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MOISTURE CONTENT VARIABILITY IN LUMBER The heartwood-sapwood comparisons in the table above show that the moisture content in logs isn’t uniform; the moisture contents of the boards sawn from logs are nonuniform as a consequence. To further complicate things, different logs even from a single species will have different moisture content profiles, especially if they were cut at different locations or at different times. Obviously we can’t think about green lumber having a single moisture value – what should we expect the moisture content distribution of drying boards to look like? One way to describe the moisture distribution of a pack of lumber would be to calculate the Average MC of the boards in the pack. To do this accurately we’d need to measure the MC of each board, but that’s neither practical nor necessary. We can often get a pretty good estimate by sampling some of the boards in the pack; based on what you already know, you might expect that the heartwood boards are probably going to be a little different from the sapwood boards; there will be a distribution of MC values. Even a package that’s entirely heartwood or entirely sapwood will have a range of values, though. In a perfect world, there will be as many boards wetter than the average as there are boards that are drier than the average. Most of the boards will have a moisture content close to the average, but some are likely to be more different than you might expect. The individual board moisture contents will vary more for packs made up of boards from multiple logs, but for fairly uniform lumber from a single log it’s possible that the distribution will be a bell-shaped curve. Here are a few hypothetical examples of what the moisture distribution might look like for green lumber. I chose to illustrate moisture variability for hickory, and I plotted heartwood, sapwood and the combination separately in the following figures. Hickory isn’t commercially available as shagbark hickory or pignut hickory or bitternut hickory, etc., so I used the average moisture content data from Air Drying of Lumber6 for combined hickory species instead of numbers from Table 1. I simulated the moisture distributions using data variability that is typical of data observed at mills.7, 8 6 Forest Products Laboratory. 1999. Air Drying of Lumber. Gen. Tech. Rep. FPL–GTR–117. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 62 p. http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr117.pdf 7 Simpson, W.T. and X. Wang. 2001. Time-Based Kiln Drying Schedule for Sugar Maple for Structural Uses. Research Note FPL-RN -0279. Madison, WI: United States Department of Agriculture Forest Service Forest Products Laboratory. 4 pages. https://books.google.com/books?id=YZNYDegP_dsC&pg=PP3&lpg=PP3&dq=Time- based+kiln+drying+schedule+for+sugar+maple+for+structural+uses&source=bl&ots=AhO6WzUX68&sig=pwAmusR 6XhjNhJFaW2XW3nb1tZc&hl=en&sa=X&ved=0ahUKEwjrgfienc3KAhVE7CYKHWbhBnwQ6AEIIjAB#v=onepage&q&f= false 35PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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