How wood seasons and burns
I wrote this column five years ago, and, upon rereading it today, it is startling to see how “unusual” weather actually does repeat itself. All the observations I noted below can be said about this past winter too!
March weather is certainly living up to its stereotype of being extreme and highly unpredictable. So far this month, we have had measurable (needing to be shoveled) snow, single-digit evening temperatures, wind gusts of 45 mph and record-breaking warmth. All of this has occurred in less than 10 days. At my house, the temperature went up more than 60 degrees in 48 hours, only to drop down to well below freezing less than 24 hours after the record warmth.
A reader emailed me recently with questions about how wood seasons and what is going on chemically in the process. I love to get email from readers, and this allows me to offer a column with some technical information that is probably useless to most people, but interesting, I hope, to some.
Living trees have as much water in them as wood by weight. A freshly cut piece of oak or hickory or beech that weighs 10 pounds right after cutting, contains at least four or five pounds of water. Ash wood has the least amount of water, but still should be seasoned for best results. Most of the water in wood is contained in tissue called xylem vessels. Ninety-five percent or more of
the tree’s wood is comprised of these straw like xylem vessels. When wood is cut, the water evaporates from the severed vessels beginning with the cut surface and the drying works inward by diffusion.
The finer wood is cut and split, the more surface area is exposed and the faster the water evaporates. Regardless of how it is stored, it will never “season” or evaporate down to less than 20 percent moisture content outdoors, even if stored under cover. Fully seasoned wood still has 20 percent moisture content and it will eventually begin to rot if not burned or kiln dried. Storing
firewood under waterproof tarps for any length of time will cause it to rot and not dry properly. Firewood can be covered with a tarp to prevent snow from accumulating, but the wood should be exposed to wind and air movement on the sides to dry properly.
Kiln-dried wood is subjected to heat and dry air, which gets the moisture content down to about 4 percent. As wood dries or seasons, it shrinks and this causes the “checking” or cracking on the cut ends that is characteristic of seasoned wood. When buying firewood, look for these cracks before buying.
Since water does not burn, it must first be evaporated (boiled off) before the wood can burn. It takes about 1,000 British Thermal Units, or BTUs, to evaporate a pound of water, so that 10 pound piece of freshly cut hickory needs about 5 or 6 thousand BTUs just to evaporate the water inside it. A pound of fully dry wood contains about 8,000 BTUs, so even after evaporating the water, that
10-pound piece of hickory still contains about 40,000 BTUs.
Of course, the wood must be heated to 451 degrees F. before it will even combust, which can be difficult in a wood stove. This is why it is impor-
tant to have some really dry wood to get the fire going before trying to burn green wood. Kilndried wood, such as leftover two by fours or firing strips, are great for starting fires.
Wood workers will sometimes soak partially dry wood in antifreeze to replace the water with the ethylene glycol and prevent cracking. Lumber that
has been freshly sawed into boards often has the board ends waxed to prevent extreme cracking at the ends of the boards. I think the term “season” has more to do with the time required to air dry. Freshly cut firewood needs at least a “season” ( six months or so) to dry before it is burned, even after it has been cut and split.
Bob Beyfuss lives and gardens in Schoharie County. Send him an e-mail to rlb14@cornell.edu.