Sweetgum

Date: 2010-03-02

Gene Wengert, wooddoc@uwalumni.com

Cherry, mahogany, or walnut in disguise?

Sweetgum

Whenever we do a little reading about the way things were in our industry in the "old days," meaning the 1920s and 1930s, we find out that one of the major species back then was sweetgum (Liquidambar styraciflua). The name liquidambar is from the common name in Mexico (indirectly from Latin, meaning liquid and amber) in reference to the fragrant resin. The yellowish colored sap (technically called gum in a hardwood tree) is exuded from bark wounds and can actually be boiled down and concentrated into a salve that will reportedly cure skin problems and other ailments. The sap can also be used for incense or mixed with tobacco to form a sleeping potion. When burned, the smoke smells aromatic and supposedly has some curative effects as well. The bark and leaves are also known to have been used for centuries as a cure for many ailments.

Maybe you have seen those golf-ball size prickly fruits that fall from the sweetgum tree along with the large, star shaped leaf? Or perhaps you have seen the five-pointed, star-shaped leaf. The tree is common from Missouri to Connecticut and all states south of there, but lumber production is primarily in the southern states.

Even though plentiful, sweetgum lumber is not seen in most sawmills and therefore not seen in most furniture plants. Yet, it is one of my favorite species. I think that it is over-looked as a premium, moderately strong wood.

In the marketplace sweetgum is sold as either sap gum (which is the light colored sapwood) or red gum (which is the reddish-brown heartwood).

Incidentally, the Australian nursery rhyme popular with Girl Scouts and others in North America that begins "Kookaburra sits in the old gum tree, eating all the gumdrops he can see." is not referring to the sweetgum tree, but to one of the Australian eucalyptus trees. However, the expression "up a gum tree" which means "in great difficulty" or "in a sticky situation" is apparently American in origin. There is also an old musical song (circa 1824) that was also danced to entitled "Possum Up a Gum Tree" which was from South Carolina and referred to the sweetgum.

Processing suggestions and characteristics

Density.

Sweetgum has a density of approximately 33 pounds per cubic foot (very close to cherry and American elm). Sweetgum KD lumber weighs about 3 pounds per board foot at 7 percent MC.

Drying.

Drying is certainly the one "problem" that we have with sweetgum. The wood has interlocked grain, meaning that the grain is sometimes diving into the piece and the adjacent area has the grain coming out of the piece of lumber. As a result, the wood shrinks lengthwise, as well as every direction, during drying. That is, it warps. Further, flatsawn red gum, as well as flatsawn sap gum from smaller trees, has a tendency to cup. Keeping gum flat during drying requires 12-inch sticker spacing, rapid drying, avoiding rewetting conditions during drying, and avoiding over-drying.

Overall, gum shrinks more than cherry, mahogany, or walnut (8 percent shrinkage from green to 6 percent MC, compared to 6, 3, and 6 percent for the other three species, respectively).

Sap gum, like most sapwood, is subject to rapid discoloration by blue stain fungi and chemical gray stain in warm weather. Red gum will honeycomb if dried too quickly.

Gluing and Machining.

Sweetgum glues fairly easily; in fact, a little easier than cherry. However, because of the interlocked grain, any change in MC will likely affect surface flatness, thereby affecting glue bond strength. The best guideline is: Glue immediately (within an hour) after ripping to avoid moisture changes and subsequent warp. (This is also required for other interlocked grain species such as mahogany.)

The wood machines easily. Sanding requires fresh (sharp) sandpaper in order to smooth the fibers in the interlocked grain areas. This is also required for other interlocked grain species such as mahogany.

Stability.

As mentioned, gum does shrink and swell a little more than some hardwood species (1 percent size change for a 3 percent MC change), but not quite as much as oak. Further, the interlocked grain can cause some unusual shrinkage problems. Therefore, it is critical to make sure that the wood’s MC is exactly on target, with little variation permitted. I suggest 6.5 percent MC to 7.0 percent MC as the acceptable target for most users. Unfortunately, some people are not very careful about controlling the final MC, so it would be advisable to check the MC yourself.

Strength.

Gum is a little higher than average in strength and stiffness than many other hardwood species; it is quite close to cherry, a little stronger than mahogany, and a little weaker than walnut. The strength (MOR) at 12 percent MC is 12,500 psi, The stiffness (MOE) is 1.64 million psi, and the hardness is 850 pounds.

Color and Grain.

Sap gum can be finished so it looks like cherry, mahogany, or walnut.

Red gum has its own wonderful, busy character often with long streaks of various dark colors and interlocked grain. This interlocked grain creates a long ribbon like appearance, very similar to mahogany. In fact, quartersawn red gum is extremely striking in its appearance.

In spite of this exciting color and "million dollar" appearance, the truth is that today when gum is used, it seldom is used as "show wood" but rather is used in unexposed areas. The concern over drying has been blown out of proportion and has led to a negative connotation when sweetgum is mentioned.

Sweetgum

Wood Species

Red alder

Fuzz after sanding

Slow drying and darker colors

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