Sawmilling Concepts - Part IV: Introduction to Drying Lumber

This is Part IV of our series about sawmilling concepts. The purpose of the series is twofold. It provides some useful tips to those who own a portable sawmill. It will also assist landowners, who may want to purchase a mill or hire a sawyer, gain a better understanding of proper sawing techniques.

Topics covered in the series include: sawing lumber, using the quarter scale, saw blade maintenance, introduction to drying lumber, determining moisture content of wood, wood drying defects, methods of drying lumber and dry kilns.

In Part IV, we examine the reasons for drying lumber.
What is lumber drying?

Lumber drying is the process of systematically removing excessive moisture from boards to a point where they will remain dimensionally stable.

Why is it necessary to dry lumber?

Logs (green wood) contain huge amounts of water when first cut. If wood is not properly dried it will shrink or swell. Lumber loses water by evaporation into air. It will eventually become stable when it reaches its final equilibrium moisture content or EMC. This depends on the surrounding environmental conditions, relative humidity and temperature.

Softwoods with more than 20% moisture content are considered green. Hardwoods are considered green until they reach the desired moisture content for the intended purpose of the wood. If left outside in most areas of North America, the minimum EMC that can be attained will be 11% to 12%. This is 4% to 5% higher than the average EMC needed for wood furniture or other interior woodwork.
Figure 1: This table shows the average green moisture content for the heartwood and sapwood by species.
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The average interior EMC in most of North America is between 6% and 8%. Therefore, it is necessary to dry the lumber to the end use moisture content conditions by artificial means (kiln drying) to keep the wood from shrinking after it is manufactured into a wood product.

Moisture in Wood and Wood Shrinkage

Free water - is the liquid water found in the interior of the wood cell. It makes up the majority of the moisture content of the wood when the tree is first cut into a log.

Bound water - is the water vapor molecules that are chemically bonded to the wood cell structure within the cell wall. When bound water begins to leave the cell wall the wood begins to shrink.

Fibre saturation point (fsp) - the point in the drying of wood when bound water first begins to leave the wood cell wall and the wood begins to shrink is called the fibre saturation point. The average fsp for most woods is when wood reaches approximately 28% moisture content (MC).

Desorption - refers to the loss of water vapor molecules at the hydrogen bonding sites. When this happens, wood shrinks.

Adsorption - refers to the taking on of water vapor molecules at the hydrogen bonding sites. When this happens, wood swells.

What effect does moisture have on wood?

When the moisture content of wood drops below the fibre saturation point, wood begins to shrink and will continue to shrink until the moisture content reaches 0%. If the conditions are right, the wood will also take on water vapor molecules and will swell back close to the original size (the size it was at fibre saturation point when it began to shrink). Once it reaches fibre saturation point, any additional moisture will not cause it to swell any more.
Figure 2:
Note the characteristic distortion and shrinkage of rectangular, round and square pieces as affected by orientation of growth rings. The tangential shrinkage is about twice as much as radial shrinkage
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Wood shrinks and swells differently in different directions. Quarter sawn boards (radial shrinkage) will shrink and swell less than plain sawn (tangential shrinkage) boards. You should be aware that shrinkage in the longitudinal direction is very little (from - 0.2% - or 0.002 times the longitudinal direction) and each different species of wood has different shrinkage values in different directions (tangential and radial).

For the most part, wood shrinkage from fsp (28% MC) to oven dry (0% MC) does not concern us. Changes in the size of wood product occur most often when interior equilibrium moisture content changes from summer to winter in most areas of the country. These changes are caused by drastic changes in temperature and relative humidity from summer to winter can range from 2% in Kansas to 7% in Maine. This causes doors that close nicely in winter to stick in summer. It can cause joints in wood to open up or crack.

Moisture Movement in Wood

In softwoods, as wood begins to dry, the liquid water begins to move from the cell lumen to the outside, by passing through the pits in the cell walls of softwoods. Also, water moves out the ends of the board through the cut longitudinal tracheids.

As the moisture content reaches fibre saturation point, movement of liquid molecules changes to movement of water vapor molecules that move by molecular action. This is a jumping action from one hydrogen-bonding (OH) site to another through the cell wall. They "jump" from one cell wall to another until they reach the outside environment. If a particular species has its pits aspirated (closed), then the liquid water in the cell lumen must first vaporize then move to the outside by molecular action also.

The hardwood structure is completely different than the softwood structure. Hardwoods have specialized cells called vessels or pores. Some hardwoods have huge pores while others have very small pores. These pores allow considerably easier movement of liquid moisture through hardwoods than softwoods. However, this doesn't mean hardwoods will kiln dry faster than softwoods. Most softwoods have considerably less water in them when they are first cut (green moisture content). Drying times are dependent on species' microstructure, green moisture content and pit aspiration changes.

How fast does wood dry?

How quickly moisture moves out of the wood is totally dependent on the wet-bulb depression. The wet-bulb depression is the difference between the dry-bulb temperature and the wet-bulb temperature. The dry-bulb temperature is the actual temperature measured with a typical thermometer.

The wet-bulb temperature is a little bit more difficult to understand. The thermometer is kept wet with a wick-like cover, from which water evaporates at a rate determined by the dryness of the air around it.

If the air is extremely humid (high relative humidity) the wet-bulb temperature will be close to the dry-bulb temperature. This creates a very small wet-bulb depression and very little moisture movement will occur from the wood to the surrounding air.

If the air is extremely dry (low relative humidity) the wet-bulb temperature will be considerably lower than the dry-bulb temperature. This creates a very high wet-bulb depression and a great deal of moisture movement will occur from the wood to the surrounding air.

If the wood is left in a constant environment (dry-bulb temperature and wet-bulb depression) for a reasonable length of time, the wood will attain a certain moisture content. Figure 3 provides the kiln operator with the information to determine that moisture content.
Figure 3: Relative humidity and equilibrium moisture content values occurring at various dry-bulb temperature and wet-bulb depressions.

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What happens if wood is not dried properly?

Before it is made into a final product, wood should be dried (as closely as possible) to the moisture content it will equalize to in the final environment (temperature and relative humidity) where it will be placed into service. For example, if the wood is to be made into a table in a house in Michigan at 7% EMC, the wood should be dried to 7% moisture content before the table is produced.
Module IV: Lumber Drying
If it is not properly dried, the wood will shrink or swell until it reaches its final EMC after the product is made. This may cause severe project deformation and possible product recall and loss of money.

Determining Moisture Content of Wood

The moisture content of wood is a ratio of the weight of the water in the wood, to the weight of the wood without any water in it. It is expressed as a percentage, and often the water in the wood can weigh more than the wood itself, making the moisture content greater than 100%.

Other key terms concerning weight are:

Specific gravity - the ratio of the weight of a material of a certain volume to the weight of an equal volume of water.

Density - the weight of a material per some unit volume, such as pounds per cubic foot, pounds per cubic yard, grams per cubic centimetre, etc.

Green weight - the weight of wood prior to drying process.

Oven Dry Weight (ODW) - the weight of wood after oven or microwave drying; all moisture has been driven out of the wood.

Moisture Meters

A variety of moisture meters are available to measure the moisture content of wood. The most common are those that measure either the electrical resistance between two points in the wood or those that measure the density of the wood with a magnetic field.

Electric Meters - require the penetration of two probes into the wood. They measure the electrical resistance between these two points. The problem is they can only measure moisture contents between 6% and fibre saturation point. They also leave two holes in the wood.

Density Meters - measure the density of the wood with the moisture in it. They require that the specific gravity of the wood be programmed into the meter to compensate for the differences in wood densities between species.

In the Next Edition

In Part V of this series, we will look at wood drying defects commonly seen in lumber operations.

Acknowledgement

The information (including illustrations) for this article is from the Instructional Guide to Wood Technology and Sawmilling, 2000. A publication of Wood-Mizer Canada Co. 217 Salem Road, Manilla, Ontario, K0M 2J0, 1-877-866-0667, <www.woodmizer.ca>.

This article appeared in the Spring/Summer 2005 (Volume 39) edition of the S&W Report the newsletter of the Ontario Woodlot Association.

(c) Ontario Woodlot Association

Visit the 'S&W Library' section of our website to read Parts I, II and IIl of Sawmilling Concepts.

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