Impregnating agents and how to avoid them

As has been illustrated in several chapters in this book, wood is, for most purposes, a good choice from the environmental point of view. However, where possible, wood should be used in a way that ensures a long lifetime and, as far as possible, not be treated with toxins or additives.

Wood and other organic materials are easily attacked by insects and fungi in damp conditions. In central and Northern Europe, six types of insects are especially attracted to timber buildings (see Table 19.1)Table 19.2. Fungus is a type of lower plant species that lacks chlorophyll. Fungi that attack buildings can be divided into two main groups, discolouring fungi and disintegrating fungi. Discolouring fungi give timber a superficial discoloration, without decreasing its strength. Disintegrating fungi attack the cell walls in timber and destroy the wood.

Spores from disintegrating fungi are ubiquitous. They spread with the wind in the same way as pollen, and attach to anything. These fungi belong to nature's renovating corps, their main operation being the breakdown of dead organic material, which regrettably includes many building materials. The optimum conditions for this phenomenon relate to dampness, temperature and acidity. Dampness in organic material needs to be from 18 to 25%. Humidity above or below these figures is not attractive to these spores. The majority of fungi, however, survive long dry periods. A temperature between 20 and 35 °C makes an attack possible, but there is no activity below 5 °C. Disintegrating fungi do not strike in environments with a high alkaline content, i.e. with a pH over 6.0. One exception is the Merulius lacrymans.

There are four principal ways to avoid attack from insects and fungi:

1. Use of high quality material in exposed locations.

2. Structural protection of exposed materials.

3. Use of non-toxic treatments: passive impregnation.

4. Use of toxic substances: active impregnation.

The toxic preservatives are usually divided into insecticides and fungicides. The concept behind them is the creation of biological toxins that kill, which frequently has unforeseen consequences for other animal species, not least humans. The main task of this chapter is to show how these substances can be avoided.

Table 19.1 Vermin



House longhorn beetle (Hylotrupes bajulus)

Does not attack heartwood in pine

Carpenter ants (Camponotus herculeanus)

Does not live on wood, but uses it as its home and lays eggs, even in pressure-impregnated wood

Common furniture beetle (Anobium punctatum)

Prefers a temperature of 20-25 °C and a relative humidity of 50%, only found in coastal areas

Woodworm (Dendrobium pertinax)

Attracted to wood that has already been attacked by fungus

Violet tanned bark beetle (Callidium violaceum)

Dependent on bark left-overs for its survival

Bark borer (Ernobius mollis) Dependent on bark for its survival

Bark borer (Ernobius mollis) Dependent on bark for its survival

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