Woodworker's Journal winter-2009, страница 61
Fungi Fighte hen building outdoor projects you really have just two choices: you can paint most any wood and expect it to last for years (like the Apple-Shaped Birdhouse shown on page 56), or you can build with species having high resistance to decay, such as redwood, cedar and cypress. Too often people choose ordinary woods for exterior use, finishing them in a penetrating oil, only to be disappointed when their project quickly deteriorates. If you have doubts, try a little experiment. Set a piece of redwood and birch outside. You'll soon notice the birch beginning to decay while the redwood develops a beautiful patina. Why do some species resist decay while others fall to ruin? The answer is found deep in the wood cells where decay is caused by fungi spores, a simple form of plant life found virtually everywhere in our air and water. Fortunately, certain species are less hospitable hosts than others. These woods have unique extractives in their heartwood that are capable of repelling or killing fungi. In the absence of these extractives, fungi quickly gets into the wood and actually dissolves the cell walls to begin the rotting process. As long as conditions for growth are present, fungi will continue destroying the internal structure of the wood. Necessary ingredients for fungi growth include a moderate temperature, oxygen and moisture. If any of these conditions aren't met, fungi growth will Cedar (Thuja occidentalis) Redwood (Sequoia sempervirens) subside or stop altogether. In fact, some species avoid decay by repelling water, thus depriving fungi of one essential element for growth. It's important to remember that sapwood is generally more prone to decay, even in circumstances less favorable to fungi growth, because it lacks the fungi-fighting extractives found in heartwood. Resistance to decay means, of course, that the process is slowed down. Unfortunately, it's never eliminated. All species differ in the type and concentration of their extractives, and this significantly influences their rates of decay. Some of the best decay-resistant woods, such as teak and mahogany, are expensive and difficult to find. If you need a hardwood with reasonable exterior qualifications, white oak is a good choice. However, all woods eventually decay as their protective extractives weaken. The difference is that species like cypress, redwood and cedar last a generation, while most woods barely survive one season. a ' J ' p V-r » ■ i %-v' Cypress (Taxodium distichum) n: |
