for some organisms is not wood. In the case of fungi (molds and mildew), the food source may be on the surface of the wood. Handling an object may leave residues which support the growth of these organisms, under the right conditions. Fungi are probably the agent of the greatest destruction of wood in nature. Often, fungi damage wood, which makes it more inviting to insects. Insects which completely consume wood include termites and beetle larvae. These insects can nearly completely consume an object or historic structure before they are detected.
Organisms which destroy wood have been found in virtually every environment. Unfortunately, the optimum temperature range of 75-90°F ( 24-32°C) is one that humans are comfortable in, so temperature is not a practical way to control the organisms. Amazingly, a newly-discovered species of termite has been discovered which even operates at sub-freezing temperature. Only sustained sub-zero temperatures, or sustained high temperatures have shown any success in arresting development or destroying wood-consuming organisms. And unfortunately, the object may be severely damaged as a result of extreme low temperature, since it may be dried out and shrink in this condition. High temperatures can cause damage to surface decoration and paints, and may cause glues to release.
Common Types of Wood Destroying Organisms
There are two main types of wood destroying organisms: fungi and insects. Infestation by either one can lead to complete destruction of an object. In fact, infestation by one usually leads to infestation by the other. Wood, fungi, and insects form a biological cycle which has been perfected over millions of years.
Fungi includes molds and mildews, and are everywhere in the environment. Fungi are simple plant-like organisms which do not have chlorophyll to produce their own food. In essence, they have a parasitic relationship with their host. They rapidly multiply in a supportive environment, and send seed-like spores out (sometimes air-borne) to extend their range. Some fungi do not affect the strength of wood by their activity--they may only stain the wood. However, some can completely destroy the wood cellulose (polymer backbone) and lignin (cellular Aglue@). Ultimately, the wood residue crumbles to a powder.
There is almost no point trying to eradicate or exclude fungi, since they are so ubiquitous. However, we can successfully control their consumption of wooden objects. Simply put, if the relative humidity is kept moderate (less than 60%), there will be virtually no problems with fungi.
Insects such as beetles in their larval stage are among the most destructive. These insects lay eggs in wood; the maturing larvae eat continuously, only emerging to mate and continue the cycle. The emerging adult will leave a small exit, or flight hole, about 1/16" (1.5mm) in diameter. While unfavorable conditions may slow their development, the usual life cycle is complete within a year.
Termites can pose a threat to furniture, but are usually more interested in the wooden building surrounding the collection. Other insects present a special problems, since they may not even use wood for food. Some ants and bees tunnel through wood creating galleries for shelter. These are not usually a problem with furniture but can be a major destructive agent in historic buildings.
Protection of Wooden Objects
From the comments above, you may gather that removal of one or more of the favorable growth conditions is sufficient to arrest the spread of biological deterioration. The simplest method to deter fungal and insect damage is to maintain and monitor a moderate environment. Since the most controllable factor is the moisture content, good collections care will include maintenance of a 40-60% relative humidity. Naturally, some special objects may require other conditions, but most will do well in this range.
Wood has variable degrees of natural resistance to decay. Over time, craftsman have selected wood for use in adverse environments, such as fence posts. Some wood lasts for years, but others would decay in one season. Wood with low resistance can be treated with preservatives. Insects and fungi show little interest in penetrating paints and varnishes to consume wood. So, those objects which are, or can be coated will be better off. However, environmental controls are still the best line of defense.
Recognition and Treatment of Deteriorated Wood
How can you recognize an active infestation, and how should you react? Insect damage is most often noticed when debris is found on the floor. This digested wood is called frass. Frass can also be loosened from old tunnels, giving the impression of new damage. Exit holes will appear freshly cut in new damage; old exit holes will have collected dust and dirt. If you have observed fresh exit holes, you should remove the object from the rest of the collection and completely wrap and seal the object in polyethylene sheeting. Inspect the rest of the collection for damage. If possible, place white paper under the wrapped object and continue to look for frass. Emerging adults may also be observed. Infested objects should be treated by a professional extermination company. Special concern should be given to any special considerations, such as the effect of the treatment on metals, paints and textiles. Extermination procedures should be carried out in the company's facility.
Fungal damage is usually noticed in the advanced stages of development. The fruiting bodies can form a dense, fuzzy-appearing mat on the surface. Infestation may also be accompanied by a distinctive, disagreeable odor. Fungal growth can be removed by wiping or vacuuming the surface. Some people are sensitive to airborne spores, so care should be taken in cleaning. Vacuuming should be conducted with a wet-dry vacuum if possible, and the filter should be replaced or thoroughly cleaned afterward. Cleaning should be done outdoors, if possible.
The cause of the insect or fungal infestation should be determined, and conditions responsible should be changed. This will prevent a reoccurrence, and safeguard the rest of the collection.
Hoadley, R. Bruce. Understanding Wood, 1980, Newton, CT: Taunton Press.
Panshin, A. J. and Carl de Zeeuw. Textbook of Wood Technology, 1980, NY: McGraw-Hill.
Story, Keith O. Approaches to Pest Management in Museums, 1985, Washington D.C.: Smithsonian Institution.
Written by Melvin J. Wachowiak, Jr., Senior Furniture Conservator, MCI.