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    Drying Oak LumberBill Smith

    SUNY ESF Wood ProductsSyracuse, NY

    What is Oak?What is Oak?Why do we care?

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    Northern red oak leaves and acorns

    White oak leaves and acorns

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    The Red and White Oaksare typically the most b d t i fabundant species, from

    Eastern hardwood forests~ half of total:

    ~ 35% red oaks~ 15% white oaks

    Red oaks

    Northern red oak Northern red oak Southern red oak Scarlet oak Cherrybark oak

    W t k Water oak Pin oak Black oak

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    White oaks

    White oak White oak Swamp white oak Overcup oak Bur oak

    Ch t t k Chestnut oak Post oak Live oak

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    We use oak for consumer and commercial applications, as well as industrial:pp , Furniture Millwork Cabinets Flooringg Railroad Crossties Pallets

    Interior use applications must be:

    Kiln dried to 6 7% MC Kiln dried to 6-7% MC (North American market; European perhaps

    somewhat higher) Without surface, end or interior checking Flat Without residual drying stress Appropriate color

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    So, whats the big deal? We know how to dry wood.Wh t i l b t k?Whats so special about oak? Its an American Wood. Attractive. Strong.g Available. Traditional.

    So, whats the big deal? We know how to dry wood.Whats so special about oak? The red and white oaks are typically high

    density woods, which means high shrinkage coefficients.

    The oaks have unique anatomical structure. Customer desire and application can often pp

    requires thicker stock. This means slow, mild drying is required

    low temperature, small WBD, high EMC.

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    Additional issues with oak - The ring-porous nature means that earlywood cells

    l d it hil l t d ll hi hare low density while latewood cells are high density.

    White oak has tyloses in the large vessels which significantly reduces permeability.

    Red, and in particular white oak have large ray cells, which create zones of weakness whichcells, which create zones of weakness which contribute to checking.

    Fast end and surface drying causes checks.

    Even more additional issues with oak - Evaporated acids from drying are corrosive. Regional differences in drying

    characteristics and value. May have bacterial infection.

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    Red Oak lumber has high market value - (well, historically); 4/4 green kiln dried

    FAS/1F $800 $1200/MBF #1 $575 $875 #2 $500 $750

    8/4 FAS/1F $1050 $2100/MBFS/ $ $ / #1 $650 $1500 #2 $1000

    White Oak lumber has high market value - (well, historically); 4/4 green kiln dried

    FAS/1F $1400 $2000/MBF #1 $650 $1100 #2 $500 $800

    8/4 FAS/1F $2000 $3300/MBFS/ $ $ / #1 $750 $1600 #2 $1200

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    Wood Anatomy

    TangentialSections

    R ll iRed Oak

    Ray cell sizeTyloses in vessels

    White Oak

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    TransverseSections

    Red Oak Ray cell sizeTyloses in vessels

    White Oak

    Red Oak

    Scanning Electron MicrographCourtesy of the SUNY ESFN.C. Brown Center for Ultrastructure Studies

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    RedOak

    Scanning Electron MicrographCourtesy of the SUNY ESFN.C. Brown Center for Ultrastructure Studies

    Hard Maple

    Scanning Electron MicrographCourtesy of the SUNY ESFN.C. Brown Center for Ultrastructure Studies

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    White Pine

    Scanning Electron MicrographCourtesy of the SUNY ESFN.C. Brown Center for Ultrastructure Studies

    Red Pine

    Scanning Electron MicrographCourtesy of the SUNY ESFN.C. Brown Center for Ultrastructure Studies

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    Red Oak

    White Oak

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    Red Oak

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    Several rules about wood:

    Wet wood with free water present (above 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 % EMC % RH % 90 80 65

    EMC % 20 16 12

    50 30 0

    9 6 0

    RH and EMC sorption isotherm

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    Several rules about wood:

    Dense wood shrinks more because there is 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 y ;dries faster where permeability is greatest. Longitudinal vs. transverse Radial vs. Tangential

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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

    Red Oak 0.56 1.35 1.82

    *S.G.grn = dry weight (oven dry) / volume (green)

    Strength (bending // to grain)S.G.grn Green 12%MC

    psiWhite Pine 0.34 4,900 8,600

    Red Oak 0.56 8,300 14,300

    *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 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% -% g % ~ 1 % / 4% MC change

    Longitudinal shrinkage of wood is very small, 0.1%

    Except for juvenile wood and reaction wood Except for juvenile wood and reaction wood (compression and tension wood)

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    Longitudinal shrinkage of boards is very small.

    Except for 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 orientationorientation.

    This is the cause of twist, bow and crook warpage!

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    So, how do we dry oak?

    Kiln Dry Kiln Dry. Air Dry, then Kiln Dry. Predry, then Kiln Dry. Vacuum Kiln Dry.

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    And what schedules do we use?

    What kind of wood is it? What kind of wood is it? Red or white oak? 4/, 5/, 6/, 8/, 10, 12/4 ? Are there published schedules?

    Wh h d l b f d? Where can schedules be found? Do we trust these schedules?

    What else do we need to know?

    What is the application? What is the application? What final MC is desired?

    Interior use? 6-7% MC Exterior use? 12-15% MC

    What is the history green, air dried, PAD, kiln dried?dried?

    Bacterially infected? Pre-existing damage surface and end checks?

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    Kiln Drying Schedules -

    The purpose is to dry as rapidly as possible The purpose is to dry as rapidly as possible, to be economical.

    However, drying must be controlled to avoid value reducing defects.

    Typically, the temperature and the relative h idit ( d EMC) t ll dhumidity (wood EMC) are controlled.

    Temperature is raised and relative humidity lowered as wood dries.

    Kiln Drying Schedules -

    So we know what we want So, we know what we want. And we know what we need. Do we always get what we want? Really, you cant always get what you want.

    B t d t h t d! But, do we get what we need!

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    Kiln Drying Schedules -

    Once upon a time there was a little girl Once upon a time, there was a little girl named Goldilocks.

    "This porridge is too hot! "This porridge is too cold,

    "Ahhh thi id i j t i ht " "Ahhh, this porridge is just right,"

    Some typical schedules -

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    As wood dries,(below FSP, 30%MC),( , ),

    it shrinks.

    But remember, wood shrinks both as a material, and

    as a board.

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    i.e.- when a board is at 40% MC, the,shell is already

    below FSP, perhaps at 16% MCperhaps at 16% MC.

    If shrinkage is restrained,because the core is still wet,

    while theshell is dryingshell is drying,

    stresses develop.

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    So,as wood dries it shrinks,,

    as wood shrinks,

    stresses developstresses develop.

    Wood Shrinkage Varies withwith

    Orientation, Density, and

    Species.

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    Development of Drying Stresses,Early in Drying

    Development of Drying Stresses,Later in Drying

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    Moisture Gradient during Drying

    Moisture Gradient during Drying

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    Stress Gradient during Drying

    Stress Gradient during Drying

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    During Stress Relief, Moisture is added to the shell to cause it Moisture is added to the shell to cause it

    to try to swell against the core, relieving the stresses.

    Stress Relief Conditioning High EMC and Temperature High EMC and Temperature

    Adds moisture High temperature moves moisture faster High temperature means wood is not as strong

    so stress relieve more readily.

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    Some typical

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