What Preservatives Are Used To Create Treated Pine Sleepers?

If you are interested in purchasing some treated pine sleepers for use around your property, you might be wondering what sorts of preservatives have been used in their treatment. There are three classes of preservative that are commonly used, including: waterborne, solvent-borne and oil-borne. Testing has found that all of these preservatives are highly toxic to decay-causing fungi and insects, and that they will remain active in the timber indefinitely.


These sorts of solutions comprise a mixture of water-soluble compounds of copper and other chemicals. Other chemicals can also be added to prevent the glowing and smoldering of the timber after a fire has occurred.

  • Copper Chrome Arsenic (CCA)
    This is the most widely used preservative for treated pine sleepers. The copper acts as a fungicide, whilst the arsenic acts as an insecticide. The chrome is added to fix the copper and arsenic, preventing them from being leached from the timber. The solution is clean, odorless, has a slight green colour and can be painted (when dry).
  • Alkaline Copper Quaternary (ACQ)
    This solution utilises copper as an agent against both fungal and insect attack, whilst it utilises quaternary ammonium compounds as an added defense (against fungi, wood boring insects and termites). It has a green-brown appearance that quickly weathers to brown.
  • Copper Azole
    This solution utilises copper as a fungicide and azole as a biocide or insecticide. After treatment, it offers a green colour that gradually weathers to a honey brown. There is only one formulation currently available, but it is thought that this will change.


These sorts of solutions are fungicides, insecticides and (in some cases) water repellent chemicals. White spirit is the solvent that is commonly used in Light Organic Solvent Preservative (LOSP) formulations.

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  • Light Organic Solvent Preservative (LOSP)
    These preservatives contain various solutions of organic fungicides and insecticides (such as copper naphthenates and synthetic pyrethroids). The process provides the timber with long lasting protection against décor and insect attack in above ground applications. The treatment is usually clear, leaving the pine’s appearance unchanged.


These sorts of solutions are a complex mixture of chemicals that are obtained from the distillation of coal tar. Being a liquid, it is normally used without the addition of a solvent (although fuel oil is sometimes added).

  • Creosote
    Timber treated with this preservative is highly water repellent and resistant to weathering. Because the solution is non-aqueous, the timber doesn’t need to be dried out after being treated. The resulting surface is generally oily and black; it cannot be painted. The oily residue will remain for some months and it gives off a strong odor.

We hope that the information provided above has given you a much better understanding of the preservatives used in treated pine sleepers. It is important to remember, however, that these preservatives do not protect pine against weathering – to achieve this, the application of a suitable surface finish (such as paint or stain) is required. Even though some sleepers will contain a wax or oil additive that acts as a water repellent, it’s not foolproof.

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