What makes a hardwood a hardwood, a softwood a softwood, and are hardwoods actually harder than softwoods?
Believe it or not, the terminology has nothing to do with the density of the wood fibers (hardness). Hardwoods are simply those species that are deciduous trees, and softwoods are conifers (evergreens). It’s just that simple. However, simple seems to rarely remain simple. While it is GENERALLY true that hardwoods tend to be harder than softwoods, the actual hardness of the wood may not follow that tendency. For example, remember those Balsa wood bridges we made in science class in middle school? Balsa is a deciduous tree, and therefore a HARDWOOD, as is the softest known wood specie, Cuipo (aka Bongo, Hameli or Macondo). Yet, they are considerably softer than, say, the softwood Douglas Fir that constitutes your average 2×4.
How does one measure such a thing objectively?
Around the turn of the last century, an Austrian-born emigrant named Gabriel Janka (1864-1932) was employed as a researcher for the Forest Products Lab of the US Department of Agriculture. His task was to objectively and scientifically measure the hardness (and therefore, durability and structural quality) of the lumber products coming out of America’s forests in that day. He came up with a methodology for testing samples of these wood species that allowed objective comparison. Janka’s hardness test consists of an instrumented mechanical press driving a .444-inch steel sphere (ball bearing approximately the size of a .44 bullet) into the surface of a between 1- to 2-inch thick sample. The amount of force required to press the sphere 1/2 its diameter into the sample’s surface is the Janka Hardness Rating. Janka’s hardness test method was adopted as the industry standard in 1906, and is measured in units of pounds in the US, kilograms in Europe and Newtons in Australia. Though, a rating is commonly referred to in a simple unit of Janka, i.e. “Cuipo is 22 Janka, Balsa is 100 Janka and Doug Fir is 660 Janka”.
The hardness rating depends on the direction of the grain. A hardness test on the face of a sample is called a side hardness rating, whereas a test on the end grain is called an end hardness rating. Unless otherwise noted on a Janka hardness chart, it is assumed that a chart is side hardness. Also, the hardness rating in the chart represents an average of a huge sample size for that particular species, to mitigate the variability of hardness within the trunk of the same tree, i.e. heartwood is harder than the “live edge” growth ring. The standard deviation is unique for each specie’s sample set, but they all median around 20%, so the accepted rule of thumb is that a number from the Janka Hardess Chart is +/-20% for that specie.
“Great, you told me some scientific history. What use is it?”
The straight Janka Hardness Rating for a specie of wood is probably useless in and of itself. I seriously doubt anyone will use that number in some sort of engineering calculation. The Janka chart allows us to compare the relative hardness of various woods during design and/or fabrication decisions. For example, on a current client project in the Wood Works, we edged Birch ply panels in Black Walnut to get a nice light-dark contrast. Black Walnut is 1010 Janka, and Birch is 1260 Janka. This means that when sanding, the Walnut will abrade away faster than the Birch, which is good to know when one is trying to flush up the trim without sanding through the veneer of the ply! Also, the flooring industry uses the Janka chart to determine material suitability and durability of engineered wood flooring products.
You can find the Janka Hardness Chart on Wikipedia, or here in our K-base: Janka Hardness Chart. Oh, and the Hardest Tree in the World?
… Australian Buloke… 5060 pounds-force!!Mike @ Doobly-Do Wood Works firstname.lastname@example.org