KILN DRYING and WOOD MOISTURE CONTENT

When trees are harvested and used for logs for log homes, the water present in the tree at the time of harvest is collected in two distinct places in the individual cells. The liquid water, known as free water, is found in the central cavity, or lumen, of the wood cell. This free water can add a tremendous amount of weight to the stem of a tree but does little to the physical and mechanical properties of that tree. However, the water found within the thin walls of the wood cells, known as bound water, has a tremendous impact on the properties of the wood. It is responsible for the shrinking, swelling, mechanical, thermal, electrical, chemical and combustion properties of wood.

The measure of the amount of water in wood is known as its moisture content (MC). The MC is the ratio of the weight of the water compared to the dry weight of the wood, expressed as a percentage. For example, if a log sample weighed 10 pounds at the time of harvest and 6 pounds after being thoroughly dried in an oven, the weight of the water, 4 pounds, would be equal to 40 percent of the original weight of the log. The weight of water contained in a freshly cut tree is often greater than the weight of only the wood of the log. In these situations, the logs’ moisture content is said to be greater than 100 percent. While that seems a little odd or just plain wrong, remember that we are comparing the weight of the water in the log with the weight of only the dry wood of that log. Once the free water in the central cavities of the cells has left the wood, the bound water within the walls of the wood cells begins its migration to the surface and the surrounding air. As it leaves, the cell walls shrink and become stronger and stiffer. The point where a cell wall’s bound water begins its migration out of the log is known as the Fiber Saturation Point (FSP).

LOG STABILITY IS THE AIM OF DIFFERENT DRYING METHODS

Logs begin drying as soon as the tree is harvested and will continue to dry until its moisture content reaches equilibrium with the atmosphere in which it is stored. This equilibrium moisture content (EMC) ranges between 5 and 30 percent, depending on the temperature and relative humidity of the air surrounding the log. At 30 percent strength properties begin to increase and logs begin to shrink, subsequently checking as far as the log’s center.

In addition drying, if carried out promptly after the felling of trees, protects timber against primary decay, fungal stain and attack by certain kinds of insects. Organisms, which cause decay and stain, generally cannot thrive in timber with a moisture content below 20%. Several, though not all, insect pests can live only in green timber. Dried wood is less susceptible to decay than green wood (above 20% moisture content).

The technique of air drying consists mainly of systematically stacking logs/timbers in a configuration that allows free air flow over their surface. Rate of drying largely depends on climatic conditions and on the air movement created by wind. If air drying is done improperly (exposed to the sun), the rate of drying may be overly rapid in the dry summer months, causing cracking and splitting, and too slow during the cold winter months. This process requires a waiting period of up to two years. In air drying, moisture contents of less than 18% are difficult to attain.

Standing dead timber, as its name infers, are trees left standing on its stump in its original growing environment after the tree has died due to natural causes, such as fire, insects and/or disease. Since these trees are left standing, potentially for years, it allows the wood ample time to thoroughly air dry. Premium logs harvested from this source are quite stable and minimize settling in a log home. One must question however, how much of the outdoors (by way of bugs, carbon, disease, decay, mold and allergens) is brought into a log home using these logs.

In kiln drying, as in air drying, unsaturated air is used as the drying medium. However, the process of kiln drying introduces heat and blowers in a controlled environment to accelerate the removal of moisture from wood. The wood is stacked in large heat chambers, called a kiln. A direct process using natural gas and/or electricity or an indirect process using steam-heated heat exchangers can be used. Deliberate control of temperature, relative humidity and air circulation is necessary to achieve effective drying. Kiln drying provides a timely means of overcoming the limitations imposed by erratic weather conditions while limiting the amount of settling that will occur following construction. In addition, the temperatures employed in kiln drying typically kill all the fungi and insects in the wood.

INDUSTRY ALERT

It is important to understand that the terms “air dried” and “kiln dried” refer to a general method of drying logs prior to use in a log structure. They do not guarantee a specific level of dryness. Logs and cants can be placed in a kiln for what seems to be an adequate period of time, then be removed and machined at a moisture content above the fiber saturation point.

Logs dry from the outside to the inside. Large timbers and logs do not have uniform moisture contents throughout their cross section, since the surface of the log dries and regains moisture at a much faster rate than its center. A log may feel dry on the outside and seasoning checks may have appeared, but an impressive and misleading test might show the outer surface to be 12 percent moisture content while the core moisture may still be 30 percent or above. As time passes and the log dries, the difference between the surface and the center of the log moisture will diminish un-uniformly and result in checking, cracking, twisting and settlement, ultimately leading to increased maintenance and expense.

For this reason, standards have been established for determining the average moisture content of large timbers and round logs. The best method is to extract a core sample but this becomes a destructive test. A more common but less telling method is to place the probe of an electronic moisture meter at a minimum depth equal to 1/6 the diameter of a round log. For a 10-inch diameter round log, the mill will likely use a moisture meter with a probe that should reach approximately 2 inches into the lateral face of the log.

 

References:

Edwin J. Burke, Ph.D., Professor of Wood Science, University of Montana.
Wikipedia

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