Kiln-dried Lumber — Light, Strong, and Profitable

As anyone who has blinked and waved their way past a smoky fire can attest, green wood contains a great deal of moisture. When a tree is alive, water enters the root system via osmosis. This moisture is the vessel through which minerals and nutrients move throughout the tree body, and eventually, around 90 percent of this water will be released by leaf stomata—the tiny pores that facilitate gas exchange between the organism and its environment.

Kiln-dried Lumber — Light, Strong, and Profitable

As soon as a tree is cut, it immediately begins to shed free water, the liquid captured in the pores and vessels of the wood. As it loses moisture, the wood contracts like a sponge. This is when dreaded warping, twisting, and cracking can occur, reducing the timber’s value as a building material of any kind. As such, properly managing the drying process is critical to producing usable timber for sale.

Air-drying is a fairly simple process in terms of tools and resources, but it requires know-how and patience. According to the USDA Forest Products Laboratory, air-drying 1” thick green boards to a 20% moisture content can take anywhere from 20 days to a full calendar year depending on the type of timber. Some woodworkers prefer air-dried timber because of its workability and rich color, but if not done properly, the wood can dry unevenly which results in an inconsistent final product. “If wood is not stabilized to its environment, and it was made into furniture,” cautions Mike Eichenberger, Senior Field Service Technician for Wood-Mizer, “the parts will become loose when the moisture lowers and warp as it gets higher.”

Each tree species has a different baseline moisture content. This is typically expressed as a percentage of the timber’s final dry weight. For instance, if a fresh-cut log weighs in at 100 kilograms then dries to 60 kilograms, the log contained 40 kilograms of moisture. The weight of the moisture (40 kilograms) divided by the log’s dry weight (60 kilograms) reveals its moisture content to be 67%. Robert McGuffy of the former Anderson-Tully Lumber Company explains this process, noting that some woods can run as high as 180% (Cottonwood), while others (White Ash) hold as little as 60%.

Due to this variance, most trees will pass through at least one kiln on their way to showroom floors. Kiln-drying works the same way as air-drying—the movement of warm, dry air across the wood’s surface draws out moisture—but the benefit of kiln-drying is it allows those curing the wood to control airflow, temperature and humidity, the variables that can make air-drying turn sour.

Eichenberger reminds his Wood-Mizer clients that their drying process should be guided by the humidity level of the environment the wood will ultimately be used in. “Inside a home where the comfortable humidity level is commonly 35 to 45%, the moisture content of the wood will be 6-8%,” Eichenberger says. As a general rule, high-density hardwoods dry more slowly, while low-density softwoods dry more quickly. Wood-Mizer kilns include species-specific schedules to simplify the process.

Reforest Teak uses a variety of kilns to produce their final product. A dehumidification kiln uses a heat pump system to pull moisture out of the air passing over the wood, and by extension, the wood itself. Dehumidification kilns recycle hot air, making them an energetically efficient choice. Vacuum kilns reduce the pressure within the kiln to such a degree that water can boil as low as 105°F. This allows water still held within the wood to easily turn to vapor and be rapidly pushed out of the wood. Wood-burning kilns take advantage of waste scraps from the milling process as a form of free, renewable energy. Igniting this readily available material provides the heat that will draw moisture out of the timber. As an added benefit, these kilns turn expensive waste into profitable energy.

Properly dried wood often sells for 30% more than green wood, weighs less, and is twice as strong as wet lumber, according to Wood-Mizer. When shipping timber to markets around the world, this means lower transport costs, increased profits, and more satisfied customers.

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