The wood torrefaction process
The technology consists of heating the wood at a very high temperature (from 190 to 240°C), in a torrefaction kiln (autoclave) at controlled atmosphere and low in oxygen. The length of the process, as well as the temperature degree sustained during a determined period, depends on the torrefied specie and the desired colour.
This technology requires a pre-drying process using conventional kiln driers, to lower the moisture level between 6% and 10%. After that, the torrefaction kiln lowers the moisture level to 0%.
Finally, the wood is placed in a conditioning chamber where the temperature is gradually lowered by controlled steam injection. The moisture level of the product is increased between 3% and 6%, in order to give back to the wood its natural flexibility. This conditioning stabilizes the wood and allows better quality manufacturing.
Torrified wood : the resulting qualities
Qualities resulting from this torrefaction are the following :
Torrefied wood does not contain any chemicals which may contaminate air, water or soil. It is perfectly safe for the users' health.
Torrefaction provides a richer hue to the wood, giving it a colour from honey blond to dark brown depending on the treatment applied. Also, the treatment enhances the wood grain.
Torrefied wood has a greater dimensional stability when exposed to atmospheric variations. There will be less shrinking, swelling and warping with changing weather and unfavourable moisture conditions.
Torrefied wood resists better to fungi and moulds which usually mark and degrade the wood. The torrefaction process destroys most of the cells on which wood insects and parasites feed. Torrefied wood is thus very interesting material for outside applications, and is as durable as cedar or chemically treated wood. And most importantly, it is completely safe for health.
Torrefied wood can be transformed, in most cases, immediately after the thermal treatment.
Important remarks about torrified wood
Torrefaction does not make the wood lifeless. Wood remains a living matter. It is therefore important to apply a good quality finish, in order to protect it from UV rays and avoid the wood eventually turning grey. Although its resistance to water and organic degradation is very high, torrefied wood still requires a minimum of maintenance.
A weaker wood
Torrefied wood is a little less resistant to static flexion and to tearing compared to non-torrefied wood. It can crumble more easily, depending on the wood species. However, this strength reduction is not very significant for most wood species. Torrefied wood must not be used for structural purposes.
Due to its hardness, torrefied wood can require some adequate cutting tools. Torrefied wood can be moulded and worked while keeping its beauty, as the process allows a uniform colour to the core, not only at the surface as opposed to for chemically treated wood.
For gluing purposes, you should make sure that the approprietated type of glue is used, as torrefied wood has a lower moister level compared to conventional wood.