Forest Products and Timber TechnologyForestrypedia

Wood Preservation, Wood Deterioration, Wood Durability, Wood Treatment

Spread the love



  • Some timbers are resistant to be attacked by biological agencies without application of chemicals. Such wood spp are considered naturally durable.
  • In contrast, some wood spp are easily deteriorated by the microorganisms and called nondurable
  • Decay is not an inherent property of the material itself. The decay is brought about the action of micro-organisms which liquefy and digest the material, using them as nutrients for their own growth.
  • Biological agents cause staining, softening, and decay. The greatest financial losses have resulted from this bio-deterioration.
  • To avoid the losses, the timber should be treated with suitable preservative particularly when it is used in contact with water. Soil and construction material.
  • Treated wood offers many times greater its service life than untreated one.


Fungal Attack:

Wood deterioration is caused by biological decay, physical and chemical decay. Biological agencies cause wood deterioration during its processing or when it is in service. Fungi, wood-boring insects and marine borers attack wood. The pattern or character of attack is often the only evidence of the identity of the organism. Fungus attack can be classified according to gross structural changes caused by the organism.

Brown Rot and White Rot:

The color of wood also becomes darker in the case of brown rots which decomposes only the cellulose and is made lighter by the white rots which attack all the constituents of the wood. These both rots are cued by Basidiomycetes.

Soft Rot, Wet Rot, and Dry Rot:

These rots are caused by Ascomycetes. The fungi attack on the cellulosic compound of cell walls 3-4 mm deep.

Insect Attack:

In addition to fungi attack, insect causes heavy losses due to deterioration of woods used in different conditions. These are pests, termites, borers, and beetles.

Pests of Forest and Mill Yards:

These include longhorn beetles and certain moths. They have fours stages of their life cycle ie egg, larva, pupa, and adult. They belong to Cerambycidae. The other pest is known as pinhole bore which makes holes in wood 0.5 – 3 mm diameter.

Pests of Seasoning Yards:

These pests of seasoning yard are Lyctus (powder post beetle). Its life cycle is 1 year and belongs to Lycitidae.

Pests of Well-Seasoned Wood:

Pests of will seasoned wood are furniture beetles, death-watch beetles, and house longhorn beetles. Both softwoods and hardwood are affected by furniture beetles. They belong to Anobium punctatum.


There are two types of termite ie dry wood termite and subterranean termites. Dry wood termites feed below the surface with granular dust through existing holes and cause more damage. On the other hand, subterranean termites live in the large colonies in the ground and remain in unbroken earth way from soil to their feeding grounds.

Marine borers (teredo/ shipworm):

They cause heavy losses when timber is used underwater.


On the basis of durability wood is classified into five classes:

Very Durable:

Timbers in this class may be used with confidence for permanent structures in contact with the ground or in water eg transmission poles, railway sleepers, bridging, timbers, foundation timbers, hut holes and fencing. The life of the wood is over 25 years in service. Eg Dalbergia sissoo, Juniperus procera, etc.


Durable wood can be used for the exposed parts of permanent structures which are not in contact with the soil they are suitable for the frames, keels, and decking of boats, making vats for a window sill, etc. the service life of the wood is from 15-25 years. Eg Cedrus deodara, Eucalyptus camaldulensis, Juniperus procera, etc  

Moderately Durable:

Wood in this class should never be used in direct contact with the soil unless given preservative treatment, but for the short periods, they can withstand exposure to damp conditions. They are suitable for those parts of permanent structures that are given protection against the weather, such as rafters’ joints. They can also be used for vehicles and for some parts of boats, deck, and beams for example. Durable timbers offer their services 10-15 years. Eg Albizzia lebbek, A. procera, Cassia fistula, Eucalyptus camaldulensis, Eugenia jambolana, Quercus incana, etc


Non-durable woods should always receive treatment if there is any risk of their becoming damp in use. They can safely be used for internal joinery and furniture, though, if they carry a high protection of sapwood, they may require long-term protection against insect attack. Such wood spp have durability form 5-1- years. Eg Abies spp, Acacia nilotica, Cedrus toona, Dalbergia sissoo, Juglans regia, Mangifera indica, Morus alba, Picea spp, Pinus roxburghii, Populus spp, etc


Timbers described as perishable require rapid extraction, conversion, and seasoning, to avoid decay in the log and while they are being dried. They should not be used for constructional work even under cover unless they have received through preservative treatment. They are often used in turnery, and as veneers in plywood for indoor use, and for making matches and chip baskets. Perishable timbers have a service life of fewer than 5 years. Eg Bombax ceiba, Fraxnius spp (Ash), etc.


Preservatives are the chemical which enhances the durability of timber treated with different methods. The following characteristics of preservatives are the most important:

  • High toxicity towards wood destroying organisms
  • Permanence in the treated wood
  • Ability to penetrate deeply in wood
  • Freedom from deleterious effects on the wood itself
  • Non-corrosive to metals.
  • Without harmful effects on the operatives and those who handle the treated wood.

There is no ideal preservative suitable for use on every kind of wood n every sort of situation. Wood preservatives are commonly classified into three main groups:

Tar Oil Preservatives:

  • The effectiveness of such preservatives against fungal decay and insect attack is fully established and well recognized.
  • The penetration and retention required for various kinds of wood and to the different dimensions are well known.
  • They tend to reduce the risk of splitting and surface checking therefore valuable for the preservation of railway sleepers.
  • They are available in large quantities and relatively cheap.
  • They are not usu corrosive to metals.
  • They can easily be applied by brushing spraying or steeping ie soaking.
  • Examples: Derivatives of coal or oil ie coal tar, creosote (distilled coaltar) Carbolineum and pentachlorophenol (PCP).

Waterborne Preservatives:

  • Such preservatives can easily be transported in solid or concentrated form and can be made up where these are to be used with the cheapest solvent water.
  • They are effective equally for fungi and insects.
  • They leave wood in clean condition
  • The treated wood can be painted even once the water has dried off.
  • They are usu odorless.
  • They can readily be combined with fire retardant.
  • Disadvantages of these preservatives are that seasoned wood rewets after treatment and cause development of drying defects.
  • Examples are:
    • Copper Sulphate (CuSO4)
    • Also, a toxic compound based upon Arsenate, Boron and Fluorine
  • The situations particularly suitable for water-borne preservatives are:
    • Mining timber
    • Timbers for building in which odors are undesirable
    • Woodwork that has subsequently to be painted
    • The in-situ treatment of timber in the building where the risks of staining plaster or paintwork.

Solvent Type Preservatives:

  • These are soluble in organic solvents
  • These are newly introduced
  • The preservatives containing toxic substances in a solvent other than water are Solvent Type Preservatives
  • Advantages: They do not cause swelling or distortion of wood as they contain no water
  • They leave the wood in clean condition
  • They do not leach out of the wood
  • The penetrate well into any permeable timber
  • Disadvantages: Disadvantages in the use of solvent type preservatives are high cost, increased flammability of wood after treatment.


Mobil oil, diesel, oil kerosene, and petroleum are not preservatives. These are rather appreciated by fungi.


  • Oil soluble preservatives are used for those wood spp which are used externally
  • Organic preservatives are used for those wood spp which are sued externally.
  • Organic preservatives are used both interior and exterior uses.
  • Waterborne preservatives are useful generally for anti-stain treatments used in control of sap stain.


  • For field tests more common, non-durable local wood spp can be easily penetrated by preservatives should be chosen. The spp should be the commonly used.
  • The wood of the stakes must be uniform and must be free from knots; cracks stain decay, insect holes or other defects which affect the uniformity of treatment.
  • The wood must have also straight grain and the annual rings must be as parallel as possible to the 5 cm side.
  • The stakes can be either clear sapwood or clear heartwood but both types of wood in the same stakes are not acceptable.
  • The stakes should be seasoned to the moisture content at which the best possible penetration of the preservative is obtained. The m.c % should be 15-25%.


Before deciding which preservative is to use it is necessary to ensure the sufficient degree of penetration? The effectiveness of the preservative treatment depends not only on the nature of the preservative but also on the amount taken up per unit area of the wood. This may be achieved by selecting the appropriate method of treatment of wood. Treatments are of five types:

Pressure method:

  • Bethel/ Full cell Process:

After preliminary vacuum creosote is injected under pressure varying from 40 to 200 lbs/ sq.inch; which is continued until the desired absorption has been obtained. The amount of creosote injected will vary from 10-25 lbs/sq.ft.
Use of Bethel Treatment:

  • Treatment of pilling
  • Bridge materials
  • Paving blocks
  • Telegraph poles
  • Timber to be used for long service.

Above materials are treated with 16 lbs of creosote/ cft.

  • The Lowry Process:

Aims to secure a good penetration with comparatively small quantities of creosote. Air dry timber is treated only. Creosote oil is forced into the timber without a preliminary vacuum until a large quantity of creosote is absorbed. A quick final vacuum is then applied and a considerable amount of injected oil is withdrawn. In standard railway tie treatment, about 2.5 gallon of creosote remains in a 6 in. * 8 ft tie and relatively more for large sizes.

  • The Rueping Process/ Empty cell Process:

Intends to secure good penetration with comparatively small quantities of oil. Compressed air is forced into wood up to a pressure of 75 lbs/ sq.inch. After which creosote oil is forced to a higher pressure. In standard practice, 1.2 to 2 gallons are left in a 6 inch * 8 inch * 8 ft railway tie.

  • Card Process:

The preserving liquid is made up of 15 to 20% of creosote/ water gas distillate/ water gas solution and the remainder 3 – 5% of ZnCl2 creosote and ZnCl2 are mixed in a centrifugal pump, and this emulsion is forced into the wood by pressure.

  • Open Tank Treatment:

Includes a number of methods of treating timber, without pressure with either creosote/ ZnCl2 the timber are put in an open tank and heated in the hot liquid for several hours. It is then quickly immersed in cold liquid. During the cooking process, a partial vacuum is produced in wood and the preservative enters under atmospheric pressure. This method is applicable to small quantities of timber in localities where large treating plants are not available.

Brushing/ Coating method:

This is the simplest, but least efficient method of applying preservative. When two or more coats are given and treatment is repeated every two or three years, some measures of protection are obtained. This method is only for timber to be used for temporary protection for a short service.

Dipping method:

For brush coating, there is a risk that the surfaces treated are not thoroughly wetted by preservative. Dipping is more effective, if the length of time of immersion is increased, the considerable amount of preservative can be taken up.

Spraying Method:

It is relatively more effective than brushing for applying preservatives to any large areas. It is used extensively for applying insecticides to roof timbers infected with woodworm enabling operations to reach woodwork that is inaccessible to brush treatment.

Sap-Replacement Method:

The principle of this method is to fix to the butt end of a freshly felled pole a sap containing a solution of preservative. The sap is connected by a rubber tube to a reservoir at a much higher level, so the hydrostatic pressure is exerted. Thus the liquid is slowly forced through the pored of wood, and at the same time, the sap is driven out at the top end of the pole.


Cost of treatment depends upon the method and preservatives used for treatment. Sometimes a good absorption is given a reasonably high services life of a method however, it is costly. But a poorly treated timber may short its service life and it is attacked by insects/ beetles and it causes more damage.
The annual cost of an item/ unit depends on the first cost (in place) divided by the number of years that the item last.
Wood Preservation
Where; A = Annual Charge; P = Cost of material plus cost of erection; R = rate of interest; N = Estimated Service life.


  • The service life of the treated wood can be often more than durable.
  • Posts and poles which decay at the ground line rapidly if untreated can be made to last 30-40 years by preservative treatment.
  • Treated wood is insect repellent.
  • It increases the durability (class of durability increases)
  • Soils built of creosoted wood last a long time and do not need painting.
  • Wood stave pipe impregnated with creosote can be used for drinking after water has run through the pipe after some time.


  • The substances which are used for fireproofing are called fire retardants.
  • Two methods are employed for this purpose ie impregnation and surface treatment.
  • Fro impregnation method mono or diammonium salts are preferably used. Generally, fuel cell process is adopted and 2 lbs of salt/ cft are required as retention. After impregnation timber should be dried before use.
  • A surface treatment technique is used for fireproofing.
  • A paint consisting ingredients: Kaolin 150 kg; Sodium silicate 112 lb; water 100 lbs.
  • Two coats are necessary to give a coverage of 20 square yard/ gallon

Uses of Fire Retardant Wood:

  • Prevents outbreak of fire
  • The rate of burning is reduced at an appreciable level
  • Fire retardants, toxic salts, are also toxic to fungi and thus to achieve protection against decay.


The most effective of the anti-stain treatments used in control of sap-stain in sawn timber have contained as active fungicides sodium salt of chlorinated phenols, sometimes mixed with borax, and organic mercury compounds. Stain control can be difficult on thicker dimensions (50 and 75mm) boards.


  • The wood of the stakes must be uniform and must be free from knots, cracks, stain, and decay insects attack.
  • Insect holes wood must also have straight grain and the annual rings must be as parallel as possible.
  • The stakes should be with either 100% sapwood or otherwise but not mixing of these two.
  • The stakes should be seasoned to the moisture content % at which the best possible penetration of preservative is obtained. It should be 15-25%.


  • A treatment with preservative is not economical when timbers are to be exposed to most severe fungus attack.
  • If the timbers are to be used for a short time, the durability is not important.
  • But on the other hand, a wood of low durability to be used in a permanent location should be treated with a suitable preservative.
  • If a user knows the average life of an untreated wood and it gives the best result after treatment to improve/ increase its service life, it must be treated.
  • If untreated timber is giving long life treatment no result in a great saving.

Image by Author 

For correction and improvements please use the comments section below.

Naeem Javid Muhammad Hassani is working as Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu ( Forestrypedia (, All Pak Notifications (, Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.

Leave a Reply

Your email address will not be published. Required fields are marked *

Translate »