The term “hardwood” refers to trees, usually flowering trees and shrubs, which produce dense heavy wood due to their thick-walled, densely packed fiber cells and abundance of tracheids and vessel elements (water conducting tissue). Hardwoods can be further categorized depending on their density into groups such as “medium heavy”, “heavy”, “very heavy” and “ironwood” for those wood types that actually sink in water. Coniferous trees are generally called “softwoods” due to their wood being mainly composed of tracheids and lack of wood fiber cells. The weight and hardness of wood is determined by its density, lignin composition and porosity of cell walls.
Wood consists of dead cells in the tree’s trunk, specifically the xylem when the bark has been stripped. Its weight comes from the cellulose and lignin surrounding the cell walls of the billions of cells making up the tree. Lignin is a phenolic polymer composed of benzene rings, this lends wood its great strength and hardness. Wood’s buoyancy comes from differing amounts and sizes of air cavities or lumens, thickness of cell walls and percentage of lignin therein. Ironwood hardwoods are composed of multitudes of long, densely packed wood fiber cells with heavily lignified cell walls and few to no lumens.
The best measurement of wood density is calculating the specific gravity (or relative density) of a specimen. Specific gravity is a dimensionless unit defined as the ratio of the density of a substance as compared to the density of water, where water is 1.0. This can be calculated by dividing a substance’s density, in grams per cubic centimeter, by the density of pure water which is 1 gram per cubic centimeter. One gram of pure water occupies the volume of one cubic centimeter meaning that any substance with a specific gravity greater than 1 will sink. Some of the heaviest hardwood trees have specific gravities of 0.80 to 0.95 including: shagbark hickory (Carya ovata), persimmon (Diospyros virginiana) and ironwood (Ostrya virginiana), canyon live oak (Quercus chrysolepis), Engelmann oak (Quercus engelmannii) and hollyleaf cherry (Prunus ilicifolia) among others. True ironwood hardwood trees have specific gravities greater than 1, such as: 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, 1.15); and ebony (Diospyros ebenum, 1.12). These seem especially dense when compared with one of the world’s softest and lightest woods the tropical West Indian balsa (Ochroma pyramidale) which has a specific gravity of 0.17.