Wood Preserver Application Methods
The application of wood preservers by industry usually involves vacuum treatment and/or pressure treatment. In the home, painting and spraying are common wood treatment methods.
The 1840s saw the first industrial technique for the application of wood preservative, and was known as the Bethell Process. Creosote was applied to wood using a vacuum and pressure treatment process. This originally occurred in airtight metal vessel, but was developed into a treating cylinder. These treating cylinders are still used today.
Pressure and Vacuum Treatment
Industrial methods of timber treatment can involve either high or low pressure, although high pressure treatment is more common. High pressure maximises the penetration of the wood preserver into the wood.
Pressure and vacuum treatment methods are used to apply creosote and most industrial chemical wood preservatives. This method can be done in two separate ways: the full cell process and the double vacuum process.
Full Cell Process
During the full cell process the wood is placed on a bogie, and pushed into the main plant chamber. This chamber is sealed, and then an initial vacuum occurs. The chamber is flooded with the wood preservative. Hydraulic pressure treatment is applied for a set period of time or until the desired amount of wood preservative has penetrated the timber. The pressure is then released, and the remaining wood preserver is pumped out. A final vacuum treatment stage then helps to dry the wood.
Empty Cell Process
The empty cell process is a variation on the full cell process. This omits the final vacuum treatment, and instead another pressure treatment is applied. This allows a larger proportion of wood preserver to be recovered and reused. The empty cell process only coats the cell wall of the wood, rather than the entire cell space as is the case with the full cell process.
Double Vacuum Process
The double vacuum process has an initial vacuum applied and held whilst the chamber is flooded with wood preservative. The vacuum is released until the correct atmospheric pressure is reached to force the wood preserver into the timber. A final vacuum treatment then recovers some of the wood preservative, and also dries the surface of the wood.
Immersion Method
The immersion method is an alternative timber treatment, although it is decreasing in favour of the double vacuum process. Freshly felled timber is sometimes treated in this way to prevent the growth of sapstain fungus. Planks are immersed in a water based solution of fungicide for a few seconds, and then stacked to dry.
Brushing and Spraying
Brushing and spraying are common home timber treatment methods. Neither of these methods allows the wood preserver to penetrate deeply into the wood, so they will have to be reapplied roughly every five years. At least two coats are needed, whether the application method is brushing or spraying.