Of all the species in the Eastern hardwood forest, the oaks (Quercus species) are the most plentiful. In North America there are more than 50 species of oak, with 20 of them being commercially important.
The high strength and beautiful grain have made oak one of the important species in the development of the United States. Oak's distinct (heavy) grain, high strength and ease of finishing make it desirable and popular for furniture and cabinets.
Telling the difference
Although the botanist or tree identification specialist will easily separate the oaks into many different species based on leaf patterns, acorn characteristics and bark appearance, when it comes to identifying and using the wood, we can make only four divisions. Oak lumber can be divided into either red oak or white oak.
White oak leaves lack the bristles or spines at the ends of the leaves, and the acorns are sweet, mature in one season and germinate in the fall when they drop. The red oak leaves are pointed at the ends of the lobes, and have bitter acorns that mature in two years and germinate in the spring after dropping the previous fall. Once the oak log is sawn into lumber, it is practically impossible to identify the exact species of the tree it came from we just can determine red or white oak.
Then, within the two major groups of red and white oak, we separate the wood into upland or lowland varieties. 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). These four groupings, red upland, red lowland, white upland and white lowland, will process differently; hence the usefulness of such separations. In the marketplace, lowland is often called "Southern" and upland is called "Appalachian" or "Northern." However, such a grouping is not always accurate from a processing standpoint. Check the ring spacing for the best separation.
Red and white oak
The separation between red and white oak can be made by examining the length of the rays; rays are cells that appear as short darker lines running lengthwise on a flatsawn surface. The rays of red oak are seldom over ½-inch long, while the rays of white oak are usually 1-1/2-inches long or longer. Microscopically, when viewed on a cross-section, the cells (called vessels) in red oak are open; in white oak, the vessels are usually (except chestnut white oak and a few other species) plugged. That is why we use white oak, but not red oak, for whiskey barrels!
When separation of red and white oak must be made without error, sodium nitrite (5 percent solution in tap water) can be used. The solution when put on oak lumber, wet or dry, cold or warm, will develop a very dark color in a short time with white oak, but will be light and will not darken with red oak.
Bacterial infections in oak
One of the more recent discoveries about oak is the presence of bacteria in the living tree that can affect subsequent processing. These bacteria are anaerobic bacteria that is, they grow in the absence of air (anaerobic = "no air"), so they will be more commonly found in wet sites rather than dry sites. Once in the tree, probably through a break in the roots caused by previous logging or grazing of cattle, the bacteria move slowly upward (perhaps only a couple of inches per year). It would be rare to find the bacteria more than 8 feet above the ground in the tree in other words, the bacteria will be confined to the lower section of the butt log. Even then, the entire cross section will typically not be infected.
As the bacteria live and grow in the tree, they secrete enzymes that slowly destroy part of the wood, making the wood weaker. Weaker wood means that when the wind blows, these infected trees can develop wind shake (also called shake). This is how most sawmillers identify this infection! Further, this weakened wood will likely develop checks, splits and honeycomb during drying. When processing the wood after drying, machining and finishing problems are likely to occur, as well.
In addition, the bacteria create chemicals that have a characteristic unpleasant, rancid odor. This odor is most noticeable when the wood is green. Such an odor when sawing is a key to identification. However, if dry wood is subjected to humid conditions, then the odor can return.
It also seems that the chemicals produced by the bacteria will at times interfere with finishing the wood product, especially when lacquer is used. In short, this wood is not good!
Red and white oak dry differently, with white oak drying more slowly and having a tendency to develop more checking (small cracks) than red oak. When purchasing oaks, especially white oak, it is critical to determine if the moisture content is correct and that there are no surface or internal checks.
Once dried, the only two major differences, and these differences are very important, between red and white oak are the appearance (color tends to be darker with more brownish hues and without reddish hues in white oak and grain pattern is heavier with white oak) and density (white oak tends to be slightly heavier). In fact, the grain of white oak is often cited as a reason why it is not used. Some may think that white oak grain is not as pretty as red oak.
Other, less important differences are that red oak is slightly more porous, so finishing can be a bit trickier for red oak. The higher density of white oak is not enough to affect gluing, machining or strength, although some people do claim that white oak is bit harder to machine and sand. Of course, bacterially infected red or white oak does not machine well at all, as mentioned earlier.
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