||.||one ton||per ton|
Weight and Heat content figures are based on seasoned wood at 20% moisture content, and 85 cu ft of wood per cord. A "cord" of wood is defined as a stack 4 feet high, 4 feet thick and 8 feet long. (A cord has about 85 cu ft of wood and not 128, because of the air spaces between the pieces). "Face cords" are often sold. These are amounts of wood that are still 4 feet high and 8 feet long, but of a lesser depth than 4 feet. Commonly, wood for sale is cut to 16 inches long, and stacked as a face cord. This is 1/3 of an actual cord, and it is also called a "rank" or "rick" or "stove cord" or "fireplace cord".
For more technical information on the amount of heat in wood, and how it is measured and calculated, see Amount of Energy in Wood.
In general, softwoods light and burn easily and quickly with a hot fire which tends to make a lot of sparks.
Hardwoods are usually harder to start but burn more evenly and quite a bit longer.
Generally, the way this drying is accomplished is by "seasoning" it. Firewood is cut to length and then seasoned (dried) in a stack, with air being able to get to it, for at least 9 months before burning. The natural 60%-70% moisture content must be reduced to about 20% to burn well. The wood cells don't lose much moisture through the bark; the moisture is most effectively removed through the cut cells at the ends of each piece.
That's why logs which have lain in the woods for years may still have a lot of moisture and may not burn well (unless cut and dried.) We have heard of people cutting up these downed trees and immediately putting them in a woodburner! And the wood burns poorly! Now you know why!
OK! So, sometimes, it turns out to be NECESSARY to burn some green wood. Which species would be best under those conditions? It turns out that the desirability is NOT the same as for seasoned wood! While they are living, various species of trees have different moisture contents. If you suitably dry them all, that difference rather disappears. But, while still green, it becomes significant.
It is possible to correlate both the heat-content of the wood fibers and the green moisture content to form a table of desirability for those situations when green wood must be burned.
to dry weight
Excess moisture is that percentage above the desirable 20% seasoned moisture content.
However, think of the reality of the situation. Fifty percent of the weight of the piece of wood is now gone, evaporated as water vapor. When we actually look at the final piece of dried wood, we have no indication of all that moisture that used to be there! All we have left is wood fibers (which represents 4/5 of what we have left) and the remaining moisture (which represents the remaining 1/5 of what we have left). In practical terms, we could describe that 1/5 moisture in the piece as being 20% moisture content. Since this approach can be used with any piece of existing wood (without having to know its previous history), this is a common way used of describing the moisture content of wood.
Do you see the confusion? For our test piece, we could very correctly describe the moisture content of the dried piece as being either 10% or 20%, and either would be true. Unfortunately, some of the sources of the numerical data in the chart above did not indicate which of these two methods they used in deriving their results.
In general, we intended these charts to be of "comparative" usefulness, so a wood burner might have a general idea of which species might be better or worse. So, as long as you are not weighing all of your wood before putting it in your stove and doing rigid scientific studies, the information should be fine and you can ignore these technical comments.
If you ARE of a technical bent, there is actually yet another method that occasionally gets used. About 1980, a researcher decided to start referring to wood moisture in a piece of wood as being the percentage of the original moisture in the piece. This is a poor approach, but his reputation in the industry caused some people to adopt this system. His system would had looked at our example piece above and said that it started out with 100% moisture, and since the dried piece ended with 1/6 of that original moisture, he would have described the dried piece as having 17% moisture content.
I guess the bottom line of all this is to just realize that when anyone states a "moisture content" of a piece of wood, just remember that that number is dependent on just which system of measuring was used! And then smile, because that level of detail is pretty much irrelevant in actually using a wood stove!
Associated with this, covering the woodpile with a tarp slightly improves this, but probably not enough to make the expense of a tarp worthwhile, except in a climate where rain and very high humidity is common. Similarly, split pieces of wood tend to dry slightly faster than full diameter logs, but again by minimal amounts.
There appears to be no value in drying firewood more than about nine months.
Return to Ken and Mary's Page