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Prepared By: Sanjay Kumar GuptaAdama University (2008)

Prevention of Mold, Stain, and Decay ( Logs, Poles, Piles, and Ties)

The wood species, geographic region, and time of the year determine what precautions must betaken to avoid serious damage from fungi in logs, poles, piles, ties, and similar thick productsduring seasoning or storage. In dry climates, rapid surface seasoning of poles and piles will retarddevelopment of mold, stain, and decay. In humid regions, such as the Gulf States, these productsoften do not air-dry fast enough to avoid losses from fungi. Pre seasoning treatments with

approved preservative solutions can be helpful in these circumstances.

For logs, rapid conversion into lumber or storage in water or under a water spray (Fig. 5) is thesurest way to avoid fungal damage. Preservative sprays promptly applied to the wood will protectmost timber species during storage for 2 to 3 months, except in severe decay hazard climates.For longer storage, an end coating is needed to prevent seasoning checks, through whichinfection can enter in to log.

Figure- 5 Spraying logs with water protects them against fungal stain and decay.

Lumber

Growth of decay fungi can be prevented in lumber and other wood products by rapidly dryingthem to a moisture content of 20% or less and keeping them dry. Standard air-drying practiceswill usually dry the wood fast enough to protect it, particularly if the protection afforded by dryingis supplemented by dip or spray treatment of the stock with an EPA approved fungicidal solution.Successful control by this method depends not only upon immediate and adequate treatment butalso upon proper handling of the lumber after treatment. However, kiln drying is the most reliable

method of rapidly reducing moisture content. Air-drying yards should be kept as sanitary and asopen as possible to air circulation (Fig.-6). Recommended practices include locating yards andsheds on well-drained ground; removing debris (which serves as a source of infection) and weeds(which reduce air circulation); and employing piling methods that permit rapid drying of the lumber and protect against wetting. In areas where termites or water-conducting fungi may betroublesome, stock to be held for long periods should be set on foundations high enough so thatthe wood can be inspected from beneath.

Figure 6- A sanitary, well-drained air-drying yard.

Bacteria

Most wood that has been wet for a considerable length of time probably will contain bacteria. Thesour smell of logs that have been held under water for several months, or of lumber cut fromthem, manifests bacterial action. Usually, bacteria have little effect on wood properties, exceptover long periods, but some may make the wood excessively absorptive. This can result inexcessive pickup of moisture, adhesive, paint, or preservative during treatment or use. This effecthas been a problem in the sapwood of millwork cut from pine logs that have been stored in

ponds. There also is evidence that bacteria developing in pine veneer bolts held under water or sprayed with water may cause noticeable changes in the physical character of the veneer,including some strength loss. Additionally, a mixture of different bacteria, as well as fungi, wasfound capable of accelerating decay of treated cooling tower slats and mine timbers

WOODS BORING INSECTS

In most instances, the damage by wood boring insects is done in the larval stage, the larvae or grubs burrowing through wood to obtain both food and shelter and often making galleries or holes. In some cases, however, the adult forms take an active part in the deterioration. In case of subterranean termites, whose adult “workers” are the destructive individual; with the matureambrosia beetles, which often penetrates the wood for the considerable distances and for thepurpose of laying eggs and rearing their young. The resultant insect holes or burrows not only

affect the appearance of the wood but may seriously impair the strength of wood.

The wood boring insects may be grouped in to two classes: (a) those that confine their activity tovarious forms of wood before it is put in to use and (b) those they attack wood in service.

The group of insects working in material in service is by far the more significant to the woodpreserving industry, since much of the deterioration is by these borers can be prevented bysuitable preservative treatment of susceptible timbers. It is also the more important group fromthe point of view of the financial losses involved, in as much as the replacement of the badlyweakened members of a structure may involve labor charges that far exceed the purchase priceof the new timbers.

The insect defects produced in wood before it is placed in service may be classified as pinholes(small, pound, usually open, Size from 1/100 to i/4 inch in diameter)or grub holes( oval, circular or irregular holes, size 3/8 in diameter). Pinholes are made by ambrosia beetles or timber worm,which attack living tree, saw logs, unseasoned lumber, timbers, and bolts. Grub holes are madeby a variety of insects, including the flat- headed borers, locust borers, pine sawyers, carpentersbees, carpenter ants, and a number of others; they infest the heartwood and softwood of all kindsof timber. Insects defects may disqualify wood for specialized uses, which involves exactingstrength requirements (vehicle and airplane stock), impermeability (tight cooperage), or appearance (furniture and cabinetwork), and yet not render unfit for less exacting service.

There are some very much important insects that are capable of deteriorating wood in service,and are consequently susceptible to control through preservative treatment and the application of other preventive measures. Carpenter bees, caddis-fly larvae, and various other insectsoccasionally cause damage to wood in service but do not appear to be of sufficient importance torequire attention by the wood preserving industry. Termites are the outstanding wood-destroyinginsects, but considerable damage is also caused by powder- post beetles and, to a lesser extent,by carpenter ants.

Figure 7. Types of insect damage most likely to occur in a building. Upper left—Termite attack; feeding galleries (often parallel to the grain) contain excrement andsoil. Upper right—Powder-post beetle attack; exit holes usually filled with woodflour and not associated with discolored wood. Lower left—Carpenter ant attack;nesting galleries usually cut across grain and are free of residue. Lower right—Beetle attack; feeding galleries (made in the wood while green) free of residue andsurrounding wood darkly stained.

TERMITES

Termites superficially resemble ants in size, general appearance, and habit of living in colonies.From the standpoint of their methods of attack on wood, termites can be grouped into two mainclasses: (a) ground-inhabiting or subterranean termites (b) wood-inhabiting or non subterraneantermites.

Subterranean Termites

Subterranean termites are responsible for most of the termite damage done to wood. Thisdamage can be prevented. The hazard of infestation is greatest (a) beneath buildings withoutbasements that were erected on a concrete slab foundation or were built over a crawl space thatis poorly drained and ventilated and (b) in any substructure wood component close to the groundor an earth fill (for example, an earth-filled porch).

The subterranean termites develop their colonies and maintain their headquarters in the ground.

They build their tunnels through earth and around obstructions to reach the wood they need for food. They also must have a constant source of moisture, whether from the wood on which theyare feeding or the soil where they nest. The worker members of the colony cause destruction of wood. At certain seasons of the year, usually spring, male and female winged forms swarm fromthe colony, fly a short time, lose their wings, mate, and if successful in locating a suitable home,start new colonies. The appearance of “flying ants” or their shed wings is an indication that atermite colony may be near and causing serious damage. Not all “flying ants” are termites;therefore, suspicious insects should be identified before investing in eradication.

Subterranean termites normally do not establish themselves in buildings by being carried there inlumber; they primarily enter from ground nests after the building has been constructed. Anintroduced species, the Formosan termite, is adept at initiating aboveground infestations andnests in structures where wood remains wet for prolonged periods, such as from roof leaks.

Telltale signs of subterranean termite presence are the earthen tubes or runways built by theseinsects over the surfaces of the foundation or other exposed areas to reach the wood above.Another sign is the swarming of winged adults early in the spring or fall. In the wood itself, thetermites make galleries that generally follow the grain, leaving a shell of sound wood to concealtheir activities. Because the galleries seldom show on the wood surfaces, probing with a pick or knife is advisable if the presence of termites is suspected.

The best protection for wood in areas where subterranean termites are prevalent is to prevent thetermites from gaining hidden access to a building. The foundations should be of concrete,pressure-treated wood or other material through which the termites cannot penetrate. With brick,

stone, or concrete block, cement mortar should be used because termites can work throughsome other kinds of mortar. Posts supporting floor girders should, if they bear directly on theground, be of concrete. If there is a basement, it should be floored with concrete. Untreated postsin such a basement should rest on concrete piers extending a few inches above the basementfloor. However, pressure-treated posts can rest directly on the basement floor. With the crawl-space type of foundation, wood floor joists should be kept at least 460 mm (18 in.) and girders

300 mm (12 in.) from the earth and good ventilation should be provided beneath the floor. A ruleof thumb is to have a minimum of 1 unit area of ventilation for every 150 units of crawlspace (for example, 1 ft2 of ventilated area for 150 ft2 of crawlspace).

Moisture condensation on the floor joists and subflooring, which may cause conditions favorableto decay and contribute to infestation by termites, can be avoided by covering the soil below witha moisture barrier, maintaining adequate ventilation, and assuming proper drainage of rainwater away from all sides of a structure.Non subterranean Termites

Non subterranean termites are of two types: 1. Dry-wood termites 2. Damp-woodtermites

Dry-wood termites:

The insects of this group are distinct from subterranean termites in that they areentirely wood inhabiting, never entering the ground, and require but little moisturefor their existence. At the time of swarming, the winged reproductives gain entranceto sound, dry wood directly from the air. Each pair of alates seeks out checks, cracks,or other natural opening in the wood, and begins to tunnel into the timber from thatpoint.

Dry wood termites are able to work in wood having a moisture content as low as 10to 12 percent( and possibly lower) and, consequently, may be found in thoroughlyair-seasoned timbers and the woodwork in upper part of the building as well as moremoist material.

The dry- wood termites are less injurious; their colonies are distinctly smaller thanthose of subterranean termites; and they do not have permanent worker caste. Nonsubterranean termites are often moved from structure to structure in infested items such asfurniture.

In constructing a building in localities where the dry-wood type of non subterranean termite isprevalent, it is good practice to inspect the lumber carefully to see that it was not infested beforearrival at the building site. Because paint is a good protection against the entrance of dry-woodtermites, exposed wood (except that which is preservative treated) should be kept covered with apaint film. Fine screen should be placed over any openings to the interior unpainted parts of thebuilding. As in the case of ground-nesting termites, dead trees, old stumps, posts, or wood debrisof any kind that could serve as sources of infestation should be removed from the premises.

If a building is infested with dry-wood termites, badly damaged wood should be replaced. If thewood is only slightly damaged or is difficult to replace, further termite activity can be arrested byinjecting a small amount of an approved pesticidal dust or liquid formulation into each nest.

Damp-wood termites:

Like the dry-wood termites, most of those belonging to the damp-wood group enter wood directlyfrom the air at the time of swarming and usually have no contact with ground. But unlike the dry-

wood termites, they are dependent upon an abundant supply of moisture for their existence andaccordingly confine their activity to damp or decayed wood.

Damp-wood termites are reported as occasionally causing serious damage to water tanks andfresh-water piling and to wood in contact with damp ground. Sometimes, they extending their galleries from moist butts into the relatively dry and sound wood also.

A TERMITE COLONY

Termites are social insects, living in colonies and dividing their essential activities amongspecialized forms or castes. There are usually three such castes in a colony, viz., thereproductives, the soldiers, and the workers.

In the early stage of development of termite colony, the sexually mature reproductives normallyconsist of a single primary king and queen, the sole ancestors of all the other members of colony.During the first year of her reproductive life the queen lays but a few eggs, but her ovariessubsequently become gradually enlarged, with a resultant elongation and swelling of her body. Inconsequence, there is a rapid increase in the egg laying power of the queen.

From the eggs laid by the queen are hatched the nymphs, which subsequently develop, throughseveral growth stages, into the adult soldiers, workers, and alates or reproductives.

Soldiers:

The soldiers, charged with the protection of colonies, are sterile, wingless, and without functionaleyes, and vary in length from less than1/4 inch (subterranean termites) to about 3/8 to 1/2 inch( dry-wood species) or even 3/4 inch( damp wood species). They are distinguished by their relatively large, colored heads and heavy jaws, which are adapted to fighting but are useless for excavating the wood or feeding. When the burrows of a colony are purposely opened to theexterior for the ejection of fecal pellets or to allow the alates to emerge or the protective shell of wood is accidentally broken down, these soldiers ward off the attacks of their natural enemies,the true ants, and of termites from other colonies. They block the small openinings with their hardhead and try to kill the invaders with their mandibles ( mandibulate soldiers) or discharge a stickysubstance ( naustiform soldiers) to stop the attack of raiding insects.

Workers:

In the subterranean termite colonies, the workers are by far the most numerous individuals. Likethe soldiers, they are sterile, winless, and blind, and their light colored bodies are less than 1/4inch in length. Although they are equipped with relatively inconspicuous jaws, their mandibles areespecially adapted to biting off fragments of wood, and this caste is responsible for all thedestruction caused by the subterranean species. Workers are assigned such duties asconstruction and enlargement of burrow and other structures that house the colony, the collectionof food, the care and feeding of the other adult forms and young, and the daily grooming of various individuals. In contrast to the subterranean termites, the dry- wood termites and damp-wood termites have no specialized worker castes, and the immature nymphs perform the various

duties of the colonies. After a period of servitude, these nymph matures and become either soldiers or alates.

Alates or Reproductive:

Alates or winged reproductives, are more nearly like ordinary insects than the other form of termites, having somewhat flattened, usually dark-colored bodies and large, black, compoundeyes, and are often called “flying ants”. They attain a length of 1/2 inch or more (up to 1 inch or more in damp-wood termites) with their wings. So long as they remain in the colony, the members

of this caste are sexually immature; it is only after they have emerged, completed their flight, andmated that they become functional reproductives. The flight, or swarms, of these alates takeplace once, and sometimes twice, each year and afford one of the outstanding indication of thepresence of termites in a structures or timbers. Only a relatively small proportion of the pairedindividuals succeed in escaping from ants, birds, and other natural enemies and in findingsuitable locations for their future activities; these dig into the earth or wood in the earth

(subterranean species) or eat their way into wood above the ground( dry-wood and damp-woodtermites), to become the primary kings and queen of the new colonies. It is thus that termiteinfestations are spread.

Termites invade wood for twofold purposes of obtaining shelter and securing the food necessaryfor their growth and development. For sustenance they are dependent chiefly on cellulose.However, they are unable to utilize this material directly, and the fragments of wood that they eatare actually digested by the thousands of Protozoa that swarm in the intestine of all the commonspecies of termites. These tiny, one-celled animals presumably break down the complex celluloseinto sugars and thus make it available to the host insects.

POWDER POST BEETLES:

The larvae or grub of powder post beetles bore the through the wood for food & shelter, leavingthe undigested part of the material in the form of powder. When an infested piece is moved or

jarred, this powdery residue falls out of the holes that were made in the surface of wood by thewinged adults as they emerged to spread the infestation. The most important of the powder postbeetles, from the standpoint of prevalence and extent of damage, are the Lyctus beetles.

The larvae, which develop from the eggs laid by the Lyctus beetles in the vessels (pores), tunnelthe interior of the sapwood and leave their irregular burrows packed with the powdery, undigestedremnants of the wood. Under normal outdoor condition, pupation takes place in the early spring,and the mature, winged beetles emerge during the late spring and early summer, and earlysummer, leaving holes 1/16 to 1/12 inch in diameter on the surface layer of the wood. However,when infested timber is located in storehouse, dwelling, or other buildings that are kept warm anddry, the adults may make their activity much earlier.

The emergence holes, and the powder like wood residues that falls out of them and often collectsin small piles at the base of, or under, infested material, serve as indication of powder-postdamage. After mating, the female beetles go in search of suitable places to deposit their eggs.They may crawl or fly, often for some distance, to sound material and thus start a new infestationbut are more apt to concentrate on nearby wood regardless of whether or not it has already beenattacked. Lyctus beetles commonly reinfest the same timber time and time again, so long as thewood continues to furnish adequate food for the larvae.

CARPENTER ANTS:

Small to large, black or brown carpenter ants are commonly found in stumps and old logs in theforest, as well as standing trees in which the wood has been exposed by basal wounds. Theymay also extend their activities to wood in service, especially after decay has begun to soften thematerial, and sometimes cause appreciable damage to wood poles, structural timbers, or even tobuildings. These true ants usually gain access directly from the soil to timber set in or on theground. They excavate the wood for shelter, rather than for food, usually making a preferentialattack on the relatively soft spring wood. They do not seriously injure the wood in the first year,

but under favourable condition may continue to enlarge their galleries until replacements or extensive repair are required.

Like the termites, these true ants live in colonies and have definite castes, and their work issometimes mistaken for that of the subterranean termites, since the burrow of the two groups of insects have considerable resemblance. However, unlike the termites, which commonly plug their

unused gallaries with a woody frass, the ants keep their runways free from all refuse.

WOODPECKERS

Wood poles occasionally suffer substantial damage from wood peckers, which sometimes cutholes in upper parts of the poles so large that they cause serious weakening, particularly if several holes are made in the same timber. The presence of creosote and the soundness of thewood seem of no concern to the woodpeckers when they start to work. No adequate explanationfor such conduct has been found and no sure preventive is known. Fortunately such attaks arenot frequent, but when they do occur in pole lines, they may cause serious loss.

MARINE BORERS:

Extensive damage is done to the submerged portion of marine piling and wharf timbers and toother wooden members of fixed and floating structures in salt or brackish water by certain watersby wood-boring animals, known under the general term of marine borers. These marine animalsare widely distributed throughout the salt water of most of the world although more prevalent anddestructive in the warm region than in the cold ones.

There are two distinct groups of marine wood-boring animals, each characteristic in its generalstructure and method of attacking wood. These are the molluscan borers, distinctly related to theoyster and clams, and the crustacean borers, which are kin to the lobsters and crabs.

UNIT: 3

TYPES OF WOOD PRESERVATIVES

There are three types of wood preservatives. 1. Oil type 2. Organic solvent type 3. Water Solvent type. Their chemical compositions are listed below:

1. Oil Type: Coal tar creosote is an oil type preservative. It is a brownish black liquid obtained asa product by distillation of coal tar. The fraction boiling from 200 to 325 degree C is condensedand used as a wood preservative. Creosote contains hundreds of compounds and it is notpossible to specify creosote based on chemical constituents. The toxicity of creosote is derivedfrom numerous compounds present even in small quantity. It is recommended to for exterior useas it is highly stable against evaporation and leaching, possesses broad spectrum of toxicity toalmost all organism and non-corrosive to metals and protects timber against splitting. It is usually

used in admixture with fuel oil, which has no toxicity. Fuel oil helps in lowering treating costs, andlends increased permanence to treated wood by preventing evaporation of creosote from treatedwood.

2. Organic Solvent Type: They are different from creosote, which needs no solvent. These arevery active chemials having high fungicidal/ insecticidal properties and are applied by dissolvingthe same in organic solvents. There are nearly two dozen compounds which have found use aspreservatives. These formulations were developed for treating joinery timbers. Certain water repellent compounds like wax, resins, an also be added to the preservative solution to impartwater repellency especially for joinery timbers. They have a high degree of penetrability and canbe applied under low vacuum or even mere dipping. These preservatives are:

Pentachlorophenol (P.C.P): It is fully chlorinated of phenol and is the oldest and most widely

used preservative in the world because of its long lasting protection and broad spectrum of efficacy. The active ingredient, a chlorinated phenol, is a crystalline solid that dissolve in variety of solvents. The performance of PCP and the properties of treated wood are influenced by thechoice of solvent. A heavy oil solvent is preferred when treated wood is to be used in groundcontact- wood treated with light oil is not as durable.

Benzene Hexachloride ( BHC) is a fully chlorinateated compound of benzene; and D.D.T.( Dichloro- Diphenyl- Trichloroethane ) also comes in this category.

Copper naphthenate: Copper napthenate is an organometallic compound formed as a reactionproduct of copper salts and petroleum derived naphthenic acids. Like pentachlorophenol, copper naphthenate can be derived in variety of solvents, but is more durable when dissolved in heavyoil.Although not as widely standardized as creosote and PCP treatments; copper naphthenate isused increasingly in the treatment of utility poles. PCP treated wood has many characteristics andproperties that similar those of creosote, except that it is ineffective against marine borer.

Long –standing concern about broad and persistent toxicity has curtailed the use of PCP in manycountries and severely restricted use elsewhere.

Synthetic Pyrethroides: Many new generation insecticides have recently come in the market.Cypermethrin and deltamethrin have shown efficacy against termites and have beenrecommended in formulations containing pentachlorophenol.Copper Zinc Abietate (resinate), Cashew Nut Shell Liquor( CNSL), Bhilawan Nut ShellLiquor(BNSL) are also preservative of this category.

3. Water Soluble Type: These preservatives are of two types as described below:

A. Leaching Type B. Fixed Type

Leaching Type preservatives are inorganic or organic salts soluble in water. These arerecommended for use under cover and provide only limited protection as these can be leachedout with rain water.

Zinc Chloride: It is quite toxic to fungi and insects but not so toxic to termites. It has fireretardant properties also. Wood treated with this tends to be hygroscopic, which may interferewith paint films.

Boric acid and Borax; these are highly toxic to all types of organism encountered in buildingtimbers and form very good preservatives for wood and wood panel products such as plywood,because of their penetrating property.

Sodium pentachlorophenate (NaPCP): Water soluble sodium salt of pentachlorophenal ishighly effective against sap stain.

Benzene-hexachloride: Water soluble powder for use as spray for prophylactic treatmentagainst borer and termites.

Fixed Type preservatives are mixture of various salts which in the presence of wood, react andform insoluble complex salts and thus get permanently fixed. Timber treated with thesecompositions can be used in outside locations also. Treated timber should, however, be allowedto dry for 1 to 2 weeks to complete the fixation process. These preservatives are applied atambient room temperature as the chemicals are precipitated at elevated temperature. To reducewaiting period especially in softwoods, which dry in 2-3 days in a kiln, treated timber is heated in

pressure cylinder after completion of treatment cycle to fix the preservative in wood. The treatedwood is very clean and can be painted, polished or waxed. Various formulations under this typeare:

Copper-Chrome-Arsenic composition (CCA) consisting of following formulation (by Weight)

Normal Minimum

Copper sulphate (CuSO4.5H2O) 37.5 35

Sodium dichromate (Na2Cr2O7.2H2O) or Potassium dichromate (K2Cr2O7) 50.0 47.5

Arsenic pentaoxide (As2O5.2H2O) 12.5 10.0

Total solid content should be more than 95 percent.

This preservative known as ASCU was developed at F.R.I. (Kamesam 1933), India and is today’sbest broad spectrum wood preservative in wide use. The basic formulation is a uniform andhomogenous mixture of the above salts reacted together at controlled temperatures. Manymodifications of the above have since been developed in the world. Most important are the oxideformulations especially developed for treatment of electricity poles in U.S.A. The usage of thisformulation is increasing every year despite the fact that some countries in Europe have droppedthis formulation. In the US, this preservative is known as Green salt and Erdalith, instead of Ascu.

Acid- Copper- Chrome- composition: It is also known as Celcure. It is reported to contain

copper-sulphate (CuSO4.5H2O) - 50 parts, potassium di-chromate- (K2Cr2O7)-45 parts, andchromic acetate (Cr2H3O3) H2O – 5 parts. It is usually recommended for impregnation of timber which would not be in contact with the ground or water and 8 kg/ m3 retention is required. Inunfavorable circumstances 12 kg/m3 is recommended.

Boliden salt: Boliden salts are reported to contain a mixture of arsenic, chromium and zinc saltswhich convert themselves into insoluble arsenate of zinc and chromium which are toxic to wooddestroying fungi and insects. It is a Swedish patented formulation and has found commercialapplication in Scadinavian countries with satisfactory results.

Copper- Chrome- Boric composition: It consists of boric acid ( H3BO3)- 1.5 parts, copper sulphate- 3 parts, sodium or potassium dichromate- 4 parts. This composition is use for thetreatment of timber required for dry locations such as door frames, panels, furniture, etc. Anabsorption of 8 to 16 kg/ m3 is recommended for the purposes. In all these compositions, it will be seen that chromium plays the part of fixing the chemicals intimber by the formulation of insoluble complex salt, having high effectiveness against fungi andinsects. It also prevents the corrosive action of other chemical like copper sulphate. It, however,has no toxicity of its own.

Ammonical- Copper- Arsenite (Chemonite): It is a well known preservative used in USA totreat refractory softwoods. A slightly modified composition using ammonium hydroxide andarsenic trioxide was developed to treat timber by dip diffusion method (Dev et al 1990). Thepreservative has given good indication of its use in the rural sector and treatment of plywoodpanels etc.

Chromated Zinc Chloride: This preservative consists of 81.5 % zinc chloride and 18.5% of

sodium dichromate. Chromated zinc chloride is claimed to be somewhat more resistant leachingthan straight zinc chloride and, therefore, to give some longer protection under damp conditions.Chromated zinc chloride shall not be heated above 160 degree F because, at higher temperaturechromium is likely to form precipitate.

AN IDEAL WOOD PRESERVATIVE

Preparation of Timber for Treatment

For satisfactory treatment and good performance, the timber must be sound and suitablyprepared. Except in specialized treating methods involving unpeeled or green material,the wood should be well peeled and either seasoned or conditioned in the cylinder before treatment. It is also highly desirable that all machining be completed beforetreatment. Machining may include incising to improve the preservative penetration inwoods that are resistant to treatment, as well as the operations of cutting or boring of holes.

Peeling

Peeling round or slabbed products is necessary to enable the wood to dry quicklyenough to avoid decay and insect damage and to permit the preservative to penetratesatisfactorily. Even strips of the thin inner bark may prevent penetration. Patches of barkleft on during treatment usually fall off in time and expose untreated wood, thuspermitting decay to reach the interior of the member.

Drying

Drying of wood before treatment is necessary to prevent decay and stain and to obtainpreservative penetration. However, for treatment with waterborne preservatives bycertain diffusion methods, high moisture content levels may be permitted. For treatmentby other methods, however, drying before treatment is essential. Drying before treatment

opens up the checks before the preservative is applied, thus increasing penetration, andreduces the risk of checks opening after treatment and exposing un-penetrated wood.

Fig- 8 Machine peeling of poles. The outer bark has been removed by hand, and theinner bark is being peeled by machine. Frequently, all the bark is removed by machine.

Conditioning of Green Products

Plants that treat wood by pressure processes can condition green material by meansother than air and kiln drying. Thus, they avoid a long delay and possible deterioration of the timber before treatment. When green wood is to be treated under pressure, one of several methods for conditioning may be selected. The steaming-and-vacuum process isused mainly for southern pines, and the Boulton or boiling-under-vacuum process is

used for Douglas-fir and sometimes hardwoods.

Incising

Wood that is resistant to penetration by preservatives may be incised before treatment topermit deeper and more uniform penetration. To incise, lumber and timbers are passedthrough rollers equipped with teeth that sink into the wood to a predetermined depth,usually 13 to 19 mm (1/2 to 3/4 in.). The teeth are spaced to give the desired distributionof preservative with the minimum number of incisions. A machine of different design isrequired for deeply incising the butts of poles, usually to a depth of 64 mm (2.5 in.)Fig-9

Figure 9 Deep incising permits better penetrationof preservative

Cutting and Framing

All cutting and boring of holes should be done prior to preservative treatment. Cuttinginto the wood in any way after treatment will frequently expose the untreated interior of

the timber and permit ready access to decay fungi or insects.

Application of Preservatives

Wood-preserving methods are of two general types: (a) pressure processes, in whichthe wood is impregnated in closed vessels under pressures considerably aboveatmospheric, and (b) nonpressure processes, which vary widely in the procedures andequipment used.

Pressure Processes

In commercial practice, wood is most often treated by immersing it in a preservative in ahigh pressure apparatus and applying pressure to drive the preservative into the wood.Pressure processes differ in details, but the general principle is the same. The wood, oncars or trams, is run into a long steel cylinder (Fig. 10), which is then closed and filledwith preservative. Pressure forces the preservative into the wood until the desiredamount has been absorbed. Considerable preservative is absorbed, with relatively deep

penetration. Three pressure processes are commonly used: full-cell, modified full-cell,and empty-cell.

Figure 10. Interior view of treating cylinder at wood-preserving plant, with a loadabout to come in.

Full-Cell

The full-cell (Bethel) process is used when the retention of a maximum quantity of preservative is desired. It is a standard procedure for timbers to be treated full-cell withcreosote when protection against marine borers is required. Waterborne preservativesare generally applied by the full-cell process, and control over preservative retention isobtained by regulating the concentration of the treating solution.

Steps in the full-cell process are essentially the following:

1. The charge of wood is sealed in the treating cylinder, and a preliminary vacuum of about 560mm (or 22in) of mercury is applied for a half-hour or more to remove the air from the cylinder and as much as possible from the wood.

2. The preservative, at ambient or elevated temperature depending on the system, isadmitted to the cylinder without breaking the vacuum.

3. After the cylinder is full, the vacuum is released and pressure (3.52 to 10.55 kg/cm2)is applied until the wood will take no more preservative or until the required retention of preservative is obtained.

4. When the pressure period is completed, the preservative is withdrawn from thecylinder.

5. A short final vacuum (380 to 508 mm of mercury) may be applied for 15 min to freethe charge from dripping preservative.

When the wood is steamed before treatment, the preservative is admitted at the end of the vacuum period that follows steaming. When the timber has received preliminaryconditioning by the Boulton or boiling-under-vacuum process, the cylinder can be filledand the pressure applied as soon as the conditioning period is complete.

Fig-11 Treating schedule for the full cell (Bethell) process

Empty cell processes (Lowry and Rueping )

For some purpose it is necessary to ensure deep penetration of preservativewith a relatively low net retention preservative. An example would be coal tarcreosote treatment for an above-ground use, such as fence rails, whichrequire retention of 8 pounds per cubic foot. A full cell process, giving fulldepth penetration of sapwood, would likely produce retention of 12 pcf (4

pounds more than required). To ensure good depth of penetration, the empty cell process forces more creosote into wood than is needed, but thenremoves the excess to leave the average retention desired.

Look at Figures 12 and 13, for the sequences followed during treatmentswith the Lowry and the Rueping empty cell processes. Both treating methodsare similar to the full cell (Bethell) process except they omit the initialvacuum stage.

The Lowry Process is named for Cuthbert Lowry (1906, U.S.A.) In thisprocess, after the wood has been closed in the cylinder, preservative ispumped in, and no air is allowed to escape. As the cylinder fills with liquid

and pressure is applied, the air in the cylinder and in the wood cells iscompressed into a smaller and smaller space. When the desired pressure isattained, air in the cells will occupy about one-tenth of the cell voids, andpreservative can gradually fill up the other nine-tenths. The process thencontinues exactly as the full cell process, but the air compressed inside thewood expands when the pressure is released, thereby forcing somepreservative out of the cells and eliminating overloading. The end result isthat many cells are “lined” with preservative rather than “filled.” The finalvacuum period can be used to extract more or less preservative as needed,so it acts as a retention control stage. The word “empty” in the term emptycell process is a poor description because cells are partly filled withpreservative, in contrast with the word “full” which appropriately describes

the full cell process.

The Lowry process is mainly used for treating wood with creosote,creosote/PCP mixtures and PCP preservatives.

The Rueping process is named for Max Rueping, (1902, Germany). Thisprocess is similar to the Lowry process. Here an air pressure higher thanatmospheric is first applied to the closed cylinder and its charge of wood. Theair pressure is generated by a compressor. A typical pressure used is four tofive times atmospheric (about 60 psi). Treatment then continues as with theLowry or full cell processes, but the amount of preservative removed (as theair compressed in the cells expands) is greater than in the Lowry process.

This provides the necessary degree of penetration with even less finalretention of preservative. The Rueping process also is mainly used withcreosote, creosote/PCP mixtures and PCP preservatives. Other benefits of empty cell processes are:

The final weight of treated wood is reduced compared to full cell treatment. Acost saving is realized from the use of less preservative chemical and carrierliquid.

The Lowry (Empty cell) Process

A Fill cylinder with preservative at atmospheric pressureB Pressurize cylinder to maximum and maintain until retention isreachedC Release pressure and withdraw preservative

D Apply final vacuumE Release final vacuum

Figure 12 - Treating schedule of the Lowry (empty cell) processRueping (Empty Cell) ProcessA Partially pressurize cylinderB Fill cylinder with preservativeC Continue pressurizing to maximum and hold until retention is reachedD Release pressure and withdraw preservativeE Apply final vacuumF Release final vacuum

Figure 13 - Treating schedule of the Ruping (empty cell) process

Modified full cell process

This process is an adaptation of the Bethel process for use with waterbornepreservatives like CCA. It, too, achieves full sapwood penetration with areduction in the weight of water left in the wood. This is important if the woodis to be shipped after treatment, without thorough air-drying or kiln-drying.

The lower weight of the treated wood is reflected in lower shipping costs.

The modified full cell process calls for a lower degree or period of initialvacuum than the full cell. By leaving more air in the cells, a greater amountof absorbed preservative is rejected when the pressure period is over. Theconcentration of CCA solutions can easily be changed, either by adding moreCCA concentrate or more water. Usually higher concentrations of CCA areused with the modified full cell than with the full cell process, and essentiallythe same weight of CCA chemical is left in the wood, but less water. A 20%reduction in overall weight of wood products treated in this way will givesimilar percentage savings in the cost of shipping un-dried products.Technical Note: Before shipping, it is essential that treated wood hasstopped dripping.A properly drained and covered drip-pad is used for this purpose. In the caseof CCA preservatives, at least 24 hours should be allowed for chemicalfixation to take place. Fixation time is appreciably longer in cold weather.

Non Pressure Method

Brush-on and spraying

The simplest treating methods do not involve expensive equipment. We couldbrush or spray a post on all surfaces, expecting capillary action to give thepreservative penetration into the wood. After several good brush coats, orsprayings and some obvious sucking-in of preservative, we would probably

find that further absorption was negligible. Additional applications wouldresult in wet wood surfaces and there would be considerable run-off. Thedeepest penetration is at the ends (transverse or cross-sections, wheretracheid ends were exposed to the preservative liquid). Radial penetrationinto the post surfaces is shallow; the rays are largely responsible for theabsorption seen in this direction. Note that all of the heartwood and much of the sapwood is left unpreserved by the brushing or spraying method. Whenthe post is part of a fence, the ground line is where the combination of air,moisture, wood and fungal source meet, and it is in this area that the risk of decay is greatest. Even a shallow split in the post will expose untreatedsapwood, and allow decay to start. Brushing and spraying are not goodtreating methods for preserving wood exposed to high risk of decay, such as

for ground contact uses.

Figure 14 a, b ,c, d Effect of treatment on penetration

Cold soaking or steeping

The term cold soaking is used when an unheated oil solution of preservative,such as penta or copper naphthenate, is used. The term steeping is used fortreatments of wood by preservatives in a water solution. The process consistsof partially filling an open tank with preservative and immersing a dry, round

fencepost in the tank. The post floats, so it must be weighted down to keep itsubmerged. The post is soaked for 24 or more hours, then removed. If we cutit lengthwise as before, the pattern of penetration using this treating methodmay look like Figure 14b. Note that both end-grain penetration and radialpenetration are better than that obtained by brushing. Cold soaking orsteeping has three advantages over brushing or spraying:

1. It allows a longer time for absorption to occurs.2. By holding the post below the surface of the preservative, a slight pressureis created which helps to force the preservative into the wood cells, therebyenhancing ordinary capillary action.3. It is easier than having to re-brush or re-spray the post with more

preservative at various intervals during the 24-hour treating period.Unfortunately, although the penetration of preservative by soaking is betterthan by brushing, the ground-line area is still insufficiently- treated to providelong-term protection.Soaking was previously used commercially for treating fence posts and rails,but pressure treatment has replaced it. Today, there are two principal usesfor this treating method:1. For exterior millwork such as window frame components. These parts getenough end-grain penetration to protect the very susceptible joints of thecompleted frame from decay in service.2. For thin wood materials like trellis slats or lath panels for fencing. Modempressure treatment, however, will give even better protection.

Thermal process or hot-and-cold bathWhen using the cold soaking or steeping method, a dry fence post wassubmerged for 24 hours in a tank of preservative at ambient (existing)temperature. With the thermal process or hot-and-cold bath, the sameequipment is used, but the preservative is heated with the post in the tank(taking precautions against fire if it is oil-based). As the fencepost heats up,air in the wood expands and bubbles out, escaping through the preservativeliquid into the atmosphere. The preservative is heated until no more airescapes from the post, and then the whole tank and submerged post isallowed to cool down. This process can also be completed in 24 hours. SeeFigure 14 c.

There are several ways to use the thermal process. It is not necessary to use just one tank in which to heat and cool the preservative for each batch of poles or posts treated. Two storage tanks and a separate treating tank can beused. One storage tank is insulated for the hot preservative and an un-insulated tank is used for the cold solution. Posts or other wood products areplaced in the treating tank and soaked in the hot preservative solution forabout 6 hours. Then the hot liquid is pumped back into the insulated tank andthe cold preservative from the other storage tank is flooded around the wood

products still in the treating tank. This system requires more equipment thancold soaking, but saves time and energy, lowers labor costs and providesbetter treatment.

The thermal process produces much better penetration than treating by thebrushing, spraying or cold soaking methods. There are several reasons for

this improvement. Much of the air in the cell cavities is forced to expand andescape from the outer post areas. When the preservative is allowed to cool,air remaining in the cells shrinks. So a partial vacuum is created in the cellsand atmospheric pressure pushing on the treating liquid helps force morepreservative into the post. For this reason, the thermal process might also beconsidered a non-equipment-induced vacuum-pressure method. Also by usingheat, viscosity of the preservative solution (especially those thick withcarriers) is reduced and it therefore penetrates the wood more readily.

The thermal process is used mainly for treating poles with creosote mixtures. The species preferred for this process are those which combine narrowsapwood bands with naturally durable heartwood.

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