During recent visits to India to promote the use of American hardwoods for use in furniture manufacturing, AHEC has been confronted with statements to the effect that American hardwoods are not fit for purpose for furniture manufacturing in India and that furniture made in India from American hardwoods is often failing. This is, of course, incorrect, and American hardwoods have been used in countries such as Vietnam, China, Turkey and Indonesia to make furniture for many years with great success. This article sets out to explain why using properly kiln-dried American hardwood lumber is the key to their successful application in India.
The basis for the statement of not being fit for purpose arises from the perceived poor performance of the wood in service after it has been manufactured into a piece of furniture, where the wood has either ‘moved’ or perhaps ‘split’. It is, therefore, the wood that gets the blame! These types of problems are caused by the lumber not being dried to the correct moisture content before manufacturing. To have the correct moisture content at the time of manufacturing is probably the most important factor in the whole furniture making process. It is crucial because wood is hygroscopic, a property that means the wood will interact with the surrounding moisture and temperature.
Wood in a piece of furniture that has not been dried properly will lose more moisture in a drier atmosphere and will always have the potential to move, especially if exposed to sudden changes of atmospheric conditions caused by an increase in temperature. The interaction between moisture and wood when it is installed or when it is in service as a piece of furniture is called movement. Movement like shrinkage happens across the cross section of the wood and it will be happening all the time during its lifespan. In fact, it never stops moving, even though you may think that it has.
Recent history shows that India used to mainly purchase hardwood logs from America, although this is now changing. The logs would be sawn to size in India and then air dried, but most-likely not properly kiln dried. Air drying is not an exact process, and it relies on the natural atmospheric conditions. Even in a hot country such as India it will take quite a long time to dry lumber outdoors. For example, as a rule of thumb, oak takes a year per one inch thickness to air dry. Air drying will only succeed in drying the lumber down to a moisture content of around 25 to 28 percent, not dry enough for furniture manufacturing.
The water or moisture content in timber is contained in two forms: ‘free water’ and ‘bound water’. The free water is contained in the hollow parts of the timber cell structure and can be removed over time by air drying. This will reduce the moisture content to around 25 to 28 percent, a figure known as the Fiber Saturation Point or FSP. At this moisture content the wood will be lighter, but no dimensional change will have occurred. The bound water is present in the cell wall structure of the timber, and it is when this moisture is removed that the wood will begin to get physically smaller in cross section. The best method of drying timber down from 25 to 28 percent to a moisture content suitable for furniture manufacturing in a controlled way is by kiln drying.
In fact, on my last trip to Delhi this year, we had a conversation with an importer who sold air-dried American hardwood lumber to this range of moisture content of around 25 percent. He did so with a disclaimer stating that if anything went wrong then it was not his problem. This is why it is critical when manufacturing furniture in India to use wood that has been properly kiln-dried in order to avoid any future drying-related failures in service. The modern day method of kiln drying at source from a sawmill in the U.S. has the benefit of being able to dry timber down to a low enough moisture content level to suit 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’, which to a degree it is. 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 given as a range usually +/- 2 percent of a central figure. For American hardwoods this would be 8 percent +/- 2 percent. The drying process is a significant contributor to the final cost of the imported lumber, so it is a fine art to get the wood dried correctly and efficiently.
*This article has been supplied by the American Hardwood Export Council (AHEC). For more information on American hardwoods and to learn more about different species and grades, please visit http://www.americanhardwood.org.
