Specific Gravity
Probably the best way to appreciate the relative hardness of different woods is the concept of "specific gravity," a numerical scale based on 1.0 for pure water. Without getting too mathematical, the specific gravity of a substance can easily be calculated by dividing its density (in grams per cubic centimeter) by the density of pure water (one gram per cubic centimeter). The brilliant Greek mathematician and inventor Archimedes discovered over 2,100 years ago that a body in water is buoyed up by a force equal to weight of the water displaced. Archimedes reportedly came upon this discovery in his bathtub, and ran out into the street without his clothing shouting "Eureka, I have found it." Since one gram of pure water occupies a volume of one cubic centimeter, anything having a specific gravity greater than 1.0 will sink in pure water. The principles of buoyancy and specific gravity are utilized in many ways, from scuba diving and chemistry to the hardness of dry, seasoned wood. Some of the heaviest hardwood trees and shrubs of the United States have specific gravities between 0.80 and 0.95; including shagbark hickory (Carya ovata), persimmon (Diospyros virginiana) and ironwood (Ostrya virginiana) of the eastern states, and canyon live oak (Quercus chrysolepis), Engelmann oak (Q. engelmannii), hollyleaf cherry (Prunus ilicifolia) and Santa Cruz Island ironwood (Lyonothamnus floribundus ssp. asplenifolius) of southern California. Although some of these trees are called ironwoods, their dense, dry wood will still float in water. Since the pure cell wall material (lignin and cellulose) of wood has a density of about 1.5 grams per cubic centimeter, even the world's heaviest hardwoods generally have specific gravities less than 1.5 due to tiny pores (lumens) within the cell walls. True ironwoods include trees and shrubs with dry, seasoned woods that actually sink in water, with specific gravities greater than 1.0. They include lignum vitae (Guaicum officinale, 1.37); quebracho (Schinopsis balansae, 1.28); pau d'arco (Tabebuia serratifolia, 1.20); knob-thorn (Acacia pallens, 1.19); desert ironwood (Olneya tesota, 1.20); Snakewood (Cecropia peltata 1.36); and ebony (Diospyros ebenum, 1.12). To appreciate the weight of these hardwoods, compare them with tropical American balsa (Ochroma pyramidale), one of the softest and lightest woods with a specific gravity of only 0.17.