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How Preserved Wood is Made

How Preserved Wood is Made

We Preserve Wood

As a natural material, wood exposed to the outdoors faces significant threats from predators such as mold, decay fungi and insects like termites and wood borers. These organisms seek to break down the wood fiber, reducing the structural soundness and serviceability of the wood.

Preservative treating creates a chemical barrier that protects the wood from these threats. The preservatives are not just on the surface, they are infused deep into the wood to provide long-lasting protection. As a result, the service life of the wood is extended from just a few years to as much as 50 years or more.

Click on the video to see how wood is preserved

The treating process utilizes pressure that forces the preservatives into the wood fiber. While there are a variety of preservatives used today, the manufacturing process is virtually the same for all.

Preparing the Wood

Preserved wood manufacturers start the process by purchasing kiln-dried lumber from sawmills or specifically selected round poles from forest companies. To meet national standards for use in applications where the wood will be in contact with the ground, Western species such as Douglas fir and Hem-Fir must be incised on all four sides prior to treating. Incising consists of making small incisions in the wood to allow proper penetration and retention of the preservative. See the publication PreserveTech - Incising for more information.

The untreated wood is then loaded onto carts, which are moved into a specially made cylinder called a retort. The retort, which can be up to 150 feet in length, is sealed and a vacuum is applied to remove the air.

Infusing Preservatives

Next, the preservative solution is pumped from storage tanks, completely filling the retort. Pressure is continuously applied within the retort to force the preservative into the cells of the wood. Processing time depends on the wood species, the size of the products treated and the preservative used, as well as the standard for the intended exposure of the final product.

The preservative is drained from the retort and a vacuum is applied to return the excess solution to storage tanks for future use. The carts are moved out of the retort onto a sealed drip pad to reclaim any residual preservative on the wood.

Quality Control

Representative core samples are taken and analyzed to ensure that quality and durability standards are met. Third-party inspection agencies also take core samples during regular visits to confirm the plant's quality control practices.

The finished preserved wood products are typically covered with a top wrap and moved to the plant's storage area for shipping.