In fact, there are now a couple of different procedures, but the traditional method is known as the “Janka hardness test,” named for Austrian wood researcher Gabriel Janka, who invented it in 1906. The test measures the force required to embed a standard-sized steel ball (0.444in/11.28mm diameter) exactly halfway into the surface of a standard-sized sample of the wood. The formal procedure, embodied since 1927 as ASTM standard D143, is not freely available, but J.J. Morlan summarizes the key points well:
Testing is done on wood from the trunk of the tree and is almost always the heartwood. With heartwood there are a handful of exceptions. One that comes to mind is Balsa {Ochroma pyramidale}. Balsa is always milled from sapwood. The standard sample as indicated in ASTM D 143 is to be at 12% moisture content, be clear {no knots}, a solid block of wood having the dimensions of at least 2″ x 2″ x 6″ long and the rate of loading will be machine set at 1/4″ per minute. Two indentations are made on the tangential surface and two indentations on the radial surface. The four indentations are then added together and divided by 4 to get the average value of the force, with the result being declared as the side hardness.When testing is done on a piece of wood with the force applied to the end grain surface, the test is of end hardness. The end hardness of wood/lumber/timber will almost always be higher {harder} than its side hardness.
Morlan also provides a lengthy list of Janka hardness values for various woods, of which the current record holder is Allocasuarina luehmannii, aka “Australian bull-oak,” which require 2.5 tons of pressure. By comparison, Ponderosa pine tests at 480 lbs on the Janka scale, and low-density balsa wood at 88 lbs. The world’s softest wood, by Janka hardness, is Cavanillesia platanifolia, aka “quipo,” at 22 lbs. The Forestry Service research note linked below gives a good historical background and general overview of the Janka test.
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