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Manufacturing lumber is a long process with several distinct stages. After the selected trees have been felled and transported to a mill, the process begins. In a series of cutting procedures, wherein logs are separated based on diameter and sectioned into lumber accordingly, wood is transformed from a tree trunk into a functional piece of lumber. However, the process doesn’t end with the cutting of wood—each piece of lumber must be sufficiently dried to allow the wood to achieve its final size before being sectioned into its final dimensions. The curing process allows the wood to release moisture, protects against decay, and helps prepare the wood to receive surface finishes.
Green Lumber and Moisture Content
When a tree is first cut down and the logs are sectioned into lumber, the resulting wood is considered “green” because it still has a considerable moisture content. However, not all trees retain the same amount of moisture, so before curing it’s important to determine what the standard moisture content is for a particular kind of wood. Moisture measurement can be done in one of two ways—either with a moisture meter or through a simple equation—but typically a moisture meter is the preferred method. Moisture meters operate by transmitting electricity between two probes inserted into the wood. The amount of electricity that flows through the wood from the first probe to the second is used to gauge how much moisture is present. The higher the moisture content, the greater the amount of electrical flow.
Green lumber can contain upwards of 130 percent moisture; cured lumber can have between 7 and 20 percent moisture. If the lumber is to be used in indoor applications, such as furniture, the moisture content should fall between 6 and 8 percent.
The Curing Process
The curing process can vary greatly in terms of time. Some people prefer do-it-yourself approaches, and either procure green lumber from a mill or produce the lumber themselves. Others prefer to purchase lumber that has already been cured.
For do-it-yourself enthusiasts, there are several ways to go about curing lumber. However, it’s important to realize that curing green lumber can take years if the curing practice isn’t expedited using a wood-kiln or an alternative method of drying. Air-drying lumber typically takes one year per inch of wood thickness.
The first step in curing green lumber in a home environment is identifying an appropriate location for the process. A garage or shed can work well as long as lumber is kept dry, otherwise it may reabsorb the moisture it is trying to release. Aside from being dry, the area should also have circulating air to help the drying process along. To avoid distortion, a few pieces of dunnage or stickers (small pieces of wood) can be placed between layers of lumber. The presence of wood in between lumber layers also enables air to circulate more freely. To gauge the drying process, use a moisture meter to track the moisture content. Let the wood sit for as many years as its thickness indicates, until the moisture content reaches the desired level. Even when curing is complete, keeping the wood in the same area will help each piece of lumber maintain a constant moisture content.
Another drying method, which speeds up the curing process considerably, involves using a wood kiln. Some mills may dry the lumber for you for an additional fee, although it may be easier to purchase lumber that is already cured if interested in using a kiln. Kilns heat the lumber so that the moisture is slowly released—wood can be left in the kiln from a few days to a few weeks, depending on the type of wood and how much moisture is present. Care must be taken not to heat the lumber too quickly, as this can cause uneven curing or create potential flaws, such as splitting.
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