|Research Paper 48||December 1986|
Donald J. Miller
Eight series of untreated posts, 16 series of nonpressure-treated posts, 11 series of pressure-treated posts, and 5 series of steel posts remain in test at the Oregon State University post farm in western Oregon. So far all posts have failed In 36 untreated series, 35 nonpressure-treated series, and 2 pressure-treated series. Causes of failures since 1949 are: fungi, 75 percent; fungi and termites. 16 percent; fungi and insects other than termites, 7 percent; and termites, 2 percent. A few steel posts have failed because of corrosion.
Some series of wood posts deserve special mention for their notable durability, sometimes the result of simple preservative treatments. Exceptionally durable series include some that were pressure-treated with creosote (series 7. 23) and untreated posts of Osage-orange (series 32). Their average lives may exceed 60 years. Soaking posts in solutions of creosote or pentachlorophenol (3 hours for black cottonwood, series 87; longer for lodgepole pine, series 86, and Douglas-fir, series 94) has substantially lengthened their lives. With that simple treatment, the average life of the lodgepole posts may exceed 50 years, and Douglas-fir and black cottonwood posts may each average about 40 years. A similarly treated series (88) of Douglas-fir posts with bark left on the upper portions should last about 36 years. Brushing undried posts with Osmosalts (series 75). a convenient treatment, will extend their average life to about 40 years.
Untreated posts. Western juniper and black locust, two of the three durable-heartwood species remaining in test, will have average service lives exceeding 30 years. No Osage-orange posts have failed since they were set 52 years ago. but their exceptional durability is mainly of academic interest since Osage-orange is not available in the Pacific Northwest. Posts of other durable-heartwood species such as Pacific yew averaged 25 years, redwood and most species of cedar averaged 19 to 24 years, and Oregon white oak averaged 18 years. Series of 28 nondurable species had lives that usually averaged from 4 to 6 years and not longer than 9 years.
Few steel posts have failed during 37 years of testing. Failures occurred only in series 61 and 70 as a result of corrosion at ground level.
Nonpressure-treated posts. For the longest service from nonpressuretreated posts, the entire post must be treated.
Double-diffusion butt treatment with solutions of copper sulfate and sodium chromate did riot increase the life of posts. Similar treatment with sodium fluoride and copper sulfate should extend post life to 27 years or more, although poorly treated tops show decay. Soaking the whole post, rather than the butt only, would improve this treatment.
Most brushed-on treatments have added only a few years to the life of Douglas-fir posts, but two coats of pentachlorophenol-diesel oil solution extended average life from 6 years (when posts are untreated) to 14 years. Diffusion treatments with Anaconda Copper Mining Company (ACM) paste or dust, or with Osmosalts, have been effective but are no longer available. Posts butt-treated with ACM paste probably will have an average life of 30 or more years despite badly decayed tops. Posts brushed completely with a slurry of Osmosalts and water and then piled under a tarpaulin for 3 weeks to allow the chemicals to diffuse into the wood have had few failures, and all tops remain sound after 36 years of testing. Their average life is estimated to be 40 years.
Treatment with sodium pentachlorophenate or sodium trichlorophenate in holes at the ground line was ineffective. Similar treatment with salt and mercuric chloride protected butts for at least 28 years, but post tops have decayed.
Cold-soaking incised butts of posts of lodgepole pine for 43 hours or more In a 5 percent solution of pentachlorophenol may extend their average lives to 50 years or longer. Similar posts of Oregon maple soaked for 24 hours should have equally long life but have suffered badly split tops. Posts of black cottonwood soaked for only 3 to 6 hours are expected to have an average life of about 40 years. Similar 40-year life is expected from Douglas-fir posts that were soaked for 6 days. Soaking in copper naphthenate (1 percent copper) was less effective, but better results might be obtained with greater concentrations (2 to 3 percent copper). Douglas-fir posts peeled only at the butt, incised, and soaked for 7 days in creosote are expected to last an average of 30 or more years; the posts have retained their bark, and tops are sound. Ponderosa pine posts soaked for 17 hours in Permatol had an average life of 19 years.
Hot-cold bath treatment of unincised butts with creosote for 6 hours extended average life of split black cottonwood posts to 22 years and could extend average life of square Douglas-fir posts beyond 35 years if the tops do not decay severely. Similar treatments with carbolineum and with creosote diluted with oil were less effective.
Pressure-treated posts. No creosote-treated posts have failed since installation 46 to 56 years ago. However, two series of posts treated with water-borne preservatives have failed. Treatment with chromated zinc chloride extended life to 20 years, while other posts treated twice with zinc-meta-arsenite lasted an average of 26 years. Average lives of posts of most series treated with other water-borne preservatives are estimated to be 40 to 50 years. Treated tops of all posts are sound.
THE POST FARM
In 1927, Professor T.J. Starker of the College of Forestry at Oregon State University (OSU) established a "post farm" to develop data on the natural durability of native woods and the effectiveness of various preservative treatments for species used as fence posts. Since the first posts were set on January 7. 1928, OSU has placed 2,662 posts in the farm. Three introduced and 25 native species in untreated condition and 8 Oregon species receiving various preservative treatments have been, or are being, tested.
The post farm is located on College of Forestry land in the Peavy Arboretum about 7 miles north of Corvallis, Oregon, on the West side of Highway 99W. Soil in the test area, located on a well-drained south slope, is Olympic silty-clay loam. The top 8 inches of soil, slightly acid (pH 5.4), have 1/2 inch or less of humus. Its organic matter and nitrogen content are 4.71 and 0.14 percent, respectively. In the past, the test site has been sprayed with herbicides to control brush.
The area typically has dry summers and rainy winters, a generally mild climate that favors growth of wood-destroying organisms throughout the year. During the past 92 years through 1984, annual precipitation averaged 42 inches, 81 percent of which fell from October through March when average monthly temperatures ranged from 39° to 53°F. Only 3 percent fell during July and August when temperatures averaged 66°F. Occasionally the temperature falls below freezing or rises above 85°F. Afternoon breezes from the Pacific Ocean cool the area almost daily during summer months.
Agents of deterioration
Since 1949, various causes of deterioration of the posts at the test site have been identified. Decay-producing fungi or fungi in combination with termites do the most damage. Discarded wings of damp-wood termites have been found at bases of some posts, and entry holes have been detected at or below ground line. However, termites alone have been the primary cause of failure in only a few instances. Carpenter ants and wood-boring beetles also contribute to the deterioration. Causes of failure in wood posts from 1950 through 1985 are:
|Fungi and termites||206||15.7|
|Fungi and insects other than termites||92||7.0|
The most vulnerable section of a post extends from a short distance above to some distance below the ground surface. This zone usually has a sustained supply of moisture and air favorable to destructive fungi. In, the test area, tops of posts may deteriorate also, but that deterioration normally proceeds more slowly.
Posts are usually installed in groups of 25, each group constituting a test series. Posts within a series are placed 2 feet apart in a row running in a northerly direction up the test slope, and test series are 3 feet apart. All posts are set 2 feet into the ground.
Before 1947, test posts ranged from 4 to 7 feet long and from 3 to 70 square inches in cross-sectional area at the ground line. Since then, posts in most series have been 5 feet long with cross sections of 7 to 27 square Inches. In 1950, post size was standardized at a length of 5 feet and cross sections were limited to 8 to 24 square inches at 2 feet from the butt ends. The cross-sectional area of each series of posts must average 14 to 18 square inches.
All posts are inspected every October. The inspector gives a moderate pull to the top of each post, then examines each post that breaks to establish the point and probable cause of failure. Deterioration of the top is rated by visual Inspection as slight, moderate, or severe. Recorded data for each series of posts are: source and species, size and type of individual posts, percentage of sapwood, processing before Installation or preservative treatment, preservative treatment (if any), date of installation, and remarks.
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INTERPRETATION OF DATA
Limitations of data
The data tabulated in this report should not be applied indiscriminately to every locality or to all service requirements for posts. Data are comparative within our test site and generally west of the Cascade Mountains. To fit other situations such as drier climate, colder climate, or summer irrigation, the data must be adjusted according to local experience.
The posts usually are not subject to stapling, nailing, or other physical forces that frequently reduce the service life of posts actually in use. Also, the arbitrary method used to determine failure, a pull at the post's top, may not be comparable to all physical forces that may be exerted on posts in actual service, but it simulates the most common force.
Influence of climate
Climate determines how long conditions suitable for decay exist in a given region. Optimal temperatures for growth of decay-producing fungi range from 75° to 90°F, and growth slows as temperature departs from optimum. If wood has a moisture content of 20 percent or less (ovendry basis), serious decay is unlikely.
In western Oregon, where favorable conditions of moisture and temperature exist for long periods, posts adequately treated with a good preservative at the butts often decay at the top long before ground-line sections are seriously weakened. Undoubtedly the long periods of dry or cold conditions in eastern Oregon retard the deterioration of post tops there.
A preservative may fall under one set of climatic conditions but may prove very successful under others. For example, a preservative readily soluble in water may leach from wood in a rainy region but not in a dry climate.
Consideration of post characteristics
Any evaluation of post service must consider characteristics of the wood. Size, amount of sapwood, and extractive constituents in the heartwood greatly Influence serviceability of untreated posts. Sapwood is not naturally insect- and decay-resistant, but extractive constituents in heartwood of a few species furnish resistance to attack by insects and fungi and usually give the wood a darker color. It should be recognized that naturally decay-resistant wood is not uniformly so; the amount of protective extractive tends to vary within and between trees of a species. Untreated posts can give long service if they contain a large amount of durable heartwood and little sapwood. Conversely, if posts are to be impregnated with preservative, an outer layer of permeable sapwood is desirable because it absorbs the preservative readily.
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PRESERVATIVE METHODS AND MATERIALS
The service life of treated wood is influenced by the effectiveness of the preservative, portion of the post treated, amount and permanence of preservative retained by the wood, and depth and uniformity of treatment. Most preservatives are effective fungicides or insecticides as long as the preservative remains present in a concentration that is adequate to combat the destructive organism.
Successful treatment provides uniform penetration into the treated area with retention of enough preservative to protect the wood under its expected conditions of use. However, high retention of preservatives does not necessarily mean full protection. For example, in some species, rapid penetration into end grain will completely protect the end of the post but provide almost no protection to the important ground-line zone. A useful remedy is to Incise (punch numerous incisions) into the side grain of the ground-line zone to enhance penetration of the preservative treatment there.
Virtually all preservatives are poisonous, and some are extremely so and also are corrosive. Many cause irritations when the chemical itself, its solutions, or its vapor touch the skin. Users should handle all preservatives carefully and frequently wash exposed parts of the body.
All preservatives should be stored in clearly labeled, closed containers and used only and exactly as recommended by the manufacturer.
The following list describes compounds that have been used, with varying degrees of success, to preserve the test posts.
ASPHALT EMULSION. Asphalt is a black-to-dark-brown, solid or semisolid material mostly composed of bitumens. The emulsion, a suspension of fine asphalt particles dispersed in water, has little or no preservative value.
BOLIDEN SALTS. This preservative contains arsenic acid, sodium arsenate, sodium bichromate, and zinc sulfate in a water solution.
CARBOLINEUM. Carbolineum, or anthracene oils, are coal-tar distillates, but the exact composition of carbolineum "B" is unknown. The specific gravity and boiling range for carbolineum are higher than for ordinary coal-tar creosote.
CHEMONITE. Chemonite solution consists of copper, arsenic, and ammonium acetate dissolved in an ammonia solution.
CHROMATED ZINC CHLORIDE. This preservative contains about 82 percent zinc chloride and 18 percent sodium bichromate in a water solution.
COPPER NAPHTHENATE. For optimum performance, solutions of this oil-soluble copper salt of naphthenic acid should contain 2 percent copper by weight. Test solutions contained 1 percent copper.
CREOSOTE, CREOSOTE OIL, OR COAL-TAR CREOSOTE. Produced by high-temperature carbonization of bituminous coal, this distillate of coal tar consists principally of liquid and solid aromatic hydrocarbons, as well as appreciable quantities of tar acids and tar bases. Its continuous boiling point begins near 392°F and ranges to at least 617°F.
CREOSOTE MIXTURES. Creosote may be mixed in varying Proportions with coal tar. Petroleum, crankcase oil, or other diluents that act as carriers for the creosote. Dilutions of more than 50 percent are less effective and therefore not recommended. Because it can cause hyperkeratosis in cattle, used crankcase oil should not be used where the animals can come in contact with it.
GASCO CREOSOTE. This distillate of tar residue from asphaltic-base petroleum oils, no longer available, was a by-product of the Production of artificial fuel gas.
OSMOSALTS. This proprietary wood preservativecontains sodium fluoride, sodium bichromate, dinitrophenol, and sometimes arsenic.
PENTACHLOROPHENOL. This is an oil-soluble chemical formed from phenol and chlorine. Solutions usually contain 5 to 7.5 percent pentachlorophenol by weight.
PERMATOL "A." A preservative containing pentachlorophenol as its toxic constituent, Permatol was developed-but since discontinued---by the Western Pine Association for the millwork industry.
SALT AND CORROSIVE SUBLIMATE. Not recommended as a preservative, this is a mixture of equal proportions, by weight, of two water-soluble compounds. The extremely poisonous mercuric chloride, or corrosive sublimate, is the toxic chemical, and the salt holds moisture.
SALT, CORROSIVE SUBLIMATE, AND ARSENOUS OXIDE. Also not recommended as a preservative is this mixture of equal proportions, by weight, of the three chemicals. The water-soluble arsenous oxide apparently contributes little, if anything, to the effectiveness of the highly toxic corrosive sublimate.
SODIUM PENTACHLOROPHENATE. This sodium salt of pentachlorophenol is water soluble.
SODIUM TRICHLOROPHENATE. This is a water-soluble salt of trichlorophenol.
TANALITH. Normally this proprietary wood preservative contains sodium fluoride, dinitrophenol. sodium chromate, and sodium arsenate In a water solution.
TREATER DUST, GRANULAR TREATER DUST, AND TREATER PASTE. These discontinued preservatives were produced by the Anaconda Copper Mining Company as by-products of copper smelting. Arsenic trioxide was the principal toxic component of the preservatives sold in dust, granular, and paste forms.
ZINC CHLORIDE. This compound has been used in a 2 to 5 percent water solution.
ZINC-META-ARSENITE. Made by dissolving zinc oxide and arsenic triaidde in water acidified with acetic acid, this preservative is no longer used commercially.
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Click here to see the tables of results referred to in this section. The most informative measure of post serviceability given in this report is average service life. Determining the service life of a series is simple when most or all of its posts have failed. However, average service life can only be estimated for those series with posts remaining. In this report, service life for such series is estimated from the number of failed posts and the service age and condition of remaining posts by a method reported by MacLean in Percentage Renewal and Average Service Life of Railway Ties, Report R886, Forest Products Laboratory, U.S. Department of Agriculture, Madison, Wisconsin.
Characteristics and service records of untreated posts are listed in Table 1 (completed series) and Table 2 (series remaining in test). The average service life of untreated posts varies greatly because of differences in the amount and durability of heartwood. Posts that are largely heartwood of durable species could have an average service life of 18 years or longer; such species include the cedars, juniper, black locust, white oak, redwood, yew, and Osage-orange. The latter species has been exceptionally durable, with no failure during 52 years of testing. Because natural durability varies greatly, all untreated woods will have a few early failures. Posts from nondurable-heartwood species, or posts that are largely sapwood, will have an average service life of 4 to 7 years; these posts should be treated with a preservative.
Nonpressure- treated posts
Characteristics and service record of nonpressure-treated posts are listed in Table 3 (completed series) and Table 4 (series remaining in test). Preservative treatments increased service life of Douglas-fir posts by the estimated amounts in Table 5. Evaluation of each treatment follows.
BORE HOLE. One or more holes three-fourths inch in diameter were drilled slanting downward about 2 inches from near ground level toward the butt of each freshly cut, unpeeled post. Holes should be spaced 5 or less inches apart and staggered vertically on the circumference to avoid serious weakening of the post. One tablespoon of a dry mixture, equal proportions by weight of salt, corrosive sublimate, and arsenous oxide, was placed in each hole. A snugly fitting wood plug sealed each hole. Effectiveness of the ground-line treatment increased with the number of holes. The treatments increased the average life of lodgepole pine posts to 18 years and that of Douglas-fir posts to more than 28 years. Similar treatments using more salt or the sodium salts of chlorinated phenols were less effective. Post tops were not protected by this method and became severely decayed. Because the chemicals applied in this treatment are very poisonous, we do not recommend using it.
BRUSHING. During hot days, two applications of preservative solution were flooded onto thoroughly air-dried posts. Oily solutions of copper naphthenate, pentachlorophenol, and creosote have added 3 to 8 years to average lives of some series of Douglas-fir posts. The best treatment was with a solution of 5 percent pentachlorophenol in diesel oil. However, because penetration and retention of preservative usually is slight, the brushing treatment is not recommended for wood in contact with soil.
CHARRING. Charring the surface of wood is not a preservative treatment. If anything, it reduces the life of posts by reducing their size at the critical ground-line area.
DOUBLE DIFFUSION. Freshly cut and peeled posts are soaked in an aqueous chemical solution for 2 or 3 days, then transferred to a similar solution of another chemical to soak for 2 or 3 more days. The chemicals diffuse into the wood where they react to form a toxic compound that is resistant to leaching. Treatments with copper sulfate and sodium chromate have not been effective. Treatments with sodium fluoride and copper sulfate, though ineffective with alder, have increased the estimated average life of Douglas-fir posts to 27 years. Lodgepole pine posts treated with zinc sulfate, arsenic acid, and sodium chromate will probably have an average life of about 25 years. Most posts treated by the double-diffusion method had decayed tops by their l1th year of life. For longer life, the entire post, rather than just the butts, should be soaked in the preservative.
HOT-COLD BATH. For this, also called the thermal treatment, dry posts are soaked in a hot (about 200°F), oily preservative solution for several hours, then either left In the solution while it cools to 100°-150°F or transferred to cool solution. The tests used several creosotes and a creosote-crankcase oil mixture. Generally effective, they prolonged the life of nondurable black cottonwood and Douglas-fir posts to as many as 22 and 18 years, respectively. A series (54) of sawed posts of Douglas-fir heartwood not dried before treatment had unaccountably good durability. Their average life could have reached 45 years or more, but their badly decayed untreated tops will cause premature failures. Posts for hot-cold bath treatment should be free of bark and thoroughly seasoned if oily solutions will be used, and should be treated full length during the cold bath.
OSMOPLASTIC BANDAGE. A strip 9 inches wide was peeled free of bark around the ground-line zone of each unseasoned post, then coated with Osmoplastic preservative and tightly wrapped with a water-resistant covering. Osmoplastic also was applied to post ends. The treatment was ineffective on posts of black cottonwood, but did increase the average life of Douglas-fir posts to 11 years. Osmoplastic bandages are not recommended for posts with nondurable heartwood.
OSMOSALTS. Peeled, unseasoned posts were fully coated with a brushed-on slurry of Osmosalts (2@ pounds of Osmosalts per pound of water). Coated posts were closely piled under a tarpaulin for 30 days to allow the preservative mixture to diffuse into the moist wood. Tested only on Douglas-fir, this simple and effective treatment has extended the life of the posts beyond 31 years and may extend it to 40 years.
SOAKING. Posts should be peeled, then thoroughly seasoned before soaking in the commonly used oil-type preservative solutions. Usually that part of the post 6 inches above and 12 inches below ground should be incised about one-half inch deep for better penetration of preservative. Post butts usually were soaked longer than tops, but the entire post may be immersed. Soaking time varied from several hours to 8 days in unheated solution. Soaking in a solution of 5 percent pentachlorophenol in diesel oil has proven an effective treatment. Soaking incised butts for 48 hours and tops for 6 hours in pentachlorophenol solution has produced an estimated average life of 30 years for Douglas-fir posts (series 64); average life of similarly treated incised lodgepole pine posts (series 86) win exceed 30 years and may reach 50 years. Absorbent posts of black cottonwood (series 68) with butts and tops soaked, respectively, only 6 hours and 1 hour, have an estimated life of 41 years. Gasco creosote, no longer available, also was effective. Douglas-fir posts, peeled only at the butts and then incised, dried, and soaked in Gasco creosote, have an estimated life of 36 years; their soaking periods were long, 7 days for butts and 2 days for tops. Copper naphthenate (1 percent copper) in diesel oil has been less effective. Treatments with water solutions of sodium pentachlorophenate (series 74) and zinc chloride (series 12) were not effective. For longest life, the full length of incised and well-seasoned posts should be soaked in an effective preservative.
TIRE TUBE WITH CHEMONITE. A section of an automobile inner tube was slipped over the butt end of an unpeeled, freshly cut post inclined on a rack so that the butt was higher than the top. The open end of the tube was elevated, and the tube was filled with a water-soluble preservative that diffused through the sapwood and finally dripped from the lower end of the post. This end-diffusion treatment has extended the estimated average life of Douglas-fir posts to 29 years, but the tops are decaying. Although posts can be treated without peeling or drying, the diffusion process is slow and each post must be treated individually.
TREATER DUST AND PASTE. These preservatives, no longer available, were tested on freshly cut Douglas-fir posts. Dust and granules were sprinkled around unpeeled posts while the postholes were backfilled with soil, or 2 to 4 pounds of paste were applied to butts of peeled posts. Two pounds of paste extended the average life of posts to 30 years; 4 pounds of paste protected butts longer but tops became severely decayed. Dust and granules extended average life to 26+ and 21 years, respectively.
Characteristics and service records of posts of all species treated by pressure processes are listed in Table 6 (completed and active series). Pressure treatments increased service life of Douglas-fir posts by the estimated amounts in Table 5.
Before such treatment, posts are air-dried, seasoned in the preservative by boiling under vacuum, or conditioned by steaming. Usually oily preservatives are heated to higher temperatures than water-borne preservative solutions. Preservative is injected into the wood under pressure in a closed vessel, and a final vacuum usually is applied to remove excess preservative. The full length of the post receives treatment.
Square posts sawed from west coast hemlock have had fewer failures than similar posts of Douglas-fir. Two series of pressure-treated Douglas-fir posts have failed. Average life of posts treated with chromated zinc chloride was 20 years; those treated with zinc-meta-arsenite had an average life of 26 years. Average life of most remaining series is expected to exceed 30 years; that of posts treated with Chemonite or Tanalith is likely to reach 40 or more years. Some series treated with creosote or creosote-petroleum have reached 56 years with no failures.
Pressure treatments have been most consistently effective in greatly increasing the service life of posts of nondurable wood. Such treatments yield longest service under severe conditions.
Notably durable posts
Oregon species and a few exotics that have untreated heartwood of notably good durability, and the most successful preservative treatments, both pressure and nonpressure processes, are listed in Table 7.
Untreated split cedar posts, long used in western Oregon where they have a history of good service, lasted 23 years on the post farm under conditions favorable for decay. Oregon white oak posts, similar to the cedar posts in girth but containing some sapwood, were somewhat less durable. Western juniper posts, commonly used in dry rangelands east of the Cascade Mountains, are expected to last for at least 30 years in the damp climate of western Oregon. Incense cedar, excluded from Table 7. had a disappointingly short average life of 14 years in the tests; however, a generally accepted classification of heartwood durability groups cedars under "Resistant or very resistant to decay," with no clear distinction between Incense and other cedars. (Forest Products Laboratory, USDA Forest Service, Madison, Wisconsin. 1967. Research Note FPL 0153)
Some durable woods included in the tests are not native to Oregon and, with the exception of redwood, most of those exotics are rare or generally unavailable as posts. Among the latter is the spectacularly durable Osage-orange, which has been in test for 52 years without failure.
The life of naturally durable, untreated posts is influenced not only by the species, but also by post girth, amount of nondurable sapwood, and, in some species such as cedars, by the post's site of origin in the tree. For example, the durability of western red cedar heartwood tends to decrease toward the pith and with height in the tree.
Early in the program, it became apparent that many of the test woods lacked natural durability and would serve adequately as fence posts for only a few years, usually less than 10, unless protected by an effective preservative treatment. The treatment minimizes the susceptibility of sapwood to decay and benefits posts that may naturally have less than normal decay resistance. Treatment is therefore most beneficial to posts that otherwise would fail early.
The pressure-treated test posts were produced commercially. Commercial treatments offer the widest selection of preservatives and can produce a product having excellent durability. Modem preservatives used by commercial plants are now more effective than some early formulations that leached more easily from treated wood. Use of less effective zinc-meta-arsenite and chromated zinc chloride (series 33, 43) has been discontinued. Average life of most series of pressure-treated posts in the tests should exceed 40 years. Square-sawed posts of west coast hemlock treated with aqueous solutions of Chemonite or Tanalith are expected to last longer than similar posts of Douglas-fir. Some series of creosoted posts (series 7, 23) have lasted for 56 years without failure.
Nonpressure methods, used in the past by do-it-yourself treaters and now restricted to certified appliers, sometimes give good results. Soaking well-dried, incised posts having absorbent sapwood in an effective preservative solution of creosote or pentachlorophenol will give posts of some test series an estimated average life exceeding 30 years; some series (83, 86) may exceed 50 years.
The best results of the few hot-cold soak treatments (series 54, 27, 18) were no better than those achieved by longer cold-soak treatments. Serviceability was impaired by failure of the untreated tops, probably also by lack of incising, and, in one test, by dilution of creosote with dirty crankcase oil. The hot-cold soak method is useful for producing nonpressure-treated posts in quantity when a faster and more controlled process is needed than can be realized from a simple cold-soaking treatment.
Nonpressure treatments with water-soluble preservatives that diffuse into moist sapwood of freshly cut and peeled posts offer the advantage of rapid processing from sturnp to treated post with no drying before treatment. The best double-diffusion treatments tested (series 101, 104) are expected to extend the average lives of Douglas-fir and lodgepole pine posts to 27 and 24 years respectively, but better treatment of post tops is recommended to avert decay there. More recent tests of modified double-diffusion methods used to treat Alaskan species (Gjovik, L.H. H.G. Roth, and H.L. Davidson. 1972. Treatment of Alaskan species by double-diffusion and modified double-diffusion methods. USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin. Research Paper FPL 182) showed that preservative penetration and retention could be remarkably improved by partially seasoning and incising posts, and by heating the solution to 190°F for the first of the two consecutive soakings. Diffusions of Osmosalts (series 75) into undried posts provides long post life, with the added benefits of simpler application and less left-over preservative for disposal. The average life of the Osmosalt-treated Douglas-fir posts in the tests is estimated to be 40 years. Unfortunately, this effective preservative, formerly used to treat mine timbers, is no longer being produced.
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TABLES OF RESULTS
This publication reports research involving preservatives. It does not imply that the treatments discussed may be administered by the general public. Preservatives can be injurious to human beings, aquatic and land animals, and plants-if they are not handled or applied properly. Crankcase oils may contain chlorinated naphthalenes that may contribute to X-disease (hyperkeratosis) in cattle. These oils are therefore not recommended for preservative treatment of wood with which cattle may come in contact. Preservatives should be used carefully, and their surplus and containers disposed of by recommended procedures. Appliers should keep the volume of contaminated waste as low as possible. A treatment system that recovers preservative for reuse from the waste stream protects the environment and reduces the high cost of disposal at a licensed site for hazardous-waste storage.
Post-farm tests include preservatives that now are available for use only by certified appliers. The restricted-use preservatives are creosote, pentachlorophenol (penta), and those containing inorganic arsenicals. Wood products treated commercially with these continue to be readily available, but solutions of creosote and pentachlorophenol formerly available to do-it-yourself treaters are no longer sold over the counter. The effectiveness of other preservatives, particularly when used in contact with soil, may be less well documented.
Oregon residents desiring to become certified appliers of restricted-use preservatives should contact the Plant Division, Oregon Department of Agriculture at 635 Capitol St. NE, Salem, OR 97310, or phone 378-3776 for information.
The mention of trade names or commercial products in this publication does not constitute endorsement or recommendation for use.
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