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Chemical Seasoning and Preservatives of Wood

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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

 

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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

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B) Water Solouble 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
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