Timber of the times

Science News, Nov 17, 2001 by Donald Girod

We read in "Isotopes reveal sources of ancient timbers" (SN: 9/29/01, p. 197) that the Chaco Anasazi builders used "large timbers" 5 meters long, 22 centimeters in diameter, and weighing 275 kilograms. As anyone who splits his own firewood could tell you, some thing is amiss here. The Handbook of Chemistry and Physics tells us that ponderosa pine has a density of about 0.5 grams per cubic centimeter, so if you do the math, you come up with a weight of about 95 kg per log. You're off by a factor of about 2.9. The clue here is "large." Nobody thinks a log 22 cm (8.5 inches) across is large.

Donald Girod
Cattaraugus, N. Y

The density value you cite is for seasoned wood, which can weigh up to 40 percent less than a freshly felled timber, says Jeffrey S. Dean, a coauthor of the research. Evidence shows that most of the smaller roof timbers in the Chaco Canyon great houses, as well as many of the larger beams, were unseasoned when used. Also, although many of the roof timbers were ponderosa pine, the ones the researchers analyzed were spruce and fir, which are denser. --S. Perkins