Weird and wonderful members of the plant kingdom - how hard is your wood?

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 - which is the specific gravity of pure water. Since one gram of pure water occupies a volume of one cubic centimetre, anything having a specific gravity greater than 1.0 will sink in pure water. Some of the heaviest hardwood trees and shrubs have specific gravities between 0.80 and 0.95 including mahogany and Santa Cruz Island ironwood. 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 centimetre, even the world's heaviest hardwoods generally have specific gravities less than 1.5 due to tiny pores, called lumens, within the cell walls, these pores not only cause the wood to be lighter, they may also take up microscopic amounts of water, which is why the specific gravity test is conducted over 24 hours - to allow such effects to manifest themselves. True ironwoods include trees and shrubs with dry, seasoned woods that actually sink in water but do not change their dry weight by very much, which means they have specific gravities greater than 1.0. They include lignum and ebony. To appreciate the weight of these hardwoods, compare them with balsa - one of the softest and lightest woods - with a specific gravity of only 0.17.

Common British woods have the following densities: Beech 0.655, Oak 0.669, Pine 0.373 (which is why pine kitchen tables soon become discoloured, the specific gravity of the wood is such that it soaks up the water used to clean it) and Walnut 0.562.

Hard wood photograph by merfam, used under a creative commons attribution licence