California black oak (Quercus kelloggii) is in the red oak group of species, but its source of supply (It grows primarily in northern California and southwest Oregon), physical properties (It is weaker than most red oaks, but machines better), appearance (It is not as red, but has tighter grain), and processing differences (Drying is difficult) merit a special discussion of this species.
The tree reaches maturity in 90 years or longer. At that age it is 50 to 100 feet high and 14 to 40 inches in diameter; the larger sizes are when the tree is growing on good sites. On poor sites, the tree is quite scrubby in appearance. The acorns were widely used as a food by Native Americans, often being dried and ground into flour. Unfortunately, many of the California black oak trees have died in recent years due to a fungal disease, commonly called “Sudden Oak Death.”
This species of red oak, also called Kellogg oak and western red oak, and in its growth area is called black oak, is tremendously underutilized. I believe that much of the reason for this underutilization is that too many people tried to use softwood drying equipment and techniques to dry this very refractory (which means prone to surface checking and cracking) wood. (California black oak is much more likely to check in drying than northern and Appalachian red oaks.) The drying results were disastrous. However, with proper “Southern red oak” drying procedures and conditions, this oak can be dried without much defect development.
Another reason for underutilization is the scarcity of NHLA trained graders in the area; the NHLA grading scheme is one of the marketing keys for any hardwood.
Finally, the typical mill supply of logs produces 38 percent No. 1 Common and Better lumber; this is marginally low and will require good markets for the lower grades in order for the sawmill to be profitable. In fact, some mills have developed proprietary grades for so-called “low grade” material; in reality, low grade is a valuable raw material for many manufacturers. Oregon State’s Wood Innovation Center can often provide technical assistance with processing this species.
The wood makes excellent lumber that is well suited for furniture, cabinets, and flooring. Developing special grading rules or adjusting the standard rules may prove to be rewarding.
Because there is not a well developed market, buyers must contact potential sawmills personally. Then try a few sample pieces of lumber that can be tested. Appreciate that most sawmills do not carry an inventory of this species, so may not be able to provide needed quantities year round. You may have to use lower grades than you are used to, or assist in locating users of lower grades if you only take the higher grade.
Processing and characteristics
This is a fairly heavy wood, but is slightly lighter than eastern red oak. The green specific gravity (SG) is 0.61; at 6 percent MC, the SG is 0.67. The weight, when dried to 6 percent MC, is 39 pounds per cubic foot or 3.3 pounds per board foot.
As mentioned, the wood dries slowly and with a high risk of checking and honeycomb. Slow shed-air-drying with plastic mesh curtains to avoid excessively rapid drying conditions is essential. Kiln drying green-from-the-saw is also acceptable quality-wise, but is too expensive time-wise. End coating is also required to prevent end cracking. Other proper drying practices are essential. Most suppliers will sell only KD stock, rather than green. It is critical to check incoming stock for preexisting checking and honeycomb before accepting a load.
Staining of the sapwood from fungi and enzymatic oxidation of the sugars is common, especially when drying is too slow. Iron stain (a reaction of iron and tannic acid) is also seen.
Shrinkage in drying of 9.7 percent is lower than for eastern red oaks.
Gluing and Machining
This wood machines well, better than eastern red oaks, although being higher in density, it does require sharp tools and proper machine set-up to avoid chip-out. Avoid drying the lumber under 5.5 percent MC.
This wood, as with all higher density species, glues with considerable difficulty. Surfaces must be freshly prepared and flat to achieve satisfactory joints with conventional adhesives.
Once dried, the wood will move substantially, although less than eastern red oaks, if there are large RH changes or if the MC is not matched to the environment’s EMC conditions. A typical final MC range is 6.0 to 7.5 percent, unless used in a humid location. It takes a 4 percent MC change to result in 1 percent size change tangentially and 7 percent MC change radially.
Due to its moderately high density, California black oak’s strength and stiffness is high, although slightly weaker than eastern red oaks. For dry wood, the ultimate strength (MOR) is 13,000 psi and hardness is 1100 pounds. Comparative red oak values are 14,300 psi and 1290 pounds.
Color and Grain
The heartwood is tan with a reddish hue, but it is not as red as many other red oaks. Because of California black oak’s slow growth, the grain is finer than most southern and many eastern red oaks. The grain pattern in lumber from open grown trees can be erratic (full of character). Also, pin knots may be common in such trees.
The grain is coarse, with the large vessels that are common in red oak giving a rough grainy feel and appearance. Stain penetration is variable at times, giving a blotchy appearance; this is due to tension wood, which is common in open-grown trees especially.