Historically wood has been used for almost anything that you can think of — fuel, shelter, weapons, fabrics, chemicals, naval stores, etc. More recent uses include building panels and cross laminated lumber. There are even plans for high-rise buildings. Not too much on the food front, although I have had some questionable meatloaf from a restaurant located next to a sawmill.
As a building material, wood has always been valued for its strength, ease of use, versatility and durability. As to durability, if wood is protected from the elements, it can last many years. However, exposed, it is susceptible to decay from a variety of agents, both plant and animal. Some species of trees, such as bald cypress, some cedars and redwood, are known for their resistance to decay, although it is the heartwood found in older trees that exhibits this resistance. Likewise, the heartwood of old-growth pine is quite durable.
But most people do not have access to such naturally resistant wood and that deficiency is addressed by using some type of preservative. There are several different preservative materials and methods to treat the wood, depending on the anticipated end use.
Let’s look at a few of those.
Surface applied coatings, such as paint, varnish or oil, are not considered wood preservatives. Although copper naphthenate, a non-restricted use pesticide, has been used as a brush on (or dip) with success for years. Disposal of wood thus treated is considered non-hazardous and disposed of the same as raw wood. Lead paint may be an exception, however.
Chemical preservatives can be classified into three broad categories: water-borne (ex. chromated copper arsenate [CCA]), oil-borne (ex. Pentachlorophenol [penta] and creosote) and light organic solvent (ex. insecticides in a light oil base, no heavy metals).
Typical uses around the homestead might include uses that are in contact with the ground, such as flowerbeds, gardens, pilings and posts. Above ground uses might include playground structures, decks, docks, feed troughs, etc.
A concern when using treated wood is leaching of the chemical, e.g., from raised garden beds into the soil and into the soil beneath playground structures.
Although CCA was the standard for several years, it and creosote and penta are not registered to treat wood for consumers. Since the restriction of CCA, borates (boric acid, oxides and salts) are often used where human contact is expected.
Borate-treated wood is an effective wood preservative and does not contain copper or other heavy metals. However, borate compounds do not become fixed in the wood and can be partially leached out if exposed repeatedly to moving water, such as during rain. When decks, docks, walkways, playground equipment, or other structures are exposed to such conditions, the wood should be sealed. Check end tags on treated wood to find out its intended use and what preservative was used.
Commercial uses of preserved wood might include utility poles, pilings, road signs, commercial docks, seawalls, livestock structures, etc. These would be items that see heavy use and limited non-commercial human contact. The above mentioned CCA, creosote and penta are often used here.
A typical method to get the preservative into the wood is pressure treatment. Wood is placed into a pressure vessel which produces a partial vacuum to prepare the wood to receive the preservative. Then the pressure in the vessel is increased and the preservative chemical is forced into the wood. By manipulating the amount of chemical, the pressure and the timing, the amount of chemical forced into the wood and the depth of the treatment can be controlled and will vary based on the desired results. Sometimes the absorption of the chemical is improved by incising, i.e., piercing the surface of the wood, making numerous shallow, slit-like holes.
Another application of treated wood utilizes a fire-retardant chemical. It is applied under pressure like the preservatives, or sometimes as a surface coating. Exposed to fire, the treated wood will char, but it will not oxidize, which slows the spread of the fire to the material. It is used mostly in commercial buildings. As more wood products are used in commercial buildings, this should become more of an issue.
Kiln Dried After Treatment (KDAT)
An issue with some treated wood is its habit of bowing and twisting once it is exposed to the elements. It is something that we just deal with, replacing the bad boards and living with the others. But when that is unacceptable, there is the option of lumber that has been kiln dried after the preservative treatment. Kiln Dried After Treatment lumber (KDAT) is a drying process that occurs after pressure treating wood — once lumber is pressure treated with chemicals, kiln drying removes excess moisture to help minimize the natural tendencies of wood to warp, cup and shrink. The finished material is more stable, easier to use, lighter than “wet” or “green” treated lumber, and it has better nail-holding and screw-holding power. It is also more expensive. It is offered as a premium product by several manufacturers.
When disposing of treated wood, old or new, there are certain standards that are supposed to be followed. Keep it separate from other waste, protect it from runoff, and do not burn it.
A word of caution. Although treated wood is safe to use, keep in mind that is not a good idea to fill your eyes or lungs with sawdust from chemically treated wood. Or to get chemically treated splinters shoved under your fingernails. Use reasonable safeguards when using treated lumber.
To learn more about preservatives, uses and precautions, here are some websites.
• National Pesticide Information Center — https://npic.orst.edu
• Wikipedia Wood Preservation — https://en.wikipedia.org/wiki/Wood_preservation
• American Wood Protection Association — https://awpa.com
• LSU AgCenter — https://lsuagcenter.com
(David Lassiter is a consulting forester from Stonewall. He can be reached at email@example.com.)