An excerpt from Drying Oak Lumber
by Eugene M. Wengert
Department of Forestry
University of Wisconsin
Part 3 (of 3) from Section 3:
Interaction of Wood and Water
When the drying of a green cell begins, the free water from a cell evaporates first. As the free water is removed, the MC of the wood is decreased from green to approximately 28% MC. At 28% MC in a cell, all the free water has been removed, but the cell wall is still fully saturated. The point at which all free water is gone, but no bound water has been removed is called the fiber saturation point (FSP). Note that the FSP of 28% MC refers to the MC of a cell and not to the average MC of a larger piece of wood. No shrinkage has occurred up to this point (that is, in drying a cell from green to FSP), unless there is collapse, which is rare in oak lumber. However, any drying below 28% MC results in removal of the bound water from the cell wall; with this removal, shrinkage occurs.
As drying continues below FSP, more and more water is removed until there is no appreciable amount of water left in the wall. This level is called 0% MC or, if done at 215o F (as detailed above), oven-dry. Shrinkage continues from FSP to 0% MC in a direct, linear proportion to the MC loss.
The cell wall always has an affinity for water. This characteristic, called hygroscopicity, means that dry wood will not stay dry if the wood is exposed to a higher RH. Therefore, if the cell wall has lost moisture and then is exposed to high relative humidities, the wall will absorb water until equilibrium between the air and the wall is obtained. So wood not only dries and shrinks when exposed to low humidities, but it also regains moisture and swells when exposed to higher humidities.
Temperature does not make the cell shrink or swell appreciably; the only factor of importance that causes shrinkage is moisture loss and the only factor that causes swelling is moisture gain. In turn, the only factor that causes the MC to change is the relative humidity (RH) of the environment. So, changes in RH cause shrinkage or swelling.
In general, the denser the wood, the more it will shrink and swell. As the amount of shrinkage is directly related to checking, the denser woods are harder to dry. Therefore, as mentioned in Section 2, density (or specific gravity) is a good predictor of drying behavior.
Wood shrinks the greatest amount in the tangential direction, as mentioned earlier. Radial shrinkage is about half of the tangential (Table 4).
Longitudinal shrinkage is usually negligible. However, there can be appreciable longitudinal shrinkage in wood in the juvenile core or in tension wood. This longitudinal shrinkage will cause bow and crook (or side bend), and may contribute to twist. As an example of why longitudinal shrinkage causes warp, consider a quartersawn piece of lumber with one edge having juvenile wood and the remainder of the piece having mature wood. The juvenile wood edge will shrink lengthwise, while the rest of the piece will shrink very little lengthwise. This difference results in crook toward the pith. The same scenario, but with the shrinkage differences being between faces rather than edges, is a major reason why lumber bows.
With the basic information on shrinkage presented in the preceding paragraphs, the shrinkage behavior, illustrated in Figure 6, should be expected as wood dries from green to 7% MC. The following explanations should help in understanding the illustrated behavior:
The round piece has become oval shaped. This is because there is more shrinkage tangentially than radially.
The square piece on the right has become diamond shaped. With the grain pattern resulting in the tangential direction being from corner to corner and with more tangential shrinkage than radial, the diamond shape results. The other square has become rectangular with the tangential dimension being slightly smaller than the radial direction as a result of the shrinkage difference.
The quartersawn lumber pieces have stayed flat, but have decreased size in width less, percentwise, than in thickness. Quartersawn lumber, when used where the MC will fluctuate, will hold paint, varnish, and other finishes better with less finish cracking than flatsawn lumber because a quartersawn surface will not move so much as a flatsawn surface would. Careful examination of the quartersawn piece with the pith shows that it is a little thicker in the center than at the edges. This results because thickness shrinkage near the pith is radial, while at the edge the piece shrinks tangentially in thickness.
The flatsawn lumber has cupped toward the bark. This is a natural tendency, because the bark face of the lumber is more tangential than the pith side. As a result the bark side will shrink more than the pith side. The difference in shrinkage will be greater if the lumber is sawn from an area closer to the pith. (Combine this information with the fact that logs are getting smaller, and the result is a greater tendency for today's lumber to cup than lumber had in the past. Further, because most lower grade lumber is from the central sections of the tree (that is, near the pith), there is a tendency for lower grade lumber to cup more than upper grade.)
The mixed grain lumber piece exhibits the same cupping tendency as flat sawn lumber, for the same reasons. However, the amount of cup is less for mixed grain lumber.
Several shrinkage problems are presented in Appendix C to assist in achieving a better understanding of shrinkage. Remember, probably 80 percent or more of the drying and processing problems of wood are shrinkage related (which in turn means MC related), so a good understanding of shrinkage is essential for a good understanding of lumber drying. The reader is encouraged to do the shrinkage exercises before preceding further.
Table 7. MC calculation procedure (for most calculators).
1. Clear machine, if necessary
2. Punch in green weight
3. Push divide key
4. Punch in oven-dry weight
5. Push minus key
6. Punch in 1
7. Push multiply key
8. Punch in 100
9. Push = and then read % MC
Note: For some calculators, it may be necessary to push = between steps 4 and 5, and between steps 6 and 7.
Table 8. Suggested species corrections for the pin-type electrical resistance moisture meter (13).
|Table 8. Suggested species corrections for the pin-type electrical resistance moisture meter (13).|
|Uncorrected Meter Reading, %|
|Note: A (+) value should be added to the meter reading; a (-) value subtracted.|
Figure 6. Typical shrinkage in wood. Differences between radial and tangential shrinkage result in warp.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison
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