Bill Smith SUNY ESF Syracuse€¦ · Fundamentals of Wood Anatomy: How Wood Dries Bill Smith SUNY...
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Fundamentals of Wood Anatomy:How Wood Dries
Bill SmithSUNY ESF Syracuse
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What is wood?
• Tree?• Log?• Boards?• Wood as a Material?• ???
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What is wood?
• Tree?• Log?• Boards?• Wood as a Material?• ALL OF THE ABOVE!
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Why do we dry wood?
• Remove moisture?• Cause controlled shrinkage?• Avoid mold and other biodeterioration?• Reduce weight?
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Why do we dry wood?
• Remove moisture?• Cause controlled shrinkage?• Avoid mold and other biodeterioration?• Reduce weight?• ALL OF THE ABOVE!
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What make wood special?
• It is an anisotropic material -– Its characteristics and properties differ with
orientation!• Strength, Stiffness, Shrinkage• Longitudinal, Tangential, Radial
• It is an hygroscopic material -– Dry wood has an affinity to adsorb and absorb
water!
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What make wood special?• It is an heterogeneous material -
– Different species– Hardwoods vs. Softwoods– Heartwood vs. Sapwood– Earlywood vs. Latewood
• (Springwood vs. Summerwood)
– Cell Type, Anatomy, Structure• fibers, tracheids, vessels, rays.
– Density, Porosity– Permeability– WOOD ANATOMY !!
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TransverseSections
Red Oak
White Oak
Ray cell sizeTyloses in vessels
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TangentialSections
Red Oak
White Oak
Ray cell sizeTyloses in vessels
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What make wood special?• It is an heterogeneous material -
– Different species– Hardwoods vs. Softwoods– Heartwood vs. Sapwood– Earlywood vs. Latewood
• (Springwood vs. Summerwood)
– Cell Type, Anatomy, Structure• fibers, tracheids, vessels, rays.
– Density, Porosity– Permeability– WOOD ANATOMY !!
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Several “rules” about wood:
• Dense wood shrinks more, because there is more wood material.– Latewood shrinks more than earlywood.
• Permeability varies with orientation; wood dries faster where permeability is greatest.– Longitudinal vs. transverse– Maple and oak shrink more than pine.– Radial vs. Tangential
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Red Oak
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Red Oak
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Hard Maple
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White Pine
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Red Pine
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Several “rules” about wood:
• Wet wood, with “free water” present (above fiber saturation, ~30%MC) is susceptible to mold, fungi and insect attack.
• Wood (as a material) shrinks as “bound water” evaporates.
• Lumber shrinks its wood (as a material) shrinks.
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Equilibrium Moisture Content and Relative Humidity
• RH %–90–80–65–50–30–0
• EMC %–20–16–12–9–6–0
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RH and EMC “sorption isotherm”
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Several “rules” about wood:
• Dense wood shrinks more, because there is more wood material.– Latewood shrinks more than earlywood.– Maple and oak shrink more than pine.
• Permeability varies with orientation; wood dries faster where permeability is greatest.– Longitudinal vs. transverse– Radial vs. Tangential
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Shrinkage - Green to 6%S.G.grn Tangential Radial
White Pine 0.34 4.9 1.7
Hard Maple 0.56 7.9 3.8
*S.G.grn = dry weight (oven dry) / volume (green)
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A good “general rule” about tangential shrinkage
• Softwoods, such as white pine shrink about 5 - 6 % from green to 0% -– ~ 1 % / 5.5% MC change
• Hardwoods, such as maple and oak shrink about 8 % from green to 0% -– ~ 1 % / 4% MC change
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Longitudinal shrinkage of wood is very small, 0.1%
• Except for juvenile wood and reaction wood (compression and tension wood)
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Longitudinal shrinkage of boards is very small.
• Except for -– boards with juvenile wood and reaction wood
(compression and tension wood)– boards with sloped grain (which means there is
a transverse component to the longitudinal orientation.
• This is the cause of twist, bow and crook warpage!
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Stiffness (bending // to grain)S.G.grn Green 12%MC
* 1,000,000 psiWhite Pine 0.34 0.99 1.25
Hard Maple 0.56 1.55 1.83
*S.G.grn = dry weight (oven dry) / volume (green)
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Strength (bending // to grain)S.G.grn Green 12%MC
* psiWhite Pine 0.34 4,900 8,600
Hard Maple 0.56 9,400 15,800
*S.G.grn = dry weight (oven dry) / volume (green)
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Electrical Resistance (maple)• MC %
– 25– 22– 19– 15– 12– 10– 8– 7
• MegOhms– 0.60– 1.23– 3.16– 16.6– 105– 690– 13,800– 72,400
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As wood dries it shrinks, as wood shrinks,
stresses develop.
If stresses exceed wood strength,failures occur.
Checks, cracks, splits, honeycomb!
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“Dry” wood, at the end of drying
“Green” wood, at the beginning of drying
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Wood is Good!
• Strong• Attractive• Useful• Economical• But, ………
– for almost every purpose and use, it must be dried properly!
–That is what we do.