Heating with Wood Part X: Seasoning Wood-It’s all cut and dried.

A cord of wood is a quiet winter comfort representing centuries worth of stored solar energy from ninety-one million miles away. Woodshed Nation releases that energy gradually knowing that whatever happens in the world outside, a home with a few cords won’t freeze for lack of fuel. It’s February, we’re gaining almost five minutes of daylight per day and wood put up last spring is pretty much seasoned, as it needed to be because along with sunlight trees store tons of water. In some species half the weight of green wood is water.

There are two kinds of water in green wood: free water and bound water. Remember in biology class when the teacher said the fundamental difference between plant and animal cells is that plant cells have walls? This is an actual application of that lesson. (Thanks Mr. Harmon, wherever you are. You tried.) Free water is stored in plant cavities—inside and between plant cells. When wood begins to dry, free water is first to go (it has weak molecular attraction to its surroundings). The wood becomes lighter but otherwise doesn’t have many changes you’d notice at the wood pile.

Once the free water is gone but while the bound water (with strong molecular attraction) is still there, wood is at the Fiber Saturation Point. When moisture goes below this point, the bound water in cell walls begins to exit. This is where wood begins checking, shrinking, warping and all those things that drive carpenters and woodworkers nuts. An interesting aspect here is that the Fiber Saturation Point of most wood species is about the same: 30 percent. The sweet spot for firewood is between 15 and 20 percent. A problem getting there is that below 30 percent the differential between wood and air is a two-way street. If it’s humid enough, moisture goes back into the wood. Humidity won’t drive the moisture content above the Fiber Saturation Point but it can get well over the 20 percent we want.

Ideal moisture content is debatable. Can wood be too dry? Some experts say ‘yes’, some say ‘no’. One article I read said ‘no’ but the author linked his reasoning to another article that said ‘yes’. Arguments against very dry wood, including kiln dried lumber, are that it burns too fast and too hot. Over firing can compromise or even ruin a woodstove. It’s inefficient because most of the heat goes up the stack. Arguments against wood with too much moisture are that it’s hard to light and keep going, creates too much smoke and it exhausts oily creosote which can stick to the chimney and potentially cause a fire. Mix very dry wood with the usual cord wood and everyone is happy.

Moisture content in wood is expressed as the percentage of water in wood to the weight of that wood once all the water is driven out of it. You can get a ballpark estimate with a moisture meter that sells for about thirty dollars. The most accurate measure of moisture content, one that commercial lumber buyers sometimes insist on before they accept a load of wood, is to weigh some of the wood, dry it completely then weigh it again. Here’s the formula: [(weight of wood – weight of oven dried wood)/weight of oven dried wood] x 100. Put another way, (weight of water in the wood/weight of oven dried wood) x 100. This is a good science experiment for kids. Say you pick out ten pounds worth of wood, you dry it in the oven until all the water is driven out of the wood, then it weighs six pounds. So, there were four pounds of water in that wood. (4 pounds/6 pounds) equals 0.67. Multiply that by 100 and you get a moisture content in the green wood of 67 percent. Once you’ve got the moisture content you can estimate how much water has to exit the wood before it gets to the desired moisture content.*

Wood begins to exhale water as soon as it’s cut with most of the moisture moving out end to end rather than through the sides. It dries faster when it’s warm but wind, even cold wind, is a bigger factor. Our long, dry cold spell in early January was great for the wood pile. In general, wood cut, split, stacked and loosely covered right after the tree was felled is going to be good firewood. Lars Mytting, author of Norwegian Wood points to a 1940’s Norwegian study that found the first weeks of seasoning were most important for long-term, quality. Wood that wasn’t treated well: logs left for weeks on wet ground, not cut and split immediately, left in high humidity right after it was cut, never preformed quite as well in the stove.

An old-time tradition is to harvest wood before Easter with the idea being that the sap hasn’t risen in the trees yet so there’s less moisture in the wood. It’s true that this practice probably started because old-time farmers didn’t have as much going on during that time of the year anyway, and that the extra sap in trees during summertime is negligible once you split the wood. “The best time to get wood is today,” and all that; but still, the earlier in the year you get wood the better. Here in Southeast Alaska April, May and June are typically the lowest humidity months. We can pretty well count on two weeks of warm, sunny, drying winds in April before tourist season starts. September, October, November are the highest humidity months. If you get wood in the fall that was cut in August the high fall humidity will inhibit drying.

Another practice is to cut deciduous trees but not limb them. Some tree crews cutting along the roadside leave alders this way and they’re worth going after. Leaves pull moisture out of the wood by capillary action. When the leaves turn brown you limb the trees, buck them up and split them.

Ways to tell if wood is seasoned

Cracks, or checks, are good. They’re caused by wood losing water and shrinking faster on the outside than on the inside. Rings on seasoned wood might be raised because rings are more dense and the wood may shrink between them. When you knock two pieces of seasoned wood together the sound should be a clack instead of a clunk. If you split a piece the middle should feel dry. Woods lose their distinctive smells and colors deepen. Spruce yellows. Alder goes tawny. The gathering light of February shines on outwardly quiet cords of seasoned wood that have undergone dramatic physical and chemical changes as they breathed hundreds of gallons water back into the atmosphere.

Note on tarps

Set them up so air can circulate under the tarp. When it’s not raining, pull the tarp off. It only takes a minute. Don’t tarp green, split wood with the tarp edges on the ground. If you do three things happen. 1) As soon as the sun shines the rest of the neighborhood starts drying out but water evaporating from your wood and the ground below it will have the inside of the tarp dripping with condensation 2) Mold will take hold. 3) A million slug pilgrimage inches towards your spa of optimal humidity. In a rainy year there’ll be one on every other piece. You stand there flicking them off your finger like boogers as you split the wood. As soon as the wood is stacked and starts drying slugs abandon ship within a day. At some point part of the tarp blows off and your wood gets wet. Say a bad word. Run out in your pajamas and re-cover it. The wood will be fine.