In the previous edition of Technical Matters, we explored the concept of natural durability of timber; how timber is able to naturally resist pests that use timber as a food source and make timber with low durability unsuitable for exterior applications. In this edition, we will look both at traditional and more modern methods of preserving timber and prolonging its service life.
Any timber species that has low natural durability and is to be used in a hazardous situation, i.e. outdoors with exposure to higher moisture levels and termites, would need to be treated with a wood preservative of some description in order to prolong its service life. Preservatives are very varied in formulation and in how they are applied to the timber. Essentially, they are made up of approved biocides, usually a fungicide and an insecticide dissolved in a ‘carrier’ liquid.
The simplest method of applying preservatives is by brush or spray, which can be messy and lead to large quantities of the preservative being lost into the surrounding area rather than on the wood itself! The uptake and penetration into the timber using these methods will be minimal. These preservatives tend to be solvent-based in order to aid penetration, as solvents penetrate into wood more readily than water.
Dipping timber in preservative and leaving it for a length of time is another option, which gives better penetration and protection, but still can be quite messy and not particularly environmentally-friendly. Dipping is a better option if you need to treat more components at one time and on a more regular basis. One common misconception is that the preservative has to penetrate all the way through to the center of the piece of timber. This is not the case.
To preserve timber what you are looking to achieve is what is termed an ‘envelope treatment’, where all faces of that component have been exposed to the preservative liquid, which is then absorbed into the timber. Ideally, you would be looking for a depth of penetration of between 3mm and 6mm depending on the hazard that that timber component will be exposed to. The higher the hazard then the more preservative is needed to provide a good service life. Important to note is that, as this is just an envelope treatment, it is essential that all machining is carried out prior to treatment.
The best method of applying conventional preservatives is in a timber treatment plant. This is a much more controlled and precise method of getting preservative liquid into timber. The process uses a combination of vacuum and pressure to force preservative into the outer cells of the wood so that a higher loading and a better depth of penetration of preservative can be achieved. The best penetration of preservative is achieved with sapwood and timber species that are easy to treat. Again, as with other methods of application, the preservative is only an envelope treatment and any cutting or machining should be carried out before treatment. Any post-treatment cutting will necessitate re-treatment.
More modern methods of improving the service life of timber have been developed in recent years. One of the most successful is thermal modification. This technology is a real alternative to traditional preservative treated timber or naturally-durable tropical timbers for higher hazard external applications.
Thermally-modified timber (TMT) is not a new concept. The ancient Vikings knew that when building defensive fencing for fortifications, poles made from timber with a burnt surface lasted longer than those without. It wasn’t until the late 1990s that Scandinavia first introduced the modern commercial method of thermal modification that we recognize today, as a way of improving the durability and stability of native softwoods. From a North American hardwood perspective, a number of species can be thermally-modified very successfully. These are hard and soft maple, tulipwood, red oak and ash in the main.
The TMT process is essentially a high intensity kiln schedule that lasts between two to three days, depending on the species and dimension of the timber. The temperature reaches between 180 and 215 degrees Celsius, depending on the level of durability required. The process requires an inert atmosphere devoid of oxygen to prevent combustion and this is usually either steam or a vacuum. During the process the chemical and physical properties of the timber undergo permanent change.
The two most beneficial effects of the TMT process are to dramatically improve both the stability and durability of low durability timber species, such as ash and tulipwood. After the process, the moisture content is around 4 - 6 percent and the equilibrium moisture content is permanently reduced, which means that it does not react to changes in humidity as drastically as untreated wood. In effect, this reduces the ability of the timber to absorb moisture, and so greatly improves dimensional stability.
Durability is improved by removing the hemicelluloses and carbohydrates from the wood, which are the main food sources for wood destroying fungi and insects. Independent testing has shown that the durability of North American hardwood species can be improved to Durability Class 1 (very durable), the highest rating possible and equal to that of tropical timber species such as teak. Thermally-modified U.S. hardwoods can now be used for exterior applications such as cladding, decking and high quality garden furniture.
The other characteristic of TMT is a change in appearance to a darker color, with color variation being influenced by temperature and species. TMT timber also machines exceptionally well, particularly soft maple. Unlike preservative treatments, the process also ensures the whole of the cross-section of the timber section is treated, meaning that this process is done to the unfinished boards and no re-treatment is required after machining.