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The Janka Hardness Test

(For values, see Janka Table)

The Janka Hardness Test was invented in 1906 by Austrian-born wood researcher Gabriel Janka {1864 - 1932} and is a variation on an earlier test used for metals. It's a measure of the force needed to drive a steel ball a certain distance into wood. The details, for anyone who cares, are given at the bottom of this page. It all boils down to a number for a given wood.

The variability of woods usually means there is a range of values but usually you'll see only one number for a wood. You should assume that there could be as much as a 5% variation up or down although it's generally less that than that. In instances of significantly different growing conditions, it could be more.

The higher the number the harder the wood. The usefulness of the number is that it gives a good idea of the absolute hardness / durability of individual woods and the relative hardness / durability of pairs or sets of woods, including how well they will stand up to heavy loads, abrasion, etc.

Example uses are knowing about whether a wood is good for

• flooring / decking / stair treads / counter tops, etc (applications where durability and wear-resistance are important)
• certain parts of musical instruments where wear-resistance is important
• other parts of musical instruments where the degree of hardness affects the tone of the wood

It is also useful for determining whether or not a wood is likely to need pre-drilling for nails & screws and how easy/difficult it will be to cut, sand, plane, etc.

Just as a general indicator of the range of Janka values:

80	balsa wood
300 => 400	soft cedars and pines, paulownia, some poplars and cottonwoods easily dented with a fingernail
1100 => 1300	various oaks
~2000	mesquite, purpleheart, hickory
~3000	various ebonies
4500	lignum vitae - you can drive nails with this stuff !
~5000	Australian bull oak

For a full table of values (over 1000 woods) see Janka Table

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THE PROCESS:
The Janka Hardness is a measure of the resistance of a sample of wood to denting. Specifically, the test measures the force required to embed an 11.28 millimeter (0.444 in) diameter steel ball halfway into a piece of clear heartwood at 12% moisture content, indicated in pounds of force per square inch. 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, which is the number normally presented.

Although the test is specifically for hardness (via denting), the results are also strongly indicative of how a wood will bear up under abrasion.


samples of the Janka testing setup: