Curing or Drying
If the wood is to be used in rough-sawn projects, construction
beams, planking or fencing, it is sometimes air-dried for
a month or two, or even used immediately. One example of a
good wood that must be used immediately is sycamore. It is
lightweight and works easily, but twists and curls unless
fastened down immediately.
Wood used for fine furniture, house trim and other projects
must first be dried. Most woods must be dried down to 6-percent
moisture. Commercial kilns are often used to kiln-dry woods.
But the simplest drying method, although the most time-consuming
method, is to air-dry wood. This is an ages-old, traditional
method. I have air-dried many board. All that's required is
a dry area with plenty of ventilation. It does, however, take
quite a bit of time. Soft woods, such as cedar or pine can
air-dry in a couple of years. The hardwoods, such as oak or
walnut may require from four to ten years, depending on the
thickness and species. Air-dried wood should be “conditioned”
for several months in your home or shop.
The first step is to coat all ends with paraffin or boiled
linseed oil. Then stack the wood on a perfectly flat surface
with “stickers,” or 1/2-by-2-inch flat pieces
of wood placed between the planks. This prevents the planks
from twisting or warping, but allows air to circulate and
gently dry the wood cells.
Small quantities of wood can also be cured and dried at home
with small kilns, including solar models. A number of small
kiln designs are available on the web from the Department
of Forest Products, Virginia Polytechnic Institute and State
University Extension. A good deal of this information is linked
through www.woodweb.com. Most of these kilns will dry woods
such as walnut or oak in two to three months.
A moisture meter is very useful for checking the stages of
the drying process.
The total amount of water in a given piece of wood is called
its moisture content (MC). Although we are accustomed to the
fact that 100% signifies the total amount of something, the
MC percent of wood can be greater than 100%. This occurs because
the water can weigh more than the wood, and the MC of wood
is usually based on the ratio of the weight of the water to
the weight of the wood after it has been dried (see Equation
Equation 1: % MC = weight of water in wood ÷ weight
of ovendry (OD) wood
The general range of moisture content for green (undried)
hardwood lumber can range between 45% and 150%. The standard
method of determining the relationship of water in wood is
weigh a wood sample before drying to obtain the combined
weight of the wood and water;
dry the wood sample in an oven at 103 ±2° Centigrade
(100°C = boiling point of water) for (approximately) 24
re-weigh the wood sample;
repeat steps 2 and 3 until the current weight equals the previous
weight (the wood sample is now ovendry (OD), sometimes referred
to as bone dry);
apply Equation 2 for determining % MC of the wood.
Equation 2: % MC = (weight of wood before drying - OD weight)
÷ OD weight
From the equation above, when the water weighs more than
the wood, the % MC will be greater than 100. The OD weight
is not a natural state for wood, and the sample must be weighed
immediately after being removed from the oven. Because wood
is a hygroscopic material (meaning that it readily takes up
and retains moisture), it is impossible to prevent moisture
from entering dry wood. As soon as the OD sample is exposed
to the air, it will start to take in moisture from the air.
Why Dry Wood?
Some important reasons to dry wood include:
Better usability. Wood shrinks as it loses moisture and swells
as it gains moisture. It should be dried to the % MC it will
have during use.
Reduced shipping costs. Dry wood weighs less (drying may reduce
its weight by one-half or more). It is more profitable to
transport wood than water.
Less likelihood of stain or decay during transit, storage,
Reduced susceptibility to insect damage.
Increased strength. As wood dries below 30% MC, most strength
Better “hold.” Nails, screws, and glue hold better
in seasoned wood.
Better finishing. Paints and finishes adhere better to seasoned
Better heat insulation. Dry wood is a better thermal insulator
than wet wood.
Better preservation. Dry wood must be used when treating with
most wood preservatives.
Added value. Drying the wood products before shipment adds
value to the product.
Wood products should be dried to a final MC about mid-range
of the expected MC of its surroundings. These can vary considerably
by product, geographic location, and the intended use of the
product (e.g., whether it will be used inside or outside).
Wood products used outside but protected from direct precipitation
will stabilize with the surrounding environment at about 12%
MC in the humid southern states, but may stabilize to as low
as 6% MC in the arid Southwest. Hardwood furniture, all paneling,
and other products used in heated buildings are estimated
to stabilize at about 8% MC. Wood products to be used inside
buildings that are only occasionally heated should be dried
to about 18% MC.
Problems in Drying Wood
There are some negative aspects to drying wood, including:
The great amount of energy that must be expended to drive
the water out of wood. As much as 80% of the total energy
requirement for a sawmill can be used in the drying operations.
The possibility of drying defects. As wood dries, it shrinks
in several dimensions. If wood is not correctly dried, the
dimensional changes will cause drying defects, including checks,
splits, warp, casehardening, and honeycomb. (These terms are
defined in the Glossary at the end of the publication.)
Some explanation of these two items is warranted because of
their importance in the wood drying process. Looking at the
relationships between water and wood can help explain how
Water and Wood
A commonly mistaken belief about lumber is that once dried
it is permanently seasoned in its final dimension. A dry piece
of wood will exchange water molecules with the surrounding
air according to the level of atmospheric relative humidity.
Loss or gain of moisture in wood products may cause such troublesome
results as shrinking or swelling, interference with paint
adhesion, and increased susceptibility to decay and stain.
Water is found in wood in three forms. Free water is found
in its liquid state in the cell cavities or lumens of wood.
Water vapor may also be present in the air within cell lumens.
Bound water is found as a part of the cell wall materials.
As wet wood dries, free water leaves the lumens before bound
water. Water can be removed from wood fairly easily up to
the point where wood reaches its fiber saturation point (FSP).
The FSP is defined as that MC where the cell wall is completely
saturated with (bound) water, but no liquid water is present
in the cell lumens.
Wood does not start to shrink until it has dried below its
FSP. FSP for most wood species falls in the range of 25 to
30% MC. It becomes increasingly hard to remove water from
wood after reaching the FSP. Remember, it is only after water
begins to leave the cell walls that the wood begins to shrink
and its strength begins to increase.
How Wood Dries
Wood will seek an equilibrium moisture content (EMC) in relation
to the relative humidity (RH) and temperature of its surroundings.
That is, as wood is dried below its FSP, the amount of moisture
leaving the wood will be determined by the relative humidity
of the atmosphere surrounding the wood. Table 1 shows the
EMC over a range of humidities. For wood to air dry, the moisture
content of the air must be less than that of the wood.
Lumber drying is usually accomplished by evaporating the moisture
from the surface of the wood. Wood dries “from the outside
in”; that is, the surface of the wood must be drier
than the interior if moisture is to be removed. Moisture will
move from an area of higher moisture content to an area of
lower moisture content within the wood. When the surface moisture
evaporates from the sides or ends, moisture moves from the
interior toward these locations. This process continues until
the wood reaches its EMC. At this point the moisture content
is equal throughout the piece of wood. Thicker lumber exposed
to the same drying conditions will take longer to reach its
EMC than thinner lumber.
Wood dries along the grain up to 15 times faster than across
the grain. Therefore, a board will dry at a faster rate from
its ends. However, because a board is usually many times longer
than it is thick, most of the moisture loss occurs across
the grain and out the surfaces of the piece. In other words,
the moisture travels across the grain at a slower rate, but
it has to cross a much shorter distance and, except near the
ends of the board, it dries more through the surfaces.
The rate at which lumber dries is controlled both by the
rate of evaporation from the surface and by the rate of movement
of the water within the piece. As long as the moisture can
move from the interior to the surface at a fast enough rate
to keep the surface moist, the drying rate will be increased
if the surface evaporation rate is increased. This can be
Increasing the air across the surface of the wood. As long
as the RH is low enough, the air will continue to dry all
exposed surfaces of the wood.
Increasing the temperature of the air surrounding the wood.
Warmer air holds more moisture; by increasing the temperature,
the moisture-carrying ability of the air is increased.
Reducing the RH of the air. Water evaporates faster into the
Lumber, usually dried in stacks called piles, should be properly
stacked for either air drying or kiln drying. Proper stacking
will take advantage of wood’s drying properties. The
lumber stack should be uniform in length. If different lengths
of lumber must be stacked together, the shorter pieces should
be placed above the longer pieces. This prevents longer lumber
from sticking out at the ends. Overhanging lumber is susceptible
to breakage and warping. Shorter lengths of lumber may also
be placed within the stack if both front and rear ends of
the stack are kept flush.
Stickers, small uniform-sized boards, allow spaces for air
to move across the lumber surfaces. They are used in stacks
to separate the lumber so that air can move through the stack
and to distribute the weight of the lumber vertically from
top to bottom. They should be placed an equal distance across
each layer of lumber and aligned on top of one another from
the bottom of the stack to the top.
If the spaces between the lumber are not equal, air will
flow more slowly through the larger spaces. Moisture on lumber
surfaces at those locations will evaporate at a slower rate,
and the lumber will dry more slowly. Stickers should be sufficiently
more wide than thick so that they are not accidentally placed
on edge between a layer of lumber.
There is no set sticker size, but the same size sticker should
be used throughout a lumber stack. One inch by 3/4 inch or
1 1/4 inches by 1 inch are practical sizes for stickers.
Stickers should be placed as far apart as possible to ensure
good air circulation. However, if stickers are placed too
far apart, the lumber will not be supported well enough. Poor
support while drying will cause the weight of the lumber in
the upper layers to sag or otherwise distort the lumber near
the bottom. Proper sticker distance is a function of the size
(especially thickness) of the lumber. Generally, a sticker
distance of about 24 to 36 inches should be sufficient for
almost any size lumber. It is important that the stickers
be placed at equal distances and straight across a layer and
that each layer have a sticker at both ends for support. Proper
sticker alignment allows air to circulate evenly across the
surfaces of the lumber and allows a more uniform drying rate
for each piece of lumber.
Commercial kiln operators need to consider a balance between
more air flow across the lumber (thicker stickers) and more
kiln capacity, that is, more layers (thinner stickers).
Air drying refers to stacking lumber and exposing it to the
outdoors. Certain controls can be used in this stage of drying
to make it more efficient. These include proper stacking,
orientation and layout of the stack, and covering the stack.
The first level of control is proper stacking. Figure 1 shows
a properly built stack of lumber for air drying. For this
example, concrete blocks are used as a foundation, but treated
timbers or used railroad ties could also be used. The stickers
are uniform in size. The stickers are aligned one on top of
the other and placed not more than 36" apart. The end
of each board rests on a sticker. On the topmost layer of
lumber, thicker and longer pieces are used to support a roof.
In this case, 4 inch by 6 inch timbers are used. This lumber
is 4 inches to 6 inches longer than the stickers.
A protective roof extends over the lumber stack by 2 to 3
inches on all sides. The roof protects the lumber from precipitation
and direct sun. The roof may be slanted for precipitation
to run off. Finally, some weight is needed to hold the roof
in place. The extra weight will also help keep the top layer
of lumber from warping as it dries. In this case additional
concrete blocks are placed on the roof for added weight.
Another control is the orientation and layout of the stack(s)
of lumber. Lumber stacked over a surface such as concrete
or asphalt where water cannot pool will dry faster than that
stacked over bare ground or ground covered with vegetation.
As an example, black asphalt can significantly increase the
rate of drying over that of vegetative-covered ground. A good
rule is never to stack lumber over vegetative-covered ground
since the bottom layer will always be exposed to air with
a higher MC.
Shorter and narrower stacks of lumber will increase the drying
rate. Stacking lumber away from buildings, trees, or other
objects that can block the wind will increase the drying rate.
Wind is not needed to force air through the stack of lumber.
Air circulation through the lumber can develop by natural
convection. Warm, dry air enters the sides and top of the
lumber stack. As the dry air moves over the lumber, it evaporates
the moisture from the surfaces. Through the process of evaporation,
the air becomes cooler, moister, and thus heavier. The heavier
air moves toward the bottom of the stack. If the prevailing
wind moves freely, the cool, moist air is blown away and replaced
with warmer, drier air. Therefore, increasing the height of
the foundation to allow more space under the pile will increase
the drying rate.
Drying lumber too fast can cause drying defects. The most
rapid drying will occur during the warmer, drier months. If
drying defects occur, several things mentioned above can be
reversed. The stack(s) of lumber can be built over concrete
or bare ground rather than asphalt to slow the drying rate.
Lumber stack(s) can be made larger (especially wider), or
the wind can be partially blocked. All of these will slow
the drying rate.
For lumber such as thick red oak that is difficult to dry
without causing seasoning checks, several additional steps
may be necessary. An end-coating material can be applied to
the ends of the lumber. End-coatings are commonly made of
a wax-base material that can be applied to the ends of boards
to retard the excessive drying rates from these points. Burlap
coverings can also be used to cover the ends of the lumber
or to cover the entire stack of lumber. It is a common practice
among commercial wood drying firms to use end-coatings, burlap,
and other materials to impede the drying rate for certain
species and sizes of lumber.
Final moisture content is determined by ambient air temperature,
relative humidity, and drying time. Air drying wood can bring
the MC down to a range of 20 to 30%. Depending on outside
conditions and lumber species and size, air drying may take
up to a year or more to obtain these moisture contents.
When large amounts of lumber are to be air-dried, pole-type
sheds can be used to achieve greater control over the drying
process. The sheds allow more control in that one or more
sides can be blocked off, thus slowing the drying process.
Drying sheds can be very simple in their construction. They
can become more complex by adding walls that can be raised
or lowered and by adding a number of fans. Fans are used to
accelerate the outside air through the building. Sides of
the shed can be blocked and fans installed at one end. The
other end of the shed is left open. Fans can be operated when
increased circulation is desired and shut off for decreased
circulation. For wood species that have a tendency to check
when drying too fast, such as oak, fans should run when the
exterior humidity is high and the air temperature is low.
Fans can be turned off when the humidity is low and the temperature
is high. This process slows the drying rate at the beginning
when some species are susceptible to checking.
After the wood has been initially dried, the fans can be
turned on when the temperature is high and the humidity is
low. When the humidity is high, the fans are turned off to
avoid reintroducing moisture into the lumber. Because no heat
is added with this type of drying (sometimes referred to as
fan pre-dryers), the final moisture content is determined
by the ambient temperatures and relative humidity. As in air
drying, the final MC range is usually 20 to 30%.
Drying wood in an insulated chamber and circulating air over
it is called kiln drying. For most end uses of wood, all of
the free water and much of the bound water should be removed.
To accomplish this in a shorter period of time, or in more
humid environments, a dry kiln must be used to dry the wood.
Almost all commercially produced lumber is dried in a kiln
before it is finally put in use.
Low-Temperature Dry Kilns
Commercial wood drying operations sometimes use a pre-dryer
to dry green wood to a MC of around 25% before drying the
wood to a lower moisture content in a dry kiln. Pre-dryers
are usually referred to as a type of low temperature kiln.
Temperatures typically range from 75 to 100°F, and relative
humidities typically range from 60 to 90%. Pre-dryers have
been used for more than 25 years in the northern latitudes
of the United States where air drying conditions are unfavorable.
More recently, pre-dryers have become established in other
areas to shorten the air drying times of some hardwoods. Pre-dryers
have controlled ventilation to regulate the drying rate. Other
advantages of pre-drying over air drying are:
more uniform MC throughout the wood,
reduction in drying defects, and
one-third or more reduction in drying times.
Unless large amounts of lumber are to be dried, building,
energy, and maintenance costs can make air drying a preference
over a pre-dryer.
Dehumidification Dry Kiln
Dehumidifiers can be viewed as a type of low temperature
wood dryer although temperatures can reach as high as 160°F.
Dry kilns that operate at these temperatures are capable of
drying most wood species at maximum drying rates. Dehumidification
kilns can dry wood to a low MC of 5 or 6%. Dehumidification
kilns operate in the following manner:
humidity (moisture in the kiln) is removed by condensation
on the cold coils of a heat pump dehumidifier;
liquid Freon® is evaporated in the coils and then cools;
water is condensed from the moist air drawn across these evaporation
the evaporated Freon® gas is compressed and the pressurized
gas attains temperatures as high as 245°F;
dehumidified air is passed over the hot coils to provide useful
energy for drying the lumber.
Vents are not needed in dehumidification kilns, as they are
in steam kilns (see below). Vents can be used as an extra
control, especially to help control temperatures in the drying
Solar Dry Kiln
In Kentucky, solar dry kilns offer a relatively inexpensive
way for the woodworker or hobbyist to dry small quantities
of wood. Drying times depend on the weather, and electricity
is needed to run kiln fans. The heat energy necessary for
drying comes from a solar collector. Depending on the chosen
design, moist air can be removed through vents or condensed
on the cold solar collector at night. Solar drying can result
in high quality lumber, primarily because the moisture gradients
in the lumber are allowed to equalize at night when drying
is not taking place. Drying times vary and are relatively
In the United States, solar drying is not a commercially
viable option due to the relatively long drying times. However,
the United States Division of Agriculture Forest Service and
others are conducting research in solar drying for Third World
countries located in the tropics. Solar drying may be an inexpensive
viable option for these countries to dry their woods before
exporting them, and thus, add value to their economies.
Elevated Temperature Kiln
Steam Kiln Drying
In a steam dry kiln, fans are used to circulate air at speeds
as high as 400 feet per minute (fpm). Drying temperatures
can reach 180°F. Heat is supplied from an oil, gas, or
wood waste-fired boiler.
Although drying the wood products before shipment adds value
to the product and lowers transportation costs, it can also
be one of the most expensive operations in terms of energy
used. The ideal situation is for a wood products mill to use
its own wood waste to fire a boiler for kiln operations, thus
reducing fuel costs.
Temperature and humidity are carefully controlled during
the drying cycle using drying schedules designed for the species,
size, and condition of the wood. It is beyond the scope of
this publication to discuss individual species and drying
schedules. A good source of information for anyone wanting
more detail can be found in the Dry Kiln Operator’s
Manual, available through the Superintendent of Documents,
Heated air is circulated over the wood, and the water on
the wood surface evaporates, raising the humidity of the air.
When the humidity of the air exceeds the level specified by
the drying schedule, the warm, moist air is vented to the
outside, and cool, drier air is brought in. Each time moist
air is vented, all the energy from the boiler is also lost.
The venting and reheating of the exchanged air consumes up
to 80% of the energy required to dry lumber.
An example can illustrate why so much energy is required.
A large kiln can hold more than 100,000 board feet of lumber.
If the “wet” lumber weighs 4,700 pounds per thousand
board feet (lb/mbf), and the dry lumber weighs 2,300 lb/mbf,
then the water removed weighs 2,400 lb for every mbf of lumber.
When drying 100,000 board feet of this lumber, the water removed
weighs 240,000 lb. A gallon of water weighs about 8.3 lb.
Almost 30,000 gallons of water had to be removed during the
drying of this one load of lumber.
As stated earlier, free water is removed from wood until
the FSP is reached. After reaching FSP, the bound water starts
to move to the surface of the wood. When this occurs, the
wood cells start to deform, and the wood begins to shrink.
The surface shrinks faster than the core, causing stresses
in the wood. In addition, shrinkage occurs at different rates
with regard to orientation of the grain. The difference in
shrinkage can result in bow, crook, cup, or twist (see the
Glossary for definitions of these terms).
When stresses are severe enough that checks occur on the
wood surface, commercial operators stress relieve the lumber.
This is typically done by rewetting the surface with wet steam
for hardwoods such as oaks. In the case of faster drying hardwoods
and most softwoods, water is used. In either case, the lumber
surface will swell slightly, relieving the stress. For some
end uses, such as construction lumber where appearances are
not important, surface checks are not a problem. Sometimes
kiln-dried lumber can absorb enough moisture when stored in
a warehouse to remove stress.
High-temperature Dry Kilns
High-temperature dry kilns operate at temperatures of 200
to 240°F. Air velocities usually exceed 800 fpm. Vents
are usually kept closed since control of the relative humidity
is not essential.
This type of kiln was developed to dry softwoods. Commercial
high-temperature kilns can dry large quantities of lumber
in one day. However, only a few species of easily dried hardwoods
can be dried in this fashion.
Small dehumidification and solar dry kilns are becoming more
popular with home woodworkers. Wood hobbyists can find plans
to build small dry kilns in their favorite trade journal.
The Tennessee Valley Authority has developed a small dry kiln
that gets its heat source from a wood-burning stove (Figure
2). The kiln will hold 2,200 board feet of 1" thick by
12' long lumber. Plans and information are available from
TVA, Norris, Tennessee.
Several state universities and the USDA Forest Service Forest
Products Laboratory have conducted research using small solar
dry kilns. Results of the research and plans for the kilns
are available from these sources. Companies, such as Wood-Mizer,
sell small, relatively inexpensive dry kilns.
If you need further information regarding the drying of lumber
or other wood products, contact your county agriculture Extension
agent or a forestry or agricultural engineering specialist
at the University of Kentucky.
Board foot - A unit of measurement for lumber and sawlogs
represented by a board 12 inches long, 12 inches wide, and
1 inch thick or the cubic equivalent. In the wood products
industry, the working unit is “1,000 board feet,”
Bound water - Water in wood that is associated with the cell
wall material. Wood does not shrink until after bound water
starts to leave the cells.
Bow - A form of warp, bow describes a deviation flatwise
from a straight line drawn from end to end of a board. If
the board is laid flat, its shape starts to form a U.
Casehardening - A condition of varying degrees of stress
set in wood such that the outer wood fibers are under compressive
stress and the inner fibers under tensile stress. These stresses
persist when the wood is uniformly dry and can cause warping
when the wood is resawn or machined.
Checks - Lengthwise separation of wood fibers that extends
across the annual growth rings. Commonly caused by stresses
during drying. Surface checks occur on flat faces of lumber
and end checks occur on the ends of lumber, logs, and other
Crook - A form of warp, crook describes a deviation edgewise
from a straight line drawn from end to end of a board. If
the board is laid on its edge, its shape starts to form a
Cup - A form of warp, cup describes a trough-like shape where
the board edges remain approximately parallel to each other.
Equilibrium moisture content (EMC) - The balance of moisture
content that wood attains at any given relative humidity and
temperature of the surrounding atmosphere.
Fiber saturation point (FSP) - The stage in the drying or
wetting of wood where the cell walls are saturated with (bound)
water and the cell cavities are free of (liquid) water. Fiber
saturation point for most wood species occurs at moisture
contents of about 25 to 30%.
Free water - Liquid water in the cell cavities of wood.
Honeycombing - Checks, often not visible on the surface,
that occur most often in the interior of the wood, usually
along the wood rays.
Hygroscopic - Readily taking up and retaining moisture. Wood
is a hygroscopic material. The forces between dry wood and
water are so great that it is impossible to prevent the gain
Kiln - A heated chamber for drying lumber, veneer, and other
wood products in which temperature and relative humidity are
Conventional-temperature - Type of kiln that typically operates
with temperatures in the range of 110 to 180°F.
Dehumidification - Type of kiln where the moisture is condensed
out of the air which is reheated rather than vented to the
Elevated temperature - Type of kiln that typically operates
with temperatures in the range of 110 to 211°F.
High-temperature - Type of kiln that typically operates with
temperatures above 212°F.
Low-temperature - Type of kiln that typically operates with
temperatures in the range of 85 to 120°F.
Vacuum - Type of kiln where lumber is dried at less than atmospheric
pressure either continuously or intermittently during the
Splits - Separation of wood fibers along the grain forming
a crack or fissure. Splits may extend partially or completely
through the wood.
Twist - A form of warp, twist describes a lengthwise “twisting”
of a board in which one corner twists out of the plane of
the other three.
Warp - Distortion in lumber and other wood products causing
departure from its original plane. Common forms of warp are
bow, crook, cup, and twist.
Planing and Finishing
Once the wood has been properly cured and dried, it must
be kept stored in a dry area and stacked perfectly flat. It
can then be planed to the correct width. Or you can create
a special molding for a very distinctive project or home interior.