Woodworker's Journal 2008-32-5, страница 40

Gross Features of Wood

The board of English yew (left) shows off its dark heartwood clearly against the sapwood.

The sapwood on the board of poplar (right) is indistinguishable from the heartwood. The edge with the bark on is called the waney or live edge. Don't be fooled by the color on the other edge. Poplar often displays strong colors in random places in the heartwood. We don't know why.

Sapwood and Heartwood

After five to 10 years of growth, the wood in the center of all trees undergoes a chemically complex change. In most trees, the transition from sapwood to heartwood is obvious because of the color change. In some, the pale color of the heartwood is hardly distinguishable from the pale color of the softwood. The chemicals that cause the change are difficult to identify and are known collectively as extractives.

Tyloses occur during this transition phase, although not in all species. They appear as glistening, fine film material that blocks the vessels. What happens is that a thin membrane that was once a part of the vessel wall collapses through tiny apertures called pits into the cavity of the vessel due to pressure differences in the tissue.

Growth Rings

The vessels of many tree species that grow in temperate regions where there is a distinct growing and resting period form a growth ring clearly seen on the transverse section. We calls these trees ring porous.

Some temperate region species, such as birch, poplar and sycamore, as well as most tropical hardwoods, such as mahogany, are diffuse porous. That is, the vessels appear in random fashion throughout the growth period.

Grain, Texture and Figure

Some confusion surrounds what is meant by grain, texture and figure when used to describe the wood surface. A good guide is that grain refers to the wood fibers relative to the length of the tree on the faces and edges of a piece of timber. Texture is the relative size and variations of the elements. Figure refers to the pattern on a board caused by the arrangement of the different elements and the nature of the grain.

Looking Closely to Understand

Studying wood with the naked eye is called gross inspection. The next level of viewing easily available to woodworkers is with a hand lens. Sold in a range of magnifications, the best for inspecting end grain is a 10-power (IOx) lens.

To make the various elements clearly visible, the end grain must be cleanly cut. I sharpened a new blade on a trim knife using an 8000 waterstone and confirmed the edge quality with the hand lens. It's not necessary to remove a large slice of tissue since the area you will look at is less than 1/4" x 1/4".

What is necessary is that the blade cuts without tearing.

The features of the wood are described on page 43.

Consider a I Ox lens a necessary member of any tool kit. Use it to inspect the cutting edges of plane blades, chisels, router bits and saw teeth, and check joint lines, surface tearout, dings and scratches.

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October 2008 Woodworker's Journal