Identifying shake
July 30, 2012 | 7:00 pm CDT

Q: What is ring shake? How do we detect it in our oak lumber? Does this shake cause a bad odor or other manufacturing problems?

A: Shake, also called wind shake and ring shake, is a separation or crack within the wood that runs parallel to the rings, rather than across the rings like a typical end crack or honeycomb.

About 20 years ago, an alert researcher named Jim Ward at the U.S. Forest Products Laboratory found that trees with this condition always had the same, anaerobic (no air) bacteria associated with the shake. The bacteria name is Clostridium, and it infects every tree species that has been studied, including wet-site species such as red oaks, white oaks, aspen, willow, cottonwood, hemlock, cypress, and white pine.

Once in the tree, the bacteria move up the stem at no more than 1-1/2 inches per year, so it is rare to see any bacterial damage except in the lower section of the butt log of an older tree. Incidentally, the bacteria doesn't infect the entire log, but seems to be in irregularly shaped columns. Much of the infected wood is in the central part of the tree, so the infection is more frequent in No. 2 and 3 Common hardwood lumber than in the upper grades.

Because the bacteria are anaerobic, we find that the moisture content of the living tree in the vicinity of the infection increases dramatically (over 100 percent MC on the oven-dry basis), with all the little air bubbles that are in green wood being replaced with water. In fact, with wood substance being 1.5 times heavier than water and with the loss of the air bubbles, badly infected logs will not float in water. So, many of the logs that are now being pulled up from the bottom of lakes and streams are probably bacterially infected.

There are several major problems with wood infected with these bacteria. First, these bacteria create an enzyme (enzymes are protein molecules) called pectinase, which can dissolve part of wood's structure. During a wind storm, it is often found that the tree cracks; hence, the common name of wind shake. Of course, the weakened wood is much more likely to end check, surface check, and honeycomb during drying. And the machining properties are terrible.

Another problem is that the bacteria create fatty acids similar to butter. When stored in the warm tree they will turn rancid and will smell obnoxiously bad.

Any adhesive that cures faster in more acidic conditions (UF for one) will be affected by these bacteria as the area around the infection becomes more acidic. Often the bacterially infected area will discolor in drying, frequently turning dark brown. If white lacquer finishes are used in the finished product, the bacterially infected areas will discolor the finish in a few weeks.

A special note: Logs stored in warm water ponds can develop a substantial infection within a month or so. It is not uncommon to find tropical veneers and plywood that have the characteristic odor and damage.

Certainly odor and ring shake are two very good identifiers of severe infections, but often the bacteria have done only a little damage so the shake and bad odor aren't obvious. In any case, bacterial infections are not a grade reducing defect in lumber!

Perhaps a visit to the sawmill providing lumber will indicate how many of their logs have shake on the ends, thereby predicting the extent of the problem at a particular supplier. High MCs at the beginning of drying are certainly a very good indicator. In the plant after drying, employees should be aware of the characteristics (odor, discoloration, shake) of the bacteria so that obvious pieces can be eliminated before causing any subsequent problems.

We can expect to see and smell more about this problem, especially as we use more lower grade lumber and harvest more wet site species.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.