Preservatives of wood:

Preservatives increase the resistance of wood to decay and increase its useful life.

There are 3 main classes of preservatives

1. Oily substances insoluble in water

2. Water-soluble salts

3. Salts carried in volatile solvent other than water

Watch a Presentation on Preservatives of wood

A) Oily Preservatives

• Coal-tar is the best known ►most widely used preservative
• Obtained from bituminous coal
• Available in many grades ► gives satisfactory results
• Insoluble in water ► hence permanent preservative
• Highly toxic to fungi
• High degree of penetration

Disadvantages of using Oily Preservatives:

1. Timber ►inflammable for a time after treatment
2. Disagreeable odor
3. Difficult to be covered with paints

B) Water Soluble Preservatives

• Zinc chloride ► most extensively used water preservative
• Readily available, clean, odorless,

C) Salts

• Another recent product is AsCu which is a copper and arsenic compound is used as a preservative ► available in the form of powder
• Odorless and leave on strains on timber
• Good fire resistant

D) Painting

• Acts not only as a preservative but it also enhances the appearance of the treated surface
• Only well seasoned timber should be painted ► moisture entrapped ► closing of timber pores by paint

Methods of Applying Wood Preservatives

Wood must have the following characteristics before preservatives are applied to it:

• Wood must be well seasoned
• Wood must be cut to size before applying preservatives

1) Painting & Dipping

• Simplest method
• Preservatives applied by mean of brush (several times)
• Timber can also be immersed in tank full of liquid (preservative)
• Penetration should hardly exceed (1/16 inch)
• Duration of immersion and temperature of preservative solution ►increased ► to increase penetration'

2) Pressure Process (Full Cell Process)

• A higher degree of penetration can be obtained by forcing the preservative into the wood
• Timber placed inside a chamber
• Air drawn out to create a vacuum
• Thus the cells are completely (almost) empty to receive the preservative
• Preservative material may be creosote oil or zinc chloride
• Preservatives pumped under a pressure of 100 to 200 psi at 120oF
• The excess preservative is removed by creating a low vacuum
• Timber preserved by this method are used in piles in saltish water, poles, sleeper

3) The Empty Cell Process

• Similar to the full cell process but no initial vacuum is created
• No attempt is made to remove the air from the cells
• The preservatives applied under a pressure of 200 psi
• The excess preservatives drains away
• A deeper penetration of preservatives ► achieved