by Eric Meier
[arve url=”https://youtu.be/6XAJyj9Au6Y” /]
In my experience, specific gravity is without a doubt the single most abused and vaguely used term in woodworking terminology.
Technically, specific gravity is a measure of the ratio of a wood’s density as compared to water. (So if a wood is of the same density as water, the specific gravity would be 1.00.) However, as with any density measurement for wood, it is greatly dependent upon the wood’s moisture content: the more moisture the wood contains, the denser it will be. The chief problem arises is that there is no standardized way between woodworkers and botanists to express specific gravity: and there is no implicit or assumed values. (At least with average dried weight, the moisture content is generally assumed to be at 12% unless otherwise noted.)
There are several ways to express specific gravity for wood—the standard within botany uses a wood’s ovendry weight (meaning a moisture content of 0%, which is the lightest the wood can ever get), and its green volume, that is, when it’s freshly cut: having the largest possible volume. This may seem like a double-standard—to calculate this density from the wood’s dry weight, and its green volume—but this standardization, commonly called the “basic specific gravity,” prevents any irregularities or inconsistencies from occurring, mainly because it uses predictable extremes (i.e., lightest weight and largest volume) to calculate the SG value. Such a combination is a real-world impossibility: it’s useful within the scientific community, but is very confusing for the woodworking community.
Other specific gravity values used in botany include using both ovendry weight and volume, called ovendry specific gravity. Another is to use the ovendry weight, and the volume of the wood at 12% MC. The problem with these scientific measurements is that they use a non-existent ideal which never truly represents a given piece of wood at any one time. Since the weight is always based on the ovendry value, it tends to produce an artificially low impression of specific gravity.
In addition to the variety of measurements used in botany, woodworkers also use various standards to gauge specific gravity—usually based on real-world wood samples. Accordingly, specific gravity measurements referenced in woodworking will usually be a pairing of green weight and green volume, 12% MC weight and 12% MC volume, and so forth.
Clear as Mud
Between scientific and woodworking standards, there are at least five different ways to express specific gravity, and oftentimes sources (particularly woodworking publications) will make no attempt at identifying which standard is being referenced. For instance, American Beech (Fagus grandifolia) could be as low as .54 for its basic specific gravity, or up to .73 for its specific gravity based on 12% MC weight and volume. (And if the wood were still green and above its fiber saturation point, its specific gravity could be over 1.00, indicating that it would sink in water.) With such a wide disparity between specific gravity values, it’s not hard to see how confusing this measurement can become when no qualifying information accompanies the value.
On this website, every effort has been made to use clear and standardized numbers for specific gravity measurements. The first number is the basic specific gravity, based on the botanical standard of ovendry weight and green volume. The second number is meant for woodworkers, and is simply a snapshot of the wood’s specific gravity at 12% MC, (that is, both 12% MC weight and volume). Water weighs 1,000 kilograms per cubic meter, so taking the wood’s density (in metric units) and dividing by 1,000 yields its specific gravity in woodworking standards.