Red Oaks a diverse group
January 13, 2020 | 12:40 pm CST
Of all the species in the eastern U.S. hardwood forest, the red oaks (Quercus species) are the most plentiful. The name "red oak" is actually the name given to a grouping of 12 major oak species. All the red oak trees are characterized by points at the ends of the leaves and bitter acorns that mature in two years and germinate in the spring after dropping the previous fall.
Red oak lumber's widespread availability, fairly low price, distinct (heavy) grain, high strength and ease of finishing are certainly several of the most important properties making it currently desirable and popular for furniture and cabinet use.
We separate red oak into two groups: upland or lowland. As a rule of thumb, lowland oaks have annual growth rings spaced more than 1/4-inch apart; upland oaks have closer rings (slower growth). Upland, in general, processes easier than lowland. However, lowland dries more slowly.
Lowland also tends to check more easily and machines more poorly at times. It also tends to have more sapwood as well.
Processing suggestions and characteristics
Density. Red oak is one of the heavier hardwoods in North America, averaging about 43 pounds per cubic foot at 7 percent MC. Willow, cherrybark and scarlet oaks are somewhat heavier. Kiln-dried, planed lumber will weigh about 2.6 pounds per board foot.
Drying. Drying must be carefully controlled, because if the wood is dried too fast, it will check and honeycomb. Certainly, end coating the lumber is critical. Drying in an open shed or warehouse pre-drying are the best drying methods. If air-dried, piles should be roofed to prevent wetting of the top layers. Plastic burlap mesh fabric can be used to slow drying when needed in the warmer months. Air drying much more than 60 to 75 days increases the risk of drying damage due to rewetting. Most of the time, air-drying degrade will exceed 10 percent; hence air-drying is not recommended. Shrinkage in drying is 9 percent, with considerable variation between species.
Final moisture contents for furniture and cabinet oak must be between 6.0 to 7.0 percent MC. No MC variation is permitted due to oak's high shrinkage.
Gluing and machining. Oak is known to be "unforgiving" when gluing. Pieces must be square and straight; surfaces to be glued must be very flat and freshly prepared; and pressure must be uniform and not too low. Any commonly used wood adhesive will perform very well with oak, although casein will produce dark glue lines.
Because of its high density, the wood is prone to developing chipped or torn grain. Knives need to be sharp and rake angles a little smaller than for the lighter-weight species. As with most species, using dull tools and worn sandpaper results in fuzzing.
Stability. Red oaks are subject to large size changes when the MC changes about 1 percent size change for each 3 percent MC change (ranging from 2.6 to 3.5 percent MC change for the different species of red oak) running across the grain parallel to the rings (tangentially), and about 1 percent size change for each 6 percent MC change (range of 5.5 to 7.5 percent MC) across the rings (radially). This big difference between tangential and radial also means that the wood is prone to cupping when the MC changes.
Strength. Red oaks are very strong and stiff. Bending strength (MOR) averages 14,000 psi. Hardness averages 1,300 pounds. Stiffness (MOE) averages 1.8 million psi. Often the lowland oaks are stronger than the upland, especially when there are more than 10 growth rings per inch.
Color and grain. The color of red oak heartwood varies from very pink (cherrybark oak) to a reddish hue (northern red oak) to light brown (black oak). The white-colored sapwood amounts are minimal except in lowland oaks, where entire pieces of 4/4 can be sapwood. Lowland sapwood often is gray due to enzymatic oxidation gray stain. Oak has large vessels in the earlywood portion of the annual growth ring, giving the surface a coarse appearance and feel. The grain is quite obvious and is called heavy or coarse.
Have something to say? Share your thoughts with us in the comments below.