In the last issue of Technical Matters, we discussed the complex relationship between moisture and wood and the need to dry wood to an acceptable moisture content level so that it can be used for commercial applications. This feature will focus on how timber is dried and also on some of the complications that can arise if timber is not dried correctly.
The simplest method of drying timber, and still probably the most common method of drying timber found in India today, is air drying. As the term implies, this method relies on the naturally occurring atmospheric conditions to dry the moisture out of the wood. Until the fairly recent development of modern steam kiln technology, this was the only method available for drying timber and it worked satisfactorily for most applications. All that air drying really involves is stacking the cut boards of timber in a pack with sticks placed horizontally between each layer to allow the air to flow over all of surfaces of the boards. And that’s about it, a cheap and simple way to dry timber.
Air drying does, however, have its drawbacks. It is not an exact process and relies solely on natural atmospheric conditions. A long, hot spell in the Indian sun can have the effect of drying the lumber too quickly, while a prolonged period of rainy weather can delay the drying process. This can also have an effect on the final quality of the timber. It also takes quite a long time to dry timber outdoors, even in a hot country like India and particularly with thicker stock material. As a general rule, oak takes a year per one inch of thickness to air dry. So it will take three years to properly air dry a three inch thick oak board.
Another issue that has been more recently encountered is the introduction of air conditioning in buildings, which can have the effect of drying the timber out while it is in service to a lower level than the natural moisture content level achieved by prior air drying outside. We talked about this in the previous article, where timber is always trying to find its Equilibrium Moisture Content (EMC) in its surrounding environment.
This is where the modern-day method of kiln drying has the benefit of being able to dry timber down to a low enough moisture content level to suit the modern indoor building conditions without causing any problems such as shrinkage or distortion. The term “kiln dried” can be misleading to the uninitiated, as it can lead to the understanding that the timber that comes out of a kiln is ‘dry’. To a degree it is, but it is never completely dry and it should be at the right moisture content for its intended application. American hardwoods tend to be kiln dried to around 7 to 9% moisture content, which is ideally suited to interior use. European hardwoods are slightly less dry, at around 10 to 12% moisture content.
Modern day kilns are usually very large chambers, capable of drying big volumes of wood at a time to make the process economical. In layman’s terms, they are really a warm and damp shed. The kiln drying cycle or ‘schedule’, as they are called, begins by placing the timber in the chamber and, in the case of American hardwoods, this may have been previously air dried, depending on the timber species and the thickness. The kiln is then gradually heated up while a high level of humidity, in the form of steam, is introduced. This is so that the cells on the outside of the boards do not lose their moisture too quickly and dry out too soon and potentially trap moisture inside the board, which could lead to irreversible damage to the timber.
As the process continues, the temperature is gradually turned up and the level of humidity is reduced, which encourages the moisture in the middle of the boards to migrate to the surface and then evaporate. Over many years’ experience by kiln manufacturers and operators, different kiln schedules have been developed for different timber species, as not all of them dry at the same rate. Lower density hardwoods, such as American tulipwood, for example, dry much faster than denser species such as American red or white oak. Even with kiln drying, it is impossible to get each board in the drying chamber to exactly the same moisture content and, therefore give a single value, so the moisture content is provided in a range usually +/- 2% of a central figure. For American hardwoods, this would be 8% +/- 2%.
The drying process is also a significant contributor to the final cost of the imported timber, so it is a fine art to get the timber dried correctly, but at the same time, efficiently. All American hardwoods are exported kiln dried, unless otherwise specified. Shipping undried timber means shipping more moisture and less wood and can lead to a whole host of other, unwanted, issues when the wood arrives at its final destination. The kiln drying process is also a way to ensure that any pests in the wood are destroyed and ordering kiln dried American hardwood lumber is one more way to ensure that what arrives is ready to use without further treatment.