The Green Thumb
Second Look (Part One)
This month, we'll begin a series of articles that will take an in-depth look at preservative treatments for wood. We'll start with a discussion of the nature of wood and the ways it is naturally and artificially rendered insect, decay and fungus resistant.
Mother knows bestMother nature has made several species of wood naturally resistant to insect infestation and fungal growth by the presence of naturally occurring oils, resins, tannins and chemicals. Of these many species, only cedar and redwood are readily available at lumberyards. Cedar has a strong odor and is intolerable to some sensitive individuals-especially when freshly cut-whereas redwood is much less odorous and tolerated by sensitive people. Of all the naturally resistant species, most exceptionally resistant are red mulberry, black locust and osage orange.
At the center of the tree-trunk (the heartwood) can be found the most resistance while at the outer layers or sapwood, the tree is least resistant. Some other high durability, resistant species are black cherry, chestnut, juniper, black walnut and pacific yew. Unlike redwood and cedar, these species typically are not commercially available except for special order.
Friend or foeWe consider insects and fungi enemies to the built environment but in nature, they play an important and vital role in the delicate ecological balance that makes life on earth possible. In effect, these biological organisms are the agents by which dead trees are recycled into the ground soil where life springs eternal. Where wood is harvested from trees as a raw material for building lumber, it may pass through many decay resistance processes before it arrives on a construction site.
Often, foresters spray live trees with insecticides, much like crops are sprayed, to control insect infestations. Also, after trees are freshly cut, they may be sprayed once again to control insect damage and/or mold growth. After milling, the rough boards may again be sprayed due to the danger of fungal attack during the storage phase for air-drying.
For kiln-dried lumber, though the temperature in a kiln is high enough to stop fungal growth, it is not high enough to kill fungus or their spores. A rise in wood's moisture content may provide the trigger for these dormant fungi/spores to start growing. Thus, kiln-dried lumber is not a guarantee for eliminating decay organisms completely from the manufacturing process.
Moisture content is the most critical aspect for keeping framing lumber fungus-free. A moisture content of plus 28 percent is usually enough to get decay fungi to start growing but a moisture content of 20 percent is enough to sustain such growth. It's recommended that wood be kept dry at levels below 20 percent moisture content and stored in locations where it will not get wet and/or experience high relative humidity.
These two factors are the main determinants as to whether fungi growth will be triggered. Moisture content in framing lumber is typically much lower than 20 percent and can be as low as 3 to 5 percent.
Most building lumber is not sprayed at each step of the harvesting/milling/storage/drying process-sometimes, it is not sprayed at all. If the wood was indeed sprayed with a chemical insecticide/fungicide anytime in the manufacturing process, it probably will be gone by the time it reaches the end-user. Such chemicals tend to adhere to the surface of the wood rather than penetrate through it. Thus, when planed smooth in the mill, chemical residue will be found in the shavings and sawdust on the mill floor-not on the processed wood's surface.
An ounce of preventionThere are a variety of chemical preservatives that are toxic to fungi, insects and, in turn, to humans. After wood has been properly dried and processed into lumber, one of four classes of chemical treatments may be applied to prevent decay, rot or infestation:
• Oily preservatives
• Solvent-soluble organic chemical
• Water-soluble salts
• Water-soluble boron compounds
Derived from coal tar or the wood tar of beech-wood trees, creosote is dark, oily and highly odorous. In the past, it was typically used to protect wooden railroad ties and utility poles. Due to health concerns, it is no longer widely available as a commercial preservative treatment. Vapors containing phenols and their ethers are emitted into the environment from the creosote preservative. These vapors are particularly acute on hot days and not just sensitive individuals find them offensive-many people suffer ill effects from breathing in these fumes. Lesions and skin cancer can result from topical contact of creosote with the skin.
Solvent-soluble organic chemical:
"Penta"-short for pentachlorophenol-can be found in small amounts in common consumer items, such as paper, soaps, leather, cotton and cleaning solutions. Like creosote, it was used as a wood preservative but is no longer widely available due to serious health concerns. Penta is absorbed into the body by inhalation, ingestion and skin absorption. A urine sample of the typical American will reveal the presence of trace amounts of penta. In Hawaii, these levels are seven times higher than in the rest of the United States.
An incident involving three dairy farmers who were poisoned by penta-treated wood and 600 cows that died from licking penta treated wood dramatizes the danger posed by this chemical compound. Penta can cause fetal death, embryotoxicity, chloracne and liver damage. For these compelling health concerns, creosote and penta are now considered wood preservative treatments of the past and have been use restricted by the Environmental Protection Agency.
In part two, we'll continue our series with an in-depth look at the third class of chemical treatments for wood: water-soluble salts.