What is the Preservation of Timber? – Know More About it!

Timber Preservation

Timber, lumber and wood are different words, but mean the same thing. Basically, they are synonyms, however, the terms timber and lumber are used in connection with buildings and construction. Wood is a natural renewable material and plays an essential role in building construction and furniture making.

With a rapid development of urban areas, the demand for timber for various purposes has also increased. Because of the limited availability of naturally durable types of wood like teak wood, sal wood etc. the supply of sustainable timber must be augmented by selected timber of lesser durability which when suitably treated would improve the lifespan under service conditions.

Therefore, preservative treatment of timber forms a significant part of the effort to conserve the natural resource and achieve their most economical utilization.

The availability of timber is ever-changing. While the supply of timber from forests is decreasing, its supply from plantation species such as rubber, eucalyptus, poplar etc. is increasing. These species are generally not durable and require adequate protection from fungi and insects.

With this is mind, let us learn all there is to learn about preservation of timber.

What is Timber Preservation?

Timber preservation is a process that protects the wood from being attacked by moisture, fungi, termites, other insects etc. The preservation of timber ensures that the wood is more durable and extends its lifespan.

The basic principle when it comes to the preservation of timber is to poison the food of the fungi by using preservatives. The success of timber preservation depends on the choice of preservatives and the methods of applying preservatives to timber.

Therefore, wood needs to be treated with suitable types of preservatives of timber before it is used. Let us look at the different types of timber preservatives.

What are the Preservatives of Timber?

Preservatives of timber are chemical substances which are applied to wood to make it resistant to attack from decaying agents.

Requirements of Good Preservatives of Timber

  • The preservatives should give the timber a pleasing appearance after treatment and should also be odourless and colourless.
  • Timber is a comparatively cheaper material hence the preservatives should also be affordable to limit the overall cost of wood.
  • The timber preservatives should not corrode or react with the building materials with which it comes in contact.
  • It should be easily available and even semi-skilled or unskilled persons should be able to apply the preservatives with ease.
  • Preservatives of timber should not be affected by heat, light etc.
  • It should not be easily washed away by water.
  • It should be efficient enough to kill fungi and insects but also be safe and harmless enough for persons and pets.
  • All types of timber preservatives that you use should have a high penetrating power, stability and durability.
  • The preservatives of timber should be such that even a small quantity can cover a large area.
  • It should offer resistance to fire, moisture and dampness.

Different Types of Timber Preservatives

The three types of preservatives for timber are used, i.e. Oil type preservatives, Organic solvent type preservatives and water soluble type preservatives.

01. Oil Type Preservatives for Preservation of Timber

Coal Tar Creosote oil or Lignite Tar Creosote oil are a fraction of coal tar distillate with a boiling point range above 200 0C. It is especially suitable for the treatment of timber that will be used outdoors.  It can be used alone or with appropriate admixtures like coal tar petroleum oil, fuel oil or any other suitable oil having a high boiling point range.

Besides, the admixture of oil gives the timber a certain amount of protection against splitting and cracking. For treatment of marine timber, or timber used in water, coal tar is recommended in place of fuel oil, for mixing with creosote.

The advantages of using creosote as a preservative are that it has a high toxicity, relatively high performance and is non-corrosive, therefore offering excellent protection against termites. However, creosote is not easy to handle because of its unpleasant odour. Also, it is challenging to paint a material that has been treated with creosote.

02. Organic Solvent Type Preservatives for Preservation of Timber

Organic solvent type preservatives are organic or inorganic salts dissolved in suitable organic solvents. The choice of the solvent depends on the solubility of the preservatives. As these preservatives are permanent, it is important to use light organic solvent to make it easier to handle the treated materials. These preservatives, however, are flammable. Some examples of organic solvent type preservatives are Copper and Zinc Naphthenates, Copper and Zinc Abietates, Trichlorophenol, Gamma-BHC (Lindane), Chlorpyrifos, Synthetic Pyrethroids etc.

03. Water-Soluble Type Preservatives for Preservation of Timber

Timber is treated with water-soluble type preservatives that have several applications, like on the outdoor deck, fences, playground equipment, structural framing etc. Water-soluble preservatives are available in two types, i.e. Water-soluble (Leachable) Type and Water-soluble (Fixed) Type.

a) Water-Soluble (Leachable) Type Preservatives for Preservation of Timber:

Water-soluble (Leachable) Type preservatives are inorganic or organic salts that are soluble in water. However, these preservatives are subject to leaching, which means that the amount of preservatives in the treated material gradually gets depleted owing to the dissolving effect of water.

Water-soluble (Leachable) Type preservatives are generally odourless and involve little fire hazard. Timber treated with these preservatives can be painted when it is dry.

Some examples of Water-soluble (Leachable) Type preservatives are Boric Acid and Borax,  Sodium Fluoride, Sodium Pentachlorophenate (Na – PCP), Gamma – BHC (Water Dispersible powder), chlorpyrifos, synthetic pyrethroids etc.

b) Water-Soluble (Fixed) Type Preservatives for Preservation of Timber:

These types of preservatives usually comprise mixtures of various water soluble salts with an added fixative salt like sodium or potassium dichromate. The role of chromium is to fix toxic elements like arsenic, copper, boron etc. in the timber so that the toxic salts become difficult for water to leach. However, the treated wood must be allowed to dry for two to three weeks to complete the fixation process.

Some example of Water-soluble (Fixed) Type preservatives is Copper-Chrome-Arsenic Composition (CCA), Acid Cupric-Chromate Composition (ACC), Copper-Chrome-Boron composition (CCB), Zinc-Meta-Arsenite, Borated Copper-Chrome-Arsenic Composition (BCCA), Ammoniacal copper Arsenite (ACA) etc.

Methods of Applying Preservatives to Timber

The efficiency of timber treatment methods depend on the choice of preservative, which ensures the required absorption and penetration. Let us discuss the various methods of timber preservation.

01. Surface Application Method

Surface application is when the timber preservatives are applied by brushing, spraying or dipping. For this method of treatment, timber is debarked thoroughly. According to the Indian Standard Code – IS – 401, the moisture content in the wood should be less than 20 percent for oil type preservatives and 20 to 30 percent for water-soluble type preservatives.

a) Brushing:

Apply Preservatives by Brush

In this method, preservatives are applied to the surface of timber with the help of a good quality brush.

b) Spraying:

Apply Preservatives by Spray

Under the spraying method, the preservative solution is filled in a spraying machine. The solution is then sprayed on the timber’s surface. The spraying method is more advanced faster and efficient when compared to the brushing method.

c) Dipping:

Here, the timber is dipped in a preservative solution for a limited defined period of time. The dipping method is better than the brushing or spraying methods, however, the performance depends upon the type of timber.

02. Soaking Treatment

Under this type of timber treatment, the wood is carefully debarked and is then dipped in a solution of preservatives for a long period of time, until an adequate amount of preservatives is absorbed.

Normally, soaking of veneers in a preservative solution for a period of one to two minutes is adequate for timber with thickness of up to 1.8 mm in case of refractory species and thickness up to 3 mm for other species. Prefinished door, furniture and timber can also be treated with light organic solvent type wood preservatives by soaking treatment.

03. Hot and Cold Process

Here, the timber is submerged in a preservative oil or solution and is then heated to about 90 0C and maintained at this temperature for a suitable period of time. The timber is then allowed to cool until the optimal amount of preservative is absorbed.

While heating, the air inside the timber expands and part of it is expelled. During cooling, the residual vapour or air in the timber contracts, creating a partial vacuum which causes the preservative to be sucked into the timber.

The hot and cold process also ensures sterilization of the timber against fungi and insects. In the absence of facilities for the pressure treatment, this process is recommended for timber made of sapwood and easily treatable heartwood with water-soluble (fixed) type preservatives.

During this process, there is a possibility of chemicals precipitating at higher temperatures and coming in contact with the extractives in the timber. To overcome this problem, a two-step treatment is used. The first is the hot treatment which is immediately followed by the cold treatment, which is the second step.

04. Pressure Process

While using the pressure process method of timber preservation, any of the types of timber preservatives can be used. As per the Indian Standard Code – IS – 401, in the case of oil type preservatives, a temperature of 80 0C to 90 0C should be maintained when the pressure is applied. This process of treatment of timber is divided into the following types:

a) Full Cell or Bethel Process Wood Preservation:

In this process, the timber is placed in an airtight impregnating chamber from which air is pumped out. The preservative, under pressure varying from 3.5 to 12.5 kg/cm2 depending on the species, size and refractory nature of timber etc., is then forced into the chamber.

The pressure is maintained until the desired preservative absorption is achieved. Finally, the pressure is released and a vacuum is created to withdraw the excess preservatives. This method is recommended for the treatment of refractory wood or timber needing high preservation retention.

b) Empty Cell Process Wood Preservation:

This process is comparatively cheaper than the full cell process, as it requires fewer preservatives to give the desired result. In this process, timber is stacked in the cylinder, the doors of which are then is then closed tightly. This process is subdivided into two methods. i.e. Lowry Process and Rueping Process

i) Lowry Process:

In this process, after the timber is loaded into the cylinder, which is then closed, preservatives are injected into the same and pressure varying from 3.5 to 12.5 kg/cm2 is applied until slightly higher than the required absorption is obtained. When the pressure is released, a certain amount of the preservatives is expelled due to the expansion of the trapped air in the cells. The cylinder is then drained of the excess preservatives by creating a vacuum.

ii) Rueping Process:

In this process, an air pressure of 2 to 5 kg/cm2 is applied for a specific period depending on the sapwood content. This pressure is maintained till the preservatives are filled in the cylinder. Once the cylinder is filled, a pressure of 3.5 to 12.5 kg/cm2 is applied until the optimal absorption is obtained.

In this case, the preservative expelled upon the release of pressure is considerable and there is also low net absorption. This process is especially recommended for treating timber of mixed species and those containing sapwood and treatable heartwood.

05. Boulton Process

The Boulton process is a combination of conditioning wet material by boiling and drawing vacuum till the moisture is removed and treating the content subsequently in the same cylinder. Oil type preservatives are used in this process.

06. Diffusion Process

The diffusion process is a method of treatment of timber and other plant materials like bamboo, while it is still green (or fresh). This method is used on lumber which is not easy to impregnate under pressure when it is dry. The diffusion process is also used when there is a danger of the timber deteriorating during seasoning, especially if air seasoning is done.

To sum up, extending the service life of timber by the application of appropriate preservatives has a significant effect when it comes to effective timber utilization. Besides, the preservation of timber contributes to a reduction in the demand for timber replacement and also helps conserve the forests.

Treatment of timber with preservatives enhances its lifespan and ensures structural integrity in many exterior applications. Treated timber can withstand fungal decay, insect damage, and enables the production of high-quality timber products.

Preservation of timber also paves for better utilization of wood as a natural resource as all types of wood species do not boast the same durability.

Must Read:

15 Popular Types of Wood used in the Construction Industry!
What are the Uses, Advantages & Disadvantages of Timber for House Construction?

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