FDM
WOOD EXPLORER
fdmonline.com
Gene Wengert
wooddoc@uwalumni.com
Lightweight, inexpensive, fine-grained overlooked hardwood:
Easter n
Cottonwood
There are several species of
cottonwood in the United
States, but the primary lumber
tree species is Populus deltoids, also
called eastern cottonwood. It grows
throughout the eastern half of the
United States, especially in wet sites.
In the West, the primary cottonwood
species is black cottonwood. However,
black cottonwood is not as desirable as
eastern cottonwood.
Many people consider eastern cottonwood a “trash species.” But when
FAS lumber prices went over $500 per
MBF the species was reconsidered.
Although the wood is light in
weight and low in many strength properties, it can be made to look like many
other species. In fact, I have heard that
more than 50 percent of the wooden
caskets made in the United States are
made with eastern cottonwood and
finished carefully to make a beautiful
wood product. This wood is not restricted to hidden locations, but makes
a wonderful show wood. ●
Processing suggestions and characteristics
Density: Eastern cottonwood averages
about 26 pounds per cubic foot at 7 percent
MC. This ranks as one of the lighter weight
hardwoods in North America, along with
aspen, basswood, butternut and alder.
Drying: Cottonwood dries quickly, but has
two major problems — certain regions of
the lumber have a tendency to warp and
collapse during drying. It is important for a
lumber buyer to specify that it be steamed
for collapse recovery.
Warp is caused because the tree has
a special type of wood cell called tension
wood. These cells are typically very weak and
also warp along the grain excessively. Close
sticker spacing and rapid drying help keep
this lumber as flat as possible.
Shrinkage in drying, not including collapse
shrinkage, is as much as 7 percent.
Low final MCs (no wetter than 7.0 percent
MC) are essential to enhance machining.
Gluing and machining: Eastern cottonwood
is easy to glue. The softness of cottonwood
means that the wood is quite forgiving when
glued. High absorptivity means that high glue
spread is required and there must be adequate
liquid in the glue mixture.
The inherent weakness of cottonwood
means that fuzzing is likely. Low MCs, sharp tools
and fresh sandpaper are the secrets to success.
Avoid excessive machine pressures; even hand
sanders can be indented into the wood.
Stability: Eastern cottonwood is subject to
modest size changes when the MC changes
— about 1 percent size change for each 3
percent MC change running across the rings
(tangentially) and about 1 percent size change
for each 7 percent MC change across the rings
(radially). Lengthwise shrinkage is as much as
3 percent in scattered, localized areas.
Strength: Cottonwood is fairly weak due to
its low density. The bending strength (MOR) is
8,500 psi (about 60 percent of oak). Stiffness
(MOE) is 1. 37 million psi and hardness
averages 430 pounds (compared to 1.82 and
1,290 for oak). The wood is noted for not
splitting when nailed close to the end.
Color and grain: Eastern cottonwood is a
light grayish white with occasionally a brownish
hue in color, with little difference between
heartwood and sapwood colors. The grain is
quite subdued and is finely textured.
