Wood hardness is often measured by the Janka scale and sometimes specific gravity (heaviness). In the USA Osage-orange, Maclura pomifera, at 2,100 on the Janka scale, and Mesquite, Prosopis spp, such as honey mesquite, Prosopis glandulosa, at 2,345, are among the most common hard American woods. The hardest woods appear to grow in the tropics, and the American variety of lignum vitae, Guaiacum sanctum or holywood, at Janka 4,400 is found in Florida and the Bahamas. It is threatened by habitat loss and not often seen other than in the Florida Keys. Many harder woods are imported from around the world, often for use as flooring.
Measuring Hardness -- the Janka Test
Two useful ways of measuring woods are the Janka Scale, which measures hardness, and Specific Gravity, which measures density. The Janka Hardness test measures the hardness of woods by rating the force needed to drive a steel ball into a plank. The measurement is given in pounds of force (lbf) in the USA and kilograms of force (kgf) in Europe. The scale goes from nominal 0 to as high as needed; the softest woods known are about 500 and the hardest woods known 2177kgf. The Janka test is very useful for the wood flooring industry because it predicts how badly a particular wood will ding and scuff in normal use. Many favourite imported hardwood floors are in the 3,000s.
Density -- Specific Gravity
Another useful test measures the density of wood. Specific gravity compares the density of one object to another; woods are rated according to whether they sink or float in water. Specific gravity under 1.0 floats, above 1.0 sinks. In general hardness goes with density and weight, so the hardest woods usually have a high specific gravity and will sink in water, but there are exceptions. Oily wood is dense, but the oil softens the wood so its Janka rating is unexpectedly low; conversely wood with many lignum cells can be hard but relatively light.
Not All Janka Scales Are the Same
A search for "Janka scale" finds many tables with a variety of woods, some giving varying measurements for apparently the same wood. No definitive Janka scale exists as many people do the measurements. Every tree grows differently depending on its location and circumstances, so one piece of wood will not test quite the same as a piece from another tree of the same species. Flooring companies tend to stop their lists at the hardest commonly available floor woods, although there are a number of harder woods such as lignum vitae, Guaiacum officinale, which are not used for floors.
Timber names are so confused that looking up species names is best. "Oak" can mean one of at least half a dozen different oak species of varying hardness. Conversely, the same tree species may be called various names; the species Piptadenia macrocarpa is known as Patagonian Rosewood, Curupay, Angico Preto and Brazilian Tiger Mahogany. The lumber trade also has a tendency to borrow names when a wood becomes scarce; when one tree becomes overlogged, the timber industry finds a similar species elsewhere and calls it a familiar name for obvious marketing reasons. But it is usually not exactly the same. Hardness and density measurements are most accurate by species.
The Hardest Floor Woods
A favourite very hard floor wood is Ipe, which means wood from one of the species of Tabebuia, typically Tabebuia serratifolia. Ipe is pronounced "epay" and is also called Brazilian Walnut and Lapacho. Janka hardness is ~3680. Ipe is often used for decking. Curupay, Piptadenia macrocarpa, Janka ~3800, is another common hardwood flooring wood used for interior floors as well as exterior. Jatoba, Hymenaea courbaril, is frequently called Brazilian Cherry, although not a member of the cherry (Prunus) family at all, because it ages like American Cherrywood does, turning from a brown colour to a darker red over time. Jatoba is around 2,300 on the Janka scale. Timber companies usually claim none of these woods are from endangered species.
Harder Yet and Hardest
Snakewood most commonly refers to Piratinera guianensis, a highly decorative wood with V-shaped markings like a snake. Janka hardness is 3,800, SG 1.3. The tree does not grow very wide and is usually available in small billets that are used for musical instruments, like guitar pieces. A number of other trees like ebony, Diospyros ebenus (Indian ebony) or Diospyros crassiflora (Africa ebony) are similarly in the 3,000s of the Janka scale and have specialised uses; ebony was used for harpsichord and piano keys.
Lignum vitae, Guaiacum officinale, also known as ironwood, has a long history of use. It has a very high resin content, Janka ~4,500 and specific gravity 1.35 (sinks in water) and was used for weather-resistant applications, such as bearings and bushings for underwater. Lignum vitae is now in the threatened category of the Endangered Species list and is not so widely available. Other Guaiacum species are sometimes sold as lignum vitae and many are over 4,000 Janka but are not as hard as Guaiacum officinale.
The hardest wood in the world is claimed to be Schinopsis brasiliensis, at Janka 4,800 with SG 1.35. The six listed species of the Schinopsis family all come from South America and are all very hard. They are known as Quebracho ( "axe-breaker"), Soto and Barauna and are used for construction and railway sleepers.
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