American elm (Ulmus americana) and slippery elm (U. rubra), two of six species of elms found in North America, are known together as soft elms. Rock, winged, cedar and September elm are known as hard elms. Hard elms are 25 percent heavier, and correspondingly stronger and stiffer.
American elm is known for the wonderful shade trees of years past. It would not be unusual to have these trees rapidly grow to 100 feet tall and spread out to shade a 60-foot radius in the cities. I can remember elm streets that appeared like dark tunnels when the elms on both sides of the street touched. Then came the Dutch elm disease, actually a fungus, that essentially destroyed all these beautiful city elms, as well as the forested American elms. Some fungal resistant trees were located and could be used for breeding. Time will tell.
Most hockey sticks are made of elm as the wood is very tough and has extensive interlocked grain. Interlocked grain means that the lengthwise grain from year to year goes different directions, basically intertwining. That makes the wood difficult to split, and it has high shock resistance.
When used above ground, the wood is resistant to decay even when permanently wet. In fact, hollowed trunks were widely used as water pipes during the medieval period in Europe.
The heavy ring pattern combined with interlocked grain results in a very bold appearance, which is why it finds widespread use in paneling. Elm is also excellent for bending without breaking or cracking. However, its interlocked grain also means that warp is likely when drying if drying isn’t perfect and that machining requires special care.
Processing suggestions and characteristics
Density: Soft elms weigh about 34 pounds per cubic foot, which is roughly 3 pounds per board foot, 1 inch thick.
Drying: Soft elms are moderately hard to dry; harder than hard maple, but slightly easier than oak. The mild drying schedules result in slightly more warp, so stacking must be perfect. Weights on the tops of piles are strongly encouraged, if flat lumber is desired or essential.
Gluing and machining: Elm wood glues without much difficulty, but not as easily as the softer species such as soft maple. The interlocked grain results in machining defects if the feed is too rapid, knives are dull, or knife angles are poor (that is, knife is too slender).
Stability: Soft elms change size by 1 percent if the moisture changes approximately 3 percent MC.
Strength: American elm has a strength (MOR) of 11,800 psi, while rock elm has a strength of 14,800. The bendability (MOE) is 1.4 million psi for American and 1.54 million psi for rock. Hardness is 830 pounds and 1,320 pounds for American and rock respectively. As mentioned, the toughness and shock resistance is very high for elms, with hard elms being better than the soft elms.
Color and grain: Sapwood is white, but may become darker due to fungal blue stain. The heartwood is light brown with tinges of red at times. The grain is quite heavy in appearance due to the obvious growth ring pattern and the interlocked grain.
This post appeared first on http://www.woodworkingnetwork.com