Specific gravity: a weighty subject
I've heard people refer to a wood's specific gravity. What are they talking about? Is it important when I'm choosing wood for a project?
—Kerry Johnson, St. Louis, Mo.
Specific gravity tells you how much the wood's cell structure contributes to its density, Kerry. It's the ratio between the density of a piece of wood after it's oven-dried and the density of enough water to equal the wood's volume. That volume for the second measurement varies with wood's moisture content, so before you can compare different wood species, a specific-gravity chart like the one shown needs to tell you whether the wood was tested when green, dried to 12 percent moisture content, or oven-dry.
Specific gravity matters because woods with high specific gravities tend to swell and shrink more across their grain than those with lower specific gravities. That can be hard on painted finishes for outdoor projects, for example. So for these applications, look for woods with low specific gravity, such as cedar or redwood.
Not all woods have a specific gravity less than 1. Lignumvitae's specific gravity of 1.05 (oven-dry weight to green volume) means it's heavier than water and sinks. Such density makes it good for mallets and other tools.