All our timber is initially air-dried. The timber is milled into planks, stacked on a flat base between stick battens and the top of the stack then covered from the sun and rain. Aside from being lower in cost to produce in terms of not having to pay extra for kiln drying green timber, it also means we are creating less of a carbon footprint. The science in air-drying is an interesting process and it’s very rewarding to find that the more attention to detail paid at the beginning, the better the quality of timber at the end.
Below are extracts from an excellent guide to timber drying – this provided us with essential information when starting up:
This is a method of drying timber by exposing it to natural atmospheric conditions. As such there is no control over drying rate as this will be determined by the prevailing weather (temperature, relative humidity, rainfall and wind speed), which will vary considerably between winter and summer. Some control over drying times and degrade can be achieved by correct stacking procedures of freshly sawn timber.
Air drying does not involve the need to burn fuels to dry wood thus conserving energy and reducing harmful atmospheric emissions
The obvious advantage of air drying is its low capital cost in comparison to kiln drying procedures. We find that it is more economical to air dry timber to 18% – 25% moisture content, and then kiln dry down to our final target MC of 10%. Note that air drying can only be used to reduce the moisture content of timber to around 12% in Summer, and it can rise up to 18 % in Winter. Therefore, we use a de-humidifier kiln to create the controlled environment as a necessary final step in the drying process, in order to produce furniture dry timber.
Kilns are closed chambers in which air temperature, relative humidity and airflow can be controlled to dry timber to specified moisture content. There are many different types of kilns such as vacuum systems, traditional heat and vent type kilns and radio frequency dryers. The cost of installing and maintaining kilns may often be prohibitive unless throughput of timber is high. However, if the value of specific species is high enough, it becomes more feasible to kiln dry green timber, although we don’t tend to do this unless we are very low on stock and take in a big order.
A rule of thumb for the UK is that 25mm (1″) thick material will take 9 months to dry, 50mm (2″) thick material will take 18 months to dry and 75mm (3″) thick material will take 24 months to reach equilibrium moisture content by air drying. In addition to thickness, drying times depend on the characteristics of the timber being dried, climatic conditions (temperature and relative humidity) and airflow through the stack. Some timbers will have higher green moisture content than others. Furthermore, heartwood and sapwood from the same tree may have very different drying characteristics.