North American elms are divided into two groups...hard (the subject of this article) and soft (American elm and slippery or red elm). Hard elms include Rock Elm (Ulmus thomasii), Winged Elm (Ulmus alata), Cedar Elm (Ulmus crassifolia), and September Elm (Ulmus serotina).
Hard elms are considerably harder (50 percent or more) than the soft elms; the group name actually makes sense! Unfortunately, all of the hard elms are subject to Dutch elm disease, so supplies of these elms are not as abundant as they once were; these trees are becoming extinct. The good news is that research has found some trees that seem to have a natural resistance. These trees, along with some controlled breeding producing some new varieties including some genetic contributions from Asian elms, seem to offer excellent hopes that the elm tree will be returning to our forests and cities in years to come.
Rock elm (also known as cork elm) is a hardwood tree that is a native to Canada and the northeastern United States. The highest quality lumber is found at mills in north-central Wisconsin, lower Michigan, and southeastern Ontario. Winged and September elms are Southern elm; Cedar elm is most common in Texas; and September elm is found mainly in. However, because the four hard elms are so similar, the lumber from all four is commonly lumped together and sold as rock elm. (In this article, rock elm data is given; the other three are quite similar.)
Many sailing ships, because of elm’s high strength, were made with rock elm timbers. Caskets have been made from elm for many centuries, both in North America and Europe. More recently, rock elm has been used for cheese boxes, vegetable boxes, furniture (especially Danish styles), and upholstery frames, among its many uses. Bending qualities are superb; more severe bending operations should consider using elm, because rejects are few, even though the lumber can be expensive and hard to obtain at times.
Processing suggestions and characteristics
Rock elm is a heavy wood, comparable to red oak, with a density at 6 percent MC of 42 pounds per cubic foot or a specific gravity of 0.65. A board foot, when freshly sawn, weighs about 5.7 pounds; when dried to 6 percent MC, a board foot weighs 3.5 pounds.
The wood dries easily, but warping is a serious risk. Stickers are usually spaced 12 inches apart. Extra weight on the tops of the piles helps to control warp. End coating is required to prevent end cracking. Overall shrinkage from green to 6 percent MC is 7.1 percent tangentially (the width in flatsawn lumber) and 3.9% radially (the thickness of flatsawn lumber).
Machining and gluing
As with all heavier weight woods, hard elm requires moderate care to develop good glue joints. That is, it is not too forgiving if everything is not perfect.
Hard elm machines fairly well for a dense wood. Tools and sandpaper must be sharp. Chip-out tear-out or grain tearing can result due to interlocked grain, especially if the wood is over-dried or tool set-up is not optimum. Turning quality is quite good.
Rock elm is quite strong. For dry wood, the ultimate strength (MOR) is 14,800 psi, stiffness (MOE) is 1.54 million psi and hardness is 1320 pounds. Comparative oak values are 14,300 psi, 1.82 million psi, and 1290 pounds.
Once dried, the wood is fairly stable when the RH changes; it takes a 7 percent MC change to result in 1 percent size change radially and 4 percent MC change tangentially.
Grain and color
The grain is quite erratic, twisting almost all the time; technically, this is called interlocked grain. This means that it is difficult to split the hard elms, because the grain is interlocked. This interlocked grain also means that impact resistance is high. On the other hand, it also means that the knives, when machining, are always going partly against the grain, which leads to chip-out. On the other hand, this grain pattern means that bending failures will be rare.
The wood itself is light brown to medium brown, with a reddish hue at times.
Have something to say? Share your thoughts with us in the comments below.