By The Metric Maven
I’ve always liked wood. My Grandfather was a carpenter, my father also knows how to work with wood. I, alas, do not. My father can generally recognize different woods, I haven’t a clue. The variety of woods that exist is astonishing. I recall reading about how the English carpenter and clockmaker John Harrison (1693-1776), used a wood called Lignum vitae, to construct wooden clocks. This wood actually has the property that is is self-lubricating, which allowed Harrison to make bearings and gears of it for his pendulum clocks. It is also one of the hardest woods in nature. In my youth I marveled at the Balsa wood rubber band driven airplanes available at the local dime store. I also have an interest in photographing wooden grave markers. An image of one of these “tombstones” I photographed is shown in the photo above.
I thought back on all this when paging through an old book called Science in Everyday Things by Engineer William C. Vergara and published in 1958. The book is a list of questions with answers, and one of them is: “Does all wood float?” What I assume he means is that given a solid cube of wood, will it float in water? When Vergara offers his answer, he informs his readers that the density of water is 62.5 pounds per cubic foot. Any wood with a density above this will sink and any below this will float. What struck me was the contrived and arbitrary nature of this value. It reminds me of the freezing point of water being defined as 32 degrees, an arbitrary magic number to be remembered on the Fahrenheit scale, because of poor measurement planning.
What struck me even more was that I was still going along with all the people who use kilograms per meter cubed. In my view, this is a vestigial Ye Olde English use of metric, which should be diminished. Density is the mass of an object divided by its volume. The everyday common volume unit allowed in metric is the liter, and a liter of water is essentially a kilogram. This means that a liter of water has a density which may be written as 1000 grams/liter. Both grams and liters are familiar units in the everyday world of an average person in a metric country. If a wood’s density is below 1000 grams/liter it will float, if it is above 1000 grams/liter it will sink. This is a nice Naughtin’s Laws friendly way to express water’s density, and it does not involve recalling a number like 62.5 pounds per cubic foot. This method essentially relates the specific gravities of the woods in an elegant fashion. It allows one to rationally list the densities of various woods in a way which one can immediately realize if they would float or not:
Wood Density (g/L)
Yellow Pine 650
Water Gum Tree 1000 (Density of water)
Black Ironwood 1040
Poison Ash 1104
Lignum Vitea 1229
Balsa is the lightest of the woods and a cube of it will clearly float in water. Yellow Pine, Maple and Hickory will also all float in water. Yellow pine was chosen for use in the caissons that were used to construct the Brooklyn Bridge. Southern Yellow Pine was chosen for it’s ability to withstand large pressure and for the considerable amount of resin it contains. This makes it very resistant to rotting. Wood from the aptly named Water Gum Tree has neutral buoyancy, that is, it has essentially the same density as water and is compelled to neither float nor sink in it. A block of Water Gum Tree wood is like a helium balloon which floats at a stable position, neither rising nor falling to the floor. The word Quebracho means “Ax Breaker.” Given its high density, this name seems appropriate.
In his last sentence Vergara states: “Since wood weighing more than 62.5 pounds per cubic foot will sink, it can be seen that many kinds of woods cannot possibly float.”
This is true for a single monolithic block of wood, but all these woods can be used to make vessels which will float, they only need to displace enough water to do so. The Civil Engineering students at Iowa State University each year create a concrete canoe. The density of concrete? It’s 2400 grams per liter. But concrete is not nearly as dense as steel which is about 8000 grams per liter. The hollow interior of a canoe or ship decreases the overall density of the ship enough to bring it well below the 1000 grams/liter threshold, which in turn allows it to float. Allowing for shaping, all wood will float.
I very much encourage the use of grams/liter for expressing density. I completely discourage the use of the cgs leftover, grams/cubic centimeter. In the case of steel its density is 8.05 g/cc. I also mostly tend to discourage the use of kilograms per cubic meter, as the units are out of the range of everyday measurement experience. It is, however, very easy to convert from kg/m3 to g/L. For instance, the density of steel is 8000 kg/m3 which is 8000 g/L. The conversion factor is one. The numerical values are the same, just change the units. It’s just that easy, as the metric system is, and should be, when it’s employed in an articulate manner. People who insist on using Ye Olde English units like pounds per cubic foot—are just dense.
 The Great Bridge, David McCullough, Simon and Schuster, New York pg. 174-175.