Types of preservatives

1. Tar oil Edit HTML

• Eg creosotes 
• Very effective 
• Derivative from wood or coal
• Suitable for external use due to noticeable colour and smell and causing timber unsuitable for painting . 
• Cheap , can be applied to timber with fairly high moisture content . 
• Reduce rate of weathering . 
• Creep into porous adjacent materials such as cement rendering and plaster . 
• Water-borne 

 Water-borne

• Consist of salts based on metal such as sodium , magnesium , zinc , copper , arsenic and boron . 
• Eg copper chrome arsenate (CCA) 
• Copper - fungicide , arsenate - insecticide 
• Very tolerant of moisture in timber , odourless , non-flammable , non-creeping , do not stain timber , may be painted on drying out 
• Fairly cheap 
• Disadvantages - swell the timber , cause corrosion of metal in contact . 
• Used as preliminary protection followed by kiln drying . 

Organic Solvent

• Organic fungicides / insecticides such as lindane , metallic napthanates and pentachlorophenol in volatile organic solvents . 
• Penetration is excellent on dry timber . 
• Brush or spray application . 
• Non-creeping , non - staining , non -corrosive to metals , non-swelling and quick drying . 
• Flammable , fire-risk until dried . 
• Expensive .
• Contain high volatile organic compound (VOC) could be hazardous to health . Require care in handling .

Boron diffusion

• Freshly felled timber is dipped into concentrated borate solution which diffuses throughout the wood by means of its own moisture . 
• Advantage on difficult timbers - which do not readily admit preservatives .