Kiln seasoning

Kiln seasoning

A number of commercial processes for seasoning of timber are available, the most common of which is kiln-drying. Kiln seasoning accelerates the process of seasoning by using external energy to drive the  moisture out. The timber is stacked in much the same way as it is for air drying, and is placed inside a chamber in which the conditions can be varied to give best seasoning results. Air is circulated around the charge (stacked timber) and the temperature and humidity can be varied to give optimum drying. Each species has different cell characteristics and therefore requires different drying schedules. Typically the timber may be in the kiln for a period of between two days to one week. All untreated structural pine and some commercial hardwoods are seasoned, mostly using kilns that are often heated by sawmill by-products or gas.

After kiln seasoning, there is often some damage to cells near the surface of the wood.  (All of the moisture passes through those cells.)  They have in fact collapsed, but can easily be ‘pumped back up’ in a reconditioning chamber.  This chamber introduces steam for a period and puts some moisture back into the outer cells and removes the effect of seasoning collapse. Kiln-seasoning of softwoods such as pine species is generally a fairly quick process, seasoning of hardwoods tends to be a much longer process. This is mainly due to the different (closed) cell structure of hardwoods.

Once the sawn hardwood material reaches fibre saturation point or slightly below (at a moisture content of about 20% to 25%), it is then placed in kilns usually for up to 10 ‑ 14 days (depending upon the thickness of the sawn timber) in order to bring the moisture content down to between 10% and 15%. This drying process must be strictly controlled and monitored in order to avoid drying degrade.