wood 4

5/25/2014 Wood - Wikipedia, the free encyclopedia

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Wood knot cross section

A section of a Yew branch showing27 annual growth rings, pale sapwoodand dark heartwood, and pith (centerdark spot). The dark radial lines aresmall knots.

extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weakenit. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not seriousdefects.[6]

Knots do not necessarily influence the stiffness of structural timber, this will depend on the size and location.Stiffness and elastic strength are more dependent upon the sound wood than upon localized defects. The breakingstrength is very susceptible to defects. Sound knots do not weaken wood when subject to compression parallel tothe grain.

In some decorative applications, wood with knots may be desirable toadd visual interest. In applications where wood is painted, such asskirting boards, fascia boards, door frames and furniture, resins presentin the timber may continue to 'bleed' through to the surface of a knot formonths or even years after manufacture and show as a yellow orbrownish stain. A knot primer paint or solution, correctly applied duringpreparation, may do much to reduce this problem but it is difficult tocontrol completely, especially when using mass-produced kiln-driedtimber stocks.

Heartwood and sapwood

Heartwood (or duramen[7]) is wood that as a result of a naturallyoccurring chemical transformation has become more resistant to decay.Heartwood formation occurs spontaneously (it is a geneticallyprogrammed process). Once heartwood formation is complete, theheartwood is dead.[8] Some uncertainty still exists as to whetherheartwood is truly dead, as it can still chemically react to decayorganisms, but only once.[9]

Usually heartwood looks different; in that case it can be seen on a cross-section, usually following the growth rings in shape. Heartwood may (ormay not) be much darker than living wood. It may (or may not) besharply distinct from the sapwood. However, other processes, such asdecay, can discolor wood, even in woody plants that do not formheartwood, with a similar color difference, which may lead to confusion.

Sapwood (or alburnum[7]) is the younger, outermost wood; in thegrowing tree it is living wood,[8] and its principal functions are to conductwater from the roots to the leaves and to store up and give backaccording to the season the reserves prepared in the leaves. However,by the time they become competent to conduct water, all xylem tracheidsand vessels have lost their cytoplasm and the cells are thereforefunctionally dead. All wood in a tree is first formed as sapwood. Themore leaves a tree bears and the more vigorous its growth, the larger thevolume of sapwood required. Hence trees making rapid growth in theopen have thicker sapwood for their size than trees of the same species