American elm (Ulmus americana) and slippery elm (U. rubra), two of six species of elms that are found in the North America, are known together as soft elms. Rock, winged, cedar and September elm are known as hard elms. The hard elms are 25 percent heavier, and correspondingly stronger and stiffer, than the soft elms. American elm is certainly know 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 getting 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 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 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 that machining requires special care.
Processing suggestions and characteristics
Density. The 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. The elms glue 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. The 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 the elms, with the hard elms being better than the soft elms.
Color and Grain. The 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.