Dave Mather Reaction Wood

    Have you ever had a piece of Butternut that fuzzed or just wouldn't clean up, or a piece of Eastern White Pine that looked more like Southern Yellow Pine with its bold grain pattern? Chances are these boards contained abnormal wood called "Reaction Wood".
    There are two types of reaction wood and they usually are produced from leaning trees or trees with large extended limbs. In softwoods it is referred to as "compression wood" and in hardwoods as "tension wood". My trick for remembering which one is which is to remember that "conifers" starts with the letter "C" so they have Compression wood. It is interesting that the compression wood forms on the lower side of the pith on a leaning tree (see picture on reverse side) whereas tension wood of the hardwoods usually forms on the upper side of the pith.
    Reaction wood is much denser than normal wood with the Specific Gravity 30% - 40% greater in compression wood and 5% - 10% greater in tension wood. Longitudinal shrinkage is also greatly increased, 10 times more than normal for compression wood and 5 times for tension wood. With this abnormal shrinkage you get warping, especially when lumber is planed, ripped, or resawed.
    Ever had a piece of wood either continually pinch the blade of a table saw or, just the opposite, widely separate as it is being fed through? That's probably compression wood. A lot of your distortions in stud walls is caused by compression wood; and, like White Pine, even grained woods appear as uneven grained.
    Conversely, the uneven grained woods like Southern Yellow Pine have duller and more lifeless latewood and its contrasting grain is evened out. It is harder to drive a nail in compression wood, there is a greater chance of it splitting, and compression wood may take a stain differently than normal wood.
    Tension wood is harder to detect. Sometimes there may be contrasting colors such as the silvery appearance in Sugar Maple and Aspen, or the light, whitish streaks in Butternut, or the darker color of tension wood in Mahogany. The tough tension wood fibers are hard to cut cleanly and thus the fuzzy appearance. Even when the wood appears smooth after sanding, there still will be microscopic fuzziness, which will result in stain being absorbed irregularly, which results in a blotchy look.


© 2001 by David Mather. All rights reserved. No parts of this publication may be reproduced in any form or by any means without the written permission of the author.