drying oak lumber - esf · drying oak lumber bill smith suny esf wood products syracuse, ny what is...
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Drying Oak LumberBill Smith
SUNY ESF Wood Products
Syracuse, 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, what’s the big deal? We know how to dry wood.Wh t’ i l b t k?What’s so special about oak?
• It’s an American Wood.
• Attractive.
• Strong.g
• Available.
• Traditional.
So, what’s the big deal? We know how to dry wood.What’s 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 i
Red OakRay 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 can’t 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 the“shell” 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 schedules -
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Some typical schedules -
Some typical schedules -
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Some typical schedules -
Schedule Control -
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Schedule Control -
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General kiln drying precautions
Airflow considerationsAirflow considerations
– Ideally ~ 300 fpm
• Instrument calibration
• Width of load
• Effect of acidity on kiln equipment
Drying Times4/4 24 – 28 days8/4 60 days +- bacterially infected material, up to double the timeA d diffi lt t d / d f tAnd very difficult to dry w/o defects
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Some typical schedules -
Degrade Issues When Drying OakDegrade Issues When Drying Oak
- surface checking
- internal checking (honeycomb vs. bottleneck checks
- final MC variabilityy
- ring shake (bacterially infected)
- water stain (iron)
Drying characteristics, schedules and other useful information can be
found in several sources.• USDA FPL schedule book.
• Dry Kiln Operator’s Manual
• Drying Hardwoods
• Local knowledge.– Look to your purchasing agent and contacts in
the field.
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