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Preservation indicates an increase in life by developing resistance to
insect attack, fungal infection and disease of timbers.
• A preservative acts like a disinfectant;
• A seasoned timber, since dried, is hygroscope and to prevent
reabsorption of moisture and to impart immunity, the tissues of
dry/seasoned wood have to be soaked with some type of a
preservative. Seasoning, therefore, prepares a timber for
preservative treatment by driving away moisture and sap.
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Choice of Preservative:
• Their toxicity and poisonous effects;
• Permanency in their effect in treated wood;
• Should not be injurious to wood tissues;
• Cheaply available and safe to handle;
• Should allow a decorative treatment;
• Should not disfigure exposed surface of timber;
• Non-inflammable;
• Should have a good covering quality.
Methods of Preservatives:
Chemical AsCu
Charring Tarring Creosoting Painting
Salt Treatment
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Charring:
• Crude method, no special preservative is used;
• Timber kept wet for 0.5~1.0 hour and then burnt to a depth of 15 mm
and cooled with water;
• A coal layer is formed on the surface which performs preservative
functions;
• The layer is not affected by fungi, moisture, or white ant;
• Used at lower ends of posts of timber.
Tarring:
• Application of a layer of hot tar on the surface;
• Generally applied to embedded ends of posts.
Creosoting:
• Creosotes are obtained by the distillation of coal, petroleum or wood
substances;
• Three types: Coal-tar creosote, water gas-tar and wood-tar creosote;
• Creosote oil is applied under pressure on wood surface;
• Used on piles, poles and railway sleepers, etc.
Painting:
• Performs both aesthetic and preservative purposes.
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Water soluble chemical salts:
• Some chemical salts which are not toxic in nature and are also soluble
in water;
• They are odorless and can be painted on drying;
• When appearance is important in wood, this type is most suitable;
• Wood treated with water-soluble salts requires to be re-dried;
• The effects of these chemicals are lost gradually and so wood
requires painting or varnishing for surface treatment;
• Cheaper than creosote treatment;
• Example: Zinc chloride treatment; Creosote oil + NaF (known as
Wolman’s salt).
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AsCu-Treatment:
• A mixture of 3 parts of 𝑪𝒖𝑺𝑶𝟒 . 𝟓𝑯𝟐 𝑶 + 4 parts of 𝑲𝟐 𝑪𝒓𝟐 𝑶𝟕 . 𝟐𝑯𝟐 𝑶 + 1
part of 𝑨𝒔𝟐 𝑶𝟓 . 𝟐𝑯𝟐 𝑶
• The solution is prepared by mixing six parts of this powder to 100
parts of water by weight;
• The solution is then sprayed on the timber surface;
• After drying, the surface needs to be waxed, varnished, polished
and painted;
• The solution is colorless.
