There are several species of cottonwood in the U.S., but the primary lumber tree species is Populus deltoides, also called eastern cottonwood. (Some botanists like to separate the Western cottonwood into a different species called Plains cottonwood or P. sargentii. So, you might here this name, but most people refer to both as just one species, eastern cottonwood.) It grows throughout the eastern half of the U.S., especially in wet sites. In the Midwest, the tree is common in river bottoms and was widely used for fuel wood, especially in the summer when it would burn quickly, cooking the food, but not burning for hours and overheating the kitchen. It is the State Tree of Kansas and Nebraska. (In the West Coast, the primary cottonwood species is black cottonwood; in the Southwestern U.S., Fremont cottonwood. In many ways, these cottonwoods are not as desirable as eastern cottonwood, due to color, wetwood, strength and defects.) The cotton-like seeds that cover the ground, fill the air, and fly seemingly for miles are most certainly the origin of the name.
Although this wood was very important to native Americans and early European settlers, until recently in many people’s mind, eastern cottonwood was a trash species, with one price for grades No. 2 Common and higher. But when FAS lumber prices went over $500 per MBF a few years ago, it was clear that this so-called “trash species” had been getting second looks by our industry.
Although the wood is light weight and low in many strength properties, it can be made to look like many other species. In fact, I have heard that over 50% of the wooden caskets made in the U.S. are actually 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% 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- - the lumber has a tendency to collapse (especially thicker lumber) and warp during drying.
Collapse is when the hollow cells that make up wood fold in on themselves. (A similar event occurs if you try to suck up a very thick milk shake with a soda straw-the straw will collapse.) Collapse can be recovered (or removed) by aggressive steaming at the end of the kiln drying cycle. In fact, if collapse is not removed, the collapsed cells can recover when exposed to moisture–especially water from water-based finishes. It is important for a lumber buyer to specify that the lumber 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 one of the easiest woods to glue. The softness of cottonwood means that the wood is quite forgiving when gluing. High absorptivity means that high glue spread is required and there must be adequate (higher than for many species) 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. Glue sizing or a wash-coat can be used to stiffen fibers before final sanding in order to achieve a fuzz-free surface.
Avoid excessive machine pressures; even pressure from hand sanders can cause compression or indentations into the wood. If the wood is compressed during handling or is collapsed in drying, even though the surface initially appears smooth, exposure to high humidity or liquid water will result in abnormal, localized swelling in the compressed areas. The resulting surface can be quite uneven and unacceptable after finishing.
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% size change for each 7% MC change across the rings (radially). Lengthwise shrinkage is as much as 3 percent in scattered, localized areas.
Strength. Cottonwood is fairly weak, as expected due to its low density. The bending strength (MOR) is 8500 psi (about 60% of oak). Stiffness (MOE) is 1.37 million psi (about 75% of oak) and hardness averages 430 pounds (about 30% of oak). The wood is noted for NOT splitting when nailed or staples close to the end.
Color and Grain. Eastern cottonwood is a light white in color with a grayish tinge (with occasionally a brownish hue), with little difference between heartwood and sapwood colors. The grain is subdued and is fine textured.
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