When dealing with any type of material one of the major concerns is the hardness of the material. The hardness determines how you need to deal with it in terms of cutting, shaping, and sizing. It also determines the durability of the material. You get an idea of how long it is likely to last.
The Janka test determines the hardness of wood. Wood is one of the softer materials as compared with steel, stone, or perhaps glass. A 44-caliber steel ball is pushed into the surface of the wood until it is half embedded. The force expressed in pounds-force (lbf) is the Janka hardness of that particular wood.
History of the Janka Hardness Test
Gabriel Janka an American scientist of Austrian origin devised the Janka test. He worked for the United States Department of Agriculture and he developed the test in 1906.
Janka developed his test based on the hardness test used on metals, called the Brinell test. In comparison to the Janka hardness test, a steel ball or punch makes an impression on the metallic test piece. Then, based on the dimensions of the indentation, a Brinell number is allotted. It denotes the extent of the hardness of the metal.
Janka created a test that he could conduct multiple times arriving at similar results for a given sample. Today the Janka hardness test is more or less the standard test for measuring the hardness of wood throughout the world.
How Does the Janka Test Work?
The Janka rating is a number that corresponds to the force required to push a steel ball halfway into the surface of wood. The test uses a steel ball of 11.28 mm (7/16 inches) in diameter. The equipment pushes the ball halfway into the wood sample and the force is measured and expressed in lbf.
For a Janka test, we usually use a piece of wood of standard dimensions typically 2’ X 2’ X 6’. The test piece should be solid, unfinished wood devoid of knots. The moisture content of the wood should be about 12%.
The ball that we use is placed on the surface and we apply force to the ball. This procedure is repeated at different points on the sample and we consider the average number as the final value.
Variation in the Janka Scale
When you conduct a Janka test on a piece of wood the results can vary slightly. This is because the grain pattern of even the same type of wood can vary from piece to piece. So, if you have a piece of flat-grained wood, it is a standard surface and you will get a particular reading.
But if you check vertical grains, you will get a typically lower reading. If you check the wood on the sides, then there again, the wood grain pattern is different. So, you can get slightly varying results depending on what part of the wood specimen you are checking.
“Good” and “Bad” Janka Rating
We wouldn’t go as far to say that a soft type of wood is “bad.” But as far as Janka hardness goes, particularly with wood for flooring, the harder the better. While selecting wood for such a purpose, you would grade the lumber available. You would then choose the hardest wood possible.
The Janka hardness scale gives you an idea of how hard a floor is. You can determine how much wear and tear the floor can take. This in turn tells you how regularly you would probably have to maintain it.
Ultimately, wood with a high Janka hardness rating is a hard variety of wood. In this context, we could consider balsa wood as a type of wood with a “bad” Janka rating. The reason here is that balsa wood has a Janka hardness rating of 100 lbf.
Balsa wood is so soft that you can dent the surface with a fingernail! So, balsa wood would be considered “bad” if you wanted to make flooring with it. But then, balsa once used to play a major role in the airplane industry due to its extreme lightness. Even today, we use balsa widely for aeromodelling purposes.
Another consideration is the exposure to traffic on flooring. You would require wood with a high Janka rating if you intended to use it in high-traffic areas. Examples of high-traffic areas would be the floor of a living room or mudroom or even the floor of a restaurant or shop.
So, when it comes to a “good” or “bad” Janka hardness rating with flooring, you would want the hardest wood possible.
Summary
The Janka hardness test is one of the simplest tests to check the hardness of wood. Today it has become the universal choice for the hardness standard of wood.
If you are looking for a relatively hard variety of wood, you don’t need the hardest wood in the world. It would always be preferable, particularly as a woodworker to choose wood between the hardest and the softest. Wood with a Janka hardness rating between 1,000 lbf. and 1,500 lbf. should be a suitable choice.
By choosing wood within this hardness range, you would get an adequately hard type of wood to withstand scratches and dents. You could also work on the wood better. The harder the wood, the more difficult it is to work with. It would be significantly tougher on your tool blades.
We hope that this brief discussion gives you a better insight into understanding Janka hardness better. You can now use this knowledge as you continue with your woodworking projects.
