The Janka hardness test measures the resistance of a type of wood to withstand denting and wear. It measures the force required to embed a 11.28 mm (0.444 inches) diameter steel ball into wood to half the ball’s diameter. Why 11.28 mm? Because a circle with a diameter of 11.28 mm has an area of 100 square millimeters.

Of course, wood hardness depends on if you’re talking about side hardness (imagine pressing against the side of a tree) or end hardness (imagine pressing into the top of a stump that you cut off). It also depends on the wood’s moisture content and a few other factors. The numbers you’ll see most often are for side hardness at 12% moisture content.

The important thing to know about the Janka scale is not so much the absolute number (which can be given in pounds-force, kilograms-force, newtons, or kilonewtons), but rather the relative difference between two species. For example, mesquite with a hardness rating of 2,345 lbf is quite a bit harder than purpleheart (1,860 lbf).

When I first started carving, I was told that the Janka rating gives a good idea of the difficulty of cutting a particular type of wood. Seeing that mesquite is harder than purpleheart, I thought that I could carve some great things from purpleheart. How wrong I was! Whereas mesquite cuts very nicely, purpleheart is very difficult to cut. It’s so hard that it damaged my knife blade.

Most oaks have a Janka hardness rating around 1,350 but, as with purpleheart, I find them more difficult to cut than mesquite. Obviously, the Janka scale doesn’t tell the whole story.

I’ve done quite a bit of searching and haven’t found anything similar to the Janka scale that gives the relative cutting resistance of different woods. Many woodworking sites will give informal measurements. For example, mesquite is said to have “low cutting resistance” and purpleheart has “moderate cutting resistance.” In those cases, it’s the resistance to cutting with a saw rather than with a knife, but it does give me a little more evidence to back up my statement that the Janka scale is not the thing to use in determining cutting resistance.

In his 1950 Ph.D. thesis, Cutting force in wood working (just the abstract is free; I haven’t been able to find a free copy of the thesis), Eero Kivimaa did a lot of research to determine the forces involved in cutting wood. He did most of his research using air-dried Finnish birch, but some tests with 21 different wood species led him to write in his Summary:

The cutting force was found to vary approximately linearly with the specific gravity of wood (air-dry Finnish Birch). The cutting force rose to a maximum when the moisture content increased up to 10-13 %, and fell again with higher moisture content. Comparison of results obtained with 21 species showed that it is possible to estimate the main cutting force of any wood species on the basis of its sp. gr.

Aha! If you look up the specific gravity of mesquite, you’ll find that it’s about 0.80. The specific gravity of purpleheart is 0.86. From Kivimaa’s research, then, you would expect purpleheart harder to cut. Southern live oak, which is pretty common around here, has a specific gravity of 0.88. Again, these woods are “softer” than mesquite on the Janka scale, but have higher specific gravity and are more difficult to carve. Hickory, too, has a lower Janka rating (1,820), but a specific gravity of 0.83.

Note that, as with Janka hardness, the force required to cut a particular type of wood will depend a lot on the moisture content. As Kivimaa pointed out, force required increased as moisture content increased to between 10 and 13 percent, and then decreased as moisture content increased. “Green” wood will cut more easily than kiln dried wood.

Cutting force required will also depend on the sharpness of the knife, the bevel angle, the angle of attack, whether you’re carving with the grain, across the grain, or on the end grain, the smoothness of the blade as is moves through the wood, and the type of cut (pressing or slicing). We can hold those values constant in order to get a relative cutting resistance measure.

I’m not convinced that specific gravity is the sole deciding factor, but it’s much better than Janka hardness in determining how hard a wood will be to cut.

A good place to find information about a particular type of wood, including its hardness and specific gravity, is The Wood Database.

Speaking of specific gravity, common wisdom is that wood floats. Whereas it’s true that most woods float, there are some species that sink. Lignum vitae, Arizona desert ironwood, some types of Ebony, and many other species have specific gravity greater than 1.0. Even kiln dried, these woods are more dense than water, meaning that they will not float in water. Black ironwood has a specific gravity of 1.49; it’s 50% more dense than water.