A wood preservative is a process or substance that when applied to wood will keep it safe from dangers such as rot, insects or water damage longer than if the wood was left untreated. There are a large number of different chemical and mechanical processes used to preserve wood. The most common substance used in non-industrial wood is copper. As far as purely mechanical processes, heat and fire treatment is most common. In all cases, the goal of a wood preservative is to remove air and water from the wood without causing it to splinter or crack.
Early wood preservation was done with pitch or tar. These substances are still used in modern wood preservation is some circumstances. Generally, the petrochemicals in the pitch and tar are applied by themselves, removing some of the mess associated with these substances.
There are several different compounds used based on the overall protection goal and use for the wood. The most common non-household substance is chromated copper arsenate (CCA). The CCA wood preservative uses copper as a fungicide and arsenic as an insecticide, while the chromium keeps them both in the wood. This makes the wood have a slight green color, a common sight on outside fences, decking material and power poles.
Since arsenic is so poisonous, many areas have moved away from new construction using CCA. In its place, most wood preservatives have gone to alkaline copper quaternary (ACQ) or copper azole (CA-B). These preservatives work in much the same way as CCA, but with less of its negative side effects. As a downside, the high amount of copper in ACQ destroys iron and steel, resulting in much more extensive construction costs.
Outside of copper, several other chemicals are used. Two of the more common non-copper compounds are borate and silicate-based. These provide adequate preservation but very little by way of insecticide. In addition, these chemicals leach out of the wood when exposed to water, making them unusable is some areas.
The copper, borate and silicate compounds are all water-based wood preservatives. Oil-based compounds, such a petrochemicals and plant oils, are common in industrial treatments. Some of these chemicals have a high toxicity to humans and a foul smell. As a result, they are only used in areas where the wood will be wet constantly and where there are not a lot of people.
Heat treatments are common in some areas as a chemical alternative. Heat as a wood preservative has mixed results; it often depends on the method used and the wood type. The basic idea is the heat changes the makeup of the wood fibers, making them more water-repellent and less appetizing to insects.