8/2/2011
Drying Oak Lumber
Bill Smith
SUNY ESF Wood Products
Syracuse, NY
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 Oaks
are typically the most
abundant
b d t species,
i from
f
Eastern hardwood forests
~ half of total:
~ 35% red oaks
~ 15% white oaks
Red oaks
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Northern red oak
Southern red oak
Scarlet oak
Cherrybark oak
W t oakk
Water
Pin oak
Black oak
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White oaks
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White oak
Swamp white oak
Overcup oak
Bur oak
Ch t t oakk
Chestnut
Post oak
Live oak
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We use oak for
consumer and commercial
applications,
pp
, as well as industrial:
•
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Furniture
Millwork
Cabinets
Flooringg
Railroad Crossties
Pallets
Interior use applications must be:
• Kiln dried to 66-7%
7% MC
– (North American market; European perhaps
somewhat higher)
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•
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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’ so special
What’s
i l about
b t oak?
k?
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•
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•
•
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.
pp
can often
• Customer desire and application
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
are low
l density
d it while
hil latewood
l t
d cells
ll are high
hi h
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 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 –
– FAS/1F
– #1
– #2
green
kiln dried
$800
$575
$500
$1200/MBF
$875
$750
$
$1050
$650
$
$2100/MBF
/
$1500
$1000
• 8/4 –
– FAS/1F
S/
– #1
– #2
White Oak lumber has high market
value - (well, historically);
• 4/4 –
– FAS/1F
– #1
– #2
green
kiln dried
$1400
$650
$500
$2000/MBF
$1100
$800
$
$2000
$750
$
$3300/MBF
/
$1600
$1200
• 8/4 –
– FAS/1F
S/
– #1
– #2
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Wood Anatomy
Tangential
Sections
Red Oak
Ray cell
R
ll size
i
Tyloses in vessels
White Oak
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Transverse
Sections
Red Oak
Ray cell size
Tyloses in vessels
White Oak
Red Oak
Scanning Electron Micrograph
Courtesy of the SUNY ESF
N.C. Brown Center for
Ultrastructure Studies
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Red
Oak
Scanning Electron Micrograph
Courtesy of the SUNY ESF
N.C. Brown Center for
Ultrastructure Studies
Hard Maple
Scanning Electron Micrograph
Courtesy of the SUNY ESF
N.C. Brown Center for
Ultrastructure Studies
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White Pine
Scanning Electron Micrograph
Courtesy of the SUNY ESF
N.C. Brown Center for
Ultrastructure Studies
Red Pine
Scanning Electron Micrograph
Courtesy of the SUNY ESF
N.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
wood, with “free
free water
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
RH and EMC “sorption isotherm”
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Several “rules” about wood:
• Dense wood shrinks more
more, because there is
more wood material.
– Latewood shrinks more than earlywood.
– Maple and oak shrink more than pine.
• Permeabilityy varies with orientation;; wood
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
White Pine 0.34
*1,000,000 psi
0.99
1.25
Red Oak
1.35
0.56
1.82
*S.G.grn = dry weight (oven dry) / volume (green)
Strength (bending // to grain)
S.G.grn
Green
12%MC
psi
White Pine 0.34
4,900
8,600
Red Oak
8,300
14,300
0.56
*S.G.grn = dry weight (oven dry) / volume (green)
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A good “general rule” about
tangential shrinkage
• Softwoods
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
g
to 0%
%– ~ 1 % / 4% MC change
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.
orientation
• This is the cause of twist, bow and crook
warpage!
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So, how do we dry oak?
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Kiln Dry.
Dry
Air Dry, then Kiln Dry.
Predry, then Kiln Dry.
Vacuum Kiln Dry.
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And what schedules do we use?
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What kind of wood is it?
Red or white oak?
4/, 5/, 6/, 8/, 10, 12/4 ?
Are there published schedules?
Wh can schedules
Where
h d l be
b found?
f
d?
Do we trust these schedules?
What else do we need to know?
• 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?
• 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
possible,
to be economical.
• However, drying must be controlled to
avoid value reducing defects.
• Typically, the temperature and the relative
h idit (wood
humidity
(
d EMC) are controlled.
t ll d
• Temperature is raised and relative humidity
lowered as wood dries.
Kiln Drying Schedules •
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So, we know what we want.
So
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 do
But,
d we gett what
h t we need!
d!
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Kiln Drying Schedules • Once upon a time
time, there was a little girl
named Goldilocks.
• "This porridge is too hot!“
• "This porridge is too cold,“
• "Ahhh,
"Ahhh this
thi porridge
id is
i just
j t right,"
i ht "
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% MC
MC.
If shrinkage is “restrained”,
because the
“core” is still wet,
while the
“shell”
shell is drying,
drying
stresses develop.
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So,
as wood dries it shrinks,,
as wood shrinks,
stresses develop
develop.
Wood Shrinkage Varies
with
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”
shell to cause it
to try to swell against the core, relieving the
stresses.
Stress Relief Conditioning
• 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 considerations
– Ideally ~ 300 fpm
• Instrument calibration
• Width of load
• Effect of acidity on kiln equipment
Drying Times
4/4 24 – 28 days
8/4 60 days +
- bacterially infected material, up to double the time
A d very diffi
And
difficult
lt to
t dry
d w/o
/ defects
d f t
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Some typical schedules Degrade Issues When Drying Oak
- surface checking
- internal checking (honeycomb vs.
bottleneck checks
- final MC variability
y
- ring shake (bacterially infected)
- water stain (iron)
Drying characteristics, schedules and
other useful information can be
found in several sources.
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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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Thank you!
• Questions?
Q ti ?
• Comments?
– Bill Smith
– wbsmith@esf.edu
– 315/470-6832
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