Mobile Edition
Print
Get the Mag
Weekly UpdatesThe initial moisture content (MC) of green lumber is not uniform. As a result, some of the lumber in a load is dried more slowly than necessary, and some lumber may be over-dried, when the drying schedule is controlled by the MC of the wettest pieces. Sorting of green lumber before drying has been investigated as a way to improve lumber quality and reduce drying time and cost. Sorting can be based on initial MC, species, provenance, sapwood vs. heartwood, density, grade, dimensions, or some combination of these parameters. Richardson (2003) found that sorting softwood lumber can lower drying costs by minimizing drying degrade and shortening drying times for lumber with low green MC when green. Research on sorting hardwood lumber is limited in the literature (Gu et al. 2004).
Methods that have been studied for sorting by initial MC include visual sorting and technologies based on weight, infrared, lasers, near infrared spectroscopy, and microwaves (Kozlik and Hamlin 1972, Friesen 1989, Taylor and So 1990, Warren and Johnson 1997, Defo et al. 2007, Moschler et al. 2007, Taylor et al. 2007).
The drying of red oak lumber is time consuming, energy intensive, and expensive. Presorting of freshly cut red oak lumber by its MC could help increase drying rate and/or reduce degrade associated with the wide variation in initial MC. Red oak, however, is often air-dried prior to placement in the kiln. It is not known if the initial relative differences in MC among green boards in a stack are maintained after air-drying, i.e., whether the initially wetter pieces are still relatively wet after air-drying. The purpose of this study was to assess whether presorting green red oak lumber by MC is beneficial if the lumber is subsequently air-dried.
Materials and methods
Freshly cut red oak (Quercus spp.) lumber (nominal thickness 1 inch or 25 mm) was sawn in mid-May of 2008 from four 8-foot and 10-foot logs at a sawmill in eastern Tennessee. The boards were of random width and mixed grade and were untrimmed for length. Immediately after cutting, the boards were covered and shipped to the nearby lab for testing. At the lab, 150 randomly selected boards were cut to a length of 6 feet (1.8 m) by removing at least 1 foot from each end. Three-inch-long (75 mm) MC samples were cut from the cut-off sections adjacent to the 6-foot long samples. The dimensions and weight of each board sample and MC sample were recorded. The boards were end coated with a wax emulsion (Anchorseal from UC Coatings, Buffalo, New York). The sample boards were stacked outside separated by profiled red oak stacking sticks and supported by bolsters. To protect the sample lumber from direct exposure to sun and rain, protection boards were used. These boards were installed as the outer course on each layer and as the top layer of the pile. A tarp was also used to cover the pile.
The approximate MCs of the sample boards were calculated based on the average of the two MC samples. The MCs of the boards were monitored regularly during air-drying by weighing each board. The boards were air-dried for 2 months starting in mid-May 2008.
[FIGURE 1 OMITTED]
Results and discussion
Over the course of the 74 days, lumber MC declined from an average of 86 percent to an average of 15 percent. The quality of the dried air-dried lumber was good, i.e., there was very little checking or warp and honeycombing was not observed on boards that were subsequently cross-cut.
To determine if the initially wetter boards remain relatively high in MC throughout the air-drying study, the data of the initial MCs were split into two groups of 75 samples: "High" ([greater than or equal to] 82.6%, the median value) and "Low" (< 82.6%). Figure 1 shows the drying of the "Low" and "High" groups over time. While a statistically significant difference was maintained between the groups even after 2 months of air-drying (p-value < 0.01 from a t-test), there was considerable overlap in the MC distribution between the two groups after only 1 week. At the end of 1 month of air-drying, the separation of the average of the two groups was 6 percent (28% vs. 22%).
This is a large enough difference to place the groups in different steps in published kiln-drying schedules (Boone et al. 1993), but it is uncertain whether attaining this level of distinction would be worth the costs of sorting and maintaining separate inventories. Future work could examine the relative drying times and degrade losses in green-sorted and air-dried lumber that is subsequently kiln-dried.
Conclusion
These data suggest that sorting of green red oak lumber by MC will bring limited benefits in terms of lasting separation of MC levels if air-drying is employed prior to kiln-drying.
Acknowledgments
This research was supported by a grant from the Wood Education and Resource Center.
Literature cited
Boone, R.S., C.J. Kozlik, P.J. Bois, and E.M. Wengert. 1993. Dry Kiln Schedules for Commercial Woods. Forest Products Soc., Madison, WI. 158 pp.
Defo, M., A.M. Taylor, and B. Bond. 2007. Determination of moisture content and density of fresh-sawn red oak lumber by near infrared spectroscopy. Forest Prod. J. 57(5):68-72.
Friesen, D. 1989. Sorting by moisture content decreases drying costs. Forest Industries 116(7):18-19.
Gu, H., T.M. Young, W.W. Moschler, and B.H. Bond. 2004. Potential sources of variation that influence the final moisture content of kiln dried hardwood lumber. Forest Prod. J. 54(11):65 70.
Kozlik, C.J. and L.W. Hamlin. 1972. Reducing variability in final moisture content of kiln-dried western hemlock lumber. Forest Prod. J. 22(7):24-36.
Moschler, W.W., G.R. Hanson, T.M. Felix, S.M. Killough, and J.B. Wilgen. 2007. Microwave moisture measurement for wood drying. Forest Prod. J. 57(10):69-74.
Richardson, M. 2003. Sorting: The solution to consistent drying. NZ Forest Industries Mag. 34(10):42-46.
Taylor, A.M., T. Young, C. Steiner, and M.K. Jeong. 2007. Hardwood lumber manufacturing optimization using NIR spectroscopy, In: Proc. of the Inter. Scientific Conf. on Hardwood Processing, Sept. 24-26, 2007, Quebec City, Canada. pp. 157-163.
Taylor, F.W. and W.T. So. 1990. Sorting southern pine lumber to improve drying. Forest Prod. J. 40(4):32-36.
Warren, S. and G. Johnson. 1997. The economic benefits of sorting SPF lumber to be kiln-dried on the basis of initial moisture content. Forest Prod. J. 47(3):58-61.
Josef Muehlbacher
Adam M. Taylor *
Timothy Young *
The authors are, respectively, Undergraduate Student, Univ. of Applied Sci. Salzburg, Kuchl, Austria (jmuehlbacher.htw2005@ fh-salzburg.ac.at); and Assistant Professor and Research Associate Professor, Dept. of Forestry, Wildlife and Fisheries, Univ. of Tennessee, Knoxville, Tennessee (AdamTaylor@utk.edu, tmyoung1@ utk.edu). This paper was received for publication in December 2008. Article No. 10555.
* Forest Products Society Member.
FEED