Timber seasoning and preservation treatment

Timber is ‘converted’ into usable building products like planks and beams. There are several ways that a single piece of log can be cut into timber, and the purpose is to maximise the amount of timber that could be used in building products. Normally they are first cut into planks or beams, and then treated and seasoned, to reduce the moisture content.

Seasoning will improve strength, hardness, durability, insect attack resistance, and better adhesion of paint and finishes as well as adhesion bond when gluing.

Natural timber seasoning

Seasoning of timber is done naturally in open-air storage, to reduce the moisture content gradually from around 40% – 180% in freshly felled logs to below 20% for modern timber usage. This process could take up to 3 months but is cost-effective.

Planks or beams are arranged in a raised storage yard, which is shaded, dry and cool with good air circulation. To provide efficient circulation, the layers of timber are separated from each other with regularly spaced strips of small pieces of thin seasoned timber, 20mm thick. In order to reduce defects warping and bending whilst seasoning, the strips should be aligned exactly vertically between layers and spaced 300 mm to 450 mm at each layer.

A group of timber in the seasoning process is called a stack, made up of timber from similar species so that they dry uniformly together.

Artificial timber seasoning

The most common artificial seasoning is using a kiln to accelerate the process of reducing the moisture content of the timber, using heat and humidity differential inside the chamber. Kiln seasoning is able to reduce seasoning time to about only 7 days compared to natural seasoning which takes months to complete.

Modern kilns are computer controlled to provide the optimal drying for different timber species and to avoid defects from accelerated drying. Likewise, planks and beams subjected to artificial seasoning are stacked similarly to the natural drying method with regularly spaced strips to ensure uniform circulation within the drying chamber.

Preservation

Preservation is a timber treatment process designed to prolong the life of timber by making it resistant to fungal and insect attacks. The most common treatment available worldwide is copper chrome arsenate (CCA) but is now generally restricted to non-human contact applications such as in toys, furniture, and handrailing. It is still being used for architectural framing and structural applications widely.

There are 3 main types of preservation treatment namely metallic compound (like CCA), salt compound, and solvent-based compound.

Preservatives are applied to timber either using pressure impregnation or external application by coating or immersion by soaking the timber in the preservative for a pre-defined time.