Properties
The two most troubling characteristics of juvenile wood are that:
- it shrinks and swells along the grain as moisture content changes, and
- strength is lower, and in some cases much lower, than mature wood of the same tree.
Shrinking and Swelling
In the module on Formation and Chemistry of Wood, it was explained that microfibrils making up the thickest layer of the walls of woody cells (the S-2 layer) are oriented almost parallel to the long axis of the cells. This orientation, it turns out, influences or controls many of the properties of wood. Among the properties affected are shrinking and swelling with moisture content change.
As you will learn in the module on Wood Drying, wood shrinks and swells as water is lost or is taken up by the cell walls. When water molecules are removed from the cell wall structure, as in drying, the microfibrils in the S-2 layer of the cell wall move more closely together, causing the cell to shrink. Conversely, when the cell wall gains moisture, water molecules move into the cell wall, causing the S-2 layer microfibrils to become further apart, thus resulting in swelling.
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Because the microfibrils in the S-2 layer are oriented essentially parallel to the long axis of the cell, shrinking and swelling is almost totally in cell diameter, corresponding to radial and tangential shrinking and swelliing.
(Adapted from: Krahmer, R. 1971. The Structure of Wood. Slide tape production of the School of Forestry, Oregon State University, Corvalis, Oregon.)
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In juvenile wood, and in a type of reaction wood called compression wood, the cellulose molecules in the thickest layer of the cell wall are oriented at a considerable angle from the along-the-grain direction. This results in dimensional change not only in tangential and radial directions, but also along the grain when moisture is lost or gained. Juvenile wood commonly shrinks as much as 1 percent along the grain as it dries, and may shrink as much as 6 percent in this direction!
(Adapted from: Espenas, L. and Graham, R.D. 1971. How Wood Dries. Slide tape production of the School of Forestry, Oregon State University, Corvalis, Oregon.)
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Shrinking or swelling along the grain can be a major problem, even when the percent shrinkage is small. Consider, for example, a 2 × 4 stud that is to be used in a wall cavity. In a stud about 8 feet (or essentially 100 inches) long, every 1 percent change in length represents about 1 inch. What would happen if green or undried studs were used in building a wall which then changed dimension by as much as a full inch as it dried? What if the change in dimension was 3 inches?
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Much more common than the example cited above is a situation in which only part of a piece of lumber's cross section is juvenile wood. In this case, one edge of the piece will tend to shrink in length much more than the other part. The result is bowing and crooking, and sometimes in spectacular fashion!
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Note that if the microfibril angle in the S-2 layer of the cell wall were perfectly parallel to the long axis of the cell, all of the shrinkage resulting from loss of water molecules would be tangential and radial, and there would be no shrinkage along the grain.
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Similarly, if all microfibrils in the cell wall were oriented perpendicular to the long axis of the cell, all of the shrinkage would be along the grain, with no radial or tangential shrinking or swelling.
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In a situation in which the microfibril is at an angle between perfectly parallel and perpendicular to the long axis of the cell, the result is development of longitudinal shrinking and swelling, with a corresponding decrease in tangential and radial shrinking and swelling. This situation describes juvenile wood. Note the relationship between tangential shrinking and longitudinal shrinking in juvenile wood. The same relationship exists with respect to radial and longitudinal shrinking. So, juvenile wood shrinks in length, but at the same time shrinks less in tangential and radial directions than does normal mature wood.
(Image courtesy: Bentsen, B.A.)
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