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 stronger and stiffer, than the soft elms.
American elm is certainly 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 a dark tunnel when the elms on both sides of the street touched. In the spring, the little propeller-like seeds would spin down. I remember the neighborhood kids splitting the seed open lengthwise about ½ inch and then sticking the propeller on our noses, so we had a nose longer than Pinnochio! I also remember parking under an elm tree and sappy drips all over the car. What a mess!
Then came the Dutch elm disease, actually a fungus, that essentially destroyed all these beautiful city elms, as well as the forested American elms as well. The good news is that some fungal-resistant trees seem to be located and could be used for breeding. Time will tell.
To hockey fans, elm is their favorite wood, as most hockey sticks are made of elm. Why use elm for these sticks? The wood is very tough and has extensive interlocked grain. Interlocked grain means that the lengthwise grain from year to year goes in different directions instead of being perfectly vertical in the tree, basically intertwining. As a result, it is very difficult to split the wood; a characteristic obviously desired for hockey sticks — the wood has high shock resistance. Likewise, interlocked grain means splitting elm for firewood can be nearly impossible.
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 (ring porous grain like oak) combined with interlocked grain results in a very bold appearance or character to the wood, which is why it finds widespread use in paneling. I am surprised that more furniture and cabinetry makers don’t use elm; it certainly has a nice appearance. The elms are also excellent bending species as they are quite easy to bend, due to the interlocked grain, without breaking or cracking. However, this interlocked grain also means that warp is likely when drying if drying isn’t perfect, and 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. Mild drying schedules result in slightly more warp, so stacking must be perfect. Weights on the tops of piles are strongly encouraged for flat lumber.
Gluing and Machining
Elms glue without much difficulty. Interlocked grain results in machining defects if the feed is too rapid, knives are dull, or angles are poor.
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. The bendability (MOE) is 1.4 million psi. Hardness is 830 pounds.
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