ENCYCLOPEDIA OF WOOD
EXAMINING A WOOD SAMPLE
Three viewing perspectives
The lOx magnification provided by a mag - nifer or hand lens allows you to examine three views of wood’s structure, represented by the hardwood log section shown at right. The transverse section lies at right angles to the grain and is visible in the end grain of stock. The tangential and radial sections are at 90° to the transverse section. The tangential section follows a straight line that is tangent to the growth rings. This section is the surface you see on the face of plain-sawn lumber. A radial section is exposed by cutting a straight line from the bark through the pith, exposing grain lines that appear as vertical strips.
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Examining wood under a microscope
At lOOx magnification, a microscope uncovers more details of the cellular structure of wood than can be seen through a hand magnifier. At left are two views of white pine, illustrating key elements in species identification. The transverse section (far left) shows the size of the tracheid cells and the transition in their density from ear-lywood to latewood. Also evident is a longitudinal resin canal. The tangential section (near left) shows the number and thickness of the rays in the wood.
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PREPARING WOOD SAMPLES FOR VIEWING WITH A LENS
Cutting tangential and radial sections
For a tangential section, mark a cutting line tangent to the growth rings on the edge of the sample. Cut along the line with a band saw, making sure your hands are not in line with the blade of the tool (right). For a radial section, make an end-to-end cut through the sample at the high point of the growth rings with the piece face down on the band saw table. To clean up the cuts for viewing, lightly smooth the surfaces with a hand plane. Avoid using sandpaper, which will crush the fibers.
Preparing a transverse section
Slice off a sliver of wood from the end grain of your sample using a sharp knife or razor blade (left). The surface should be smooth and even. If the wood is particularly dense and difficult to cut, first soak the end grain for a short time in hot water.
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WOOD IDENTIFICATION METHODS
and can be revived by moistening a dry wood sample.
Checking a sample for hardness by running a fingernail along the grain and noting the degree of indentation can help differentiate similar species such as butternut and black walnut.
The standard tool for macroscopic viewing of wood is a lOx hand lens. Choose one with built-in illumination for sharp resolution. Examine samples in good light, holding the lens close to one eye and moving the surface to be studied into focus. Note the distribution and shape of features such as vessels, tracheids, resin canals, earlywood, latewood, pores and medullary rays. The rela
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Although identifying wood requires careful observation of the appropriate features of a sample, practice makes the job easier. First measure the width of the growth rings, and note the color and luster of the wood. Remember that wood exposed to sunlight and air changes color, so the hue of a freshly cut sample may be different after it has dried. Luster is not a common feature of many woods, but it can help distinguish between species that are otherwise alike in color, texture and weight. Although odor, like luster, is distinctive for only a few woods, it can be a useful key to identification, particularly among softwoods. Odor is most pronounced in freshly cut lumber,
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tive diameter of vessels (in hardwood) or tracheids (in softwood) is important in determining the texture of the wood; the larger these cells, the coarser the wood. The distribution of pores within the growth rings will also tell you whether a hardwood is ring-, diffuse-, semi-ring - or semi-diffuse-porous. When viewing end grain, choose an area of average growth rate, avoiding defects like cross grain and knots.
With softwoods, look for resin canals; they are only present in pine, spruce, larch and Douglas-fir.
If you are looking for rays—an important feature of hardwoods—they are best seen on a transverse or tangential surface.
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COMPARING MAGNIFIED VIEWS OF TWO WOOD SAMPLES
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The two photos above show what the end grain, or transverse sections, of two different hardwood samples would look like under the magnification of a hand lens. A ring-porous hardwood (above, left) features rows of relatively large pores in the earlywood and clusters of smaller pores in the latewood. The vertical bars interrupting the pores are medullary rays. A semi-ring-porous wood (above, right) shows little distinction between the earlywood and latewood. Here, the pores are evenly distributed throughout the tissue.
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