Janka hardness divides wood species into hard and soft and helps us choose wood that is resistant to printing and wear

Hardness is the property that divides wood species into hard and soft. It tells us how easy a species is to work, how resistant it is to imprinting and wear, how it behaves when sanded, milled or when nails and screws are driven in or out. The best known method for determining the hardness of wood is the Janka test. Janka hardness, like densityare relative values, determined under certain conditions. Wood is not a homogeneous material, its properties are influenced by structure, cutting method, grain direction, water content, etc. Even relative, Janka hardness helps us to choose the right wood to best withstand the stresses. It is the most important property when choosing wood for flooring.

Janka test determines wood resistance to printing and wear

The Janka test was created in 1906 by the Austrian-born American scientist Gabriel Janka. It is in fact an adaptation for wood of the hardness test for metals, developed in 1900 by the Swedish engineer Johan August Brinell. The Brinell test involves pushing a metal ball with a certain force against the surface of a metal sample. After the force has ceased, the trace left by the ball on the surface is measured. The hardness of the material was calculated according to the diameter of the trace, the force applied and the time the force was applied.

Gabriel Janka designed a special test for wood, with fewer variables. He used a metal ball with a diameter of 11.28 mm that was pushed halfway into the wood (half of the ball). The diameter of the ball was chosen so that the mark left in the wood after penetrating halfway was a circle with an area of 1 cm² (100 mm²). The mark left by the ball was thus eliminated as a variable and was the same regardless of species. The hardness of the wood is given by the force required to push the balls into the wood.

In 1927 the method was standardised in the USA and became the main method for determining the hardness of wood (ASTM D1037). In the meantime, the method has been adopted all over the world, with the specification that the unit of measurement in which it is expressed differs from continent to continent, which often creates confusion. In the USA Janka hardness is expressed in pounds-force (lbf), in Europe in kilograms-force (kgf), in Australia and New Zealand in Newton (N) or kiloNewton (kN). The following equivalences are used to make the transformations: 1 kg=2.2 lb and 1 kgf=9.81 N.

Sometimes hardness is also expressed in terms of surface area (kgf/cm², lbf/cm²) because hardness was originally expressed as the pressure exerted on the wood sample and the units of measurement for pressure were used. When the standardisation was carried out, the expression as pressure was abandoned and the expression as force applied to the wood was changed to force applied to the wood, the units of measurement being those for force.

Janka hardness scale divides wood into soft, medium and hard

By testing similar samples, the relative hardness of the species was determined. The samples were of equal size, with straight grain and no knots, taken from the heartwood, and the force was applied perpendicular to the grain. The moisture content of the wood samples was chosen 12%. Tests carried out under controlled conditions provide quantifiable measurements that allow an accurate comparison of wood hardness by species and even growing area. The results have led to lists showing Janka hardness for wood species worldwide.

According to this classification the softest wood is balsa, with a relative hardness of 220 kgf, and the hardest is Australian buloke, a specific and rare oak with a hardness of 11,200 kgf. A fairly rich list of wood species and hardnesses expressed in lbf and N can be found here.

The Janka scale can be used to classify wood species according to hardness. The higher the number, the higher the hardness of the species. A rather relative classification and some examples of native wood below:

• soft and very soft wood - under 250 kgf (poplar, lime, brad, molid)
• medium softwood - between 250 and 500 kgf (willow, arin, tea, ulm, black pine, lariță)
• medium hardwood - between 500 and 650 kgf (oak, gorun, cherry, nuc)
• hardwood - between 650 and 1000 kgf (fagfrasin, plum, apple, pear, carpen, sip, yew)
• very hard wood - over 1000 kgf (acacia)

The hardness of wood decreases with increasing humidity. The wetter the wood, the less resistant it is to printing and wear. Hardness is also influenced by growth defects, density, grain direction, cutting method. The Janka test can be determined on samples cut along the grain or on grain ends. In general, hardness is higher on fibre ends. In the case of along-fibre, determinations can be made on radial or tangential cuts, but the difference between the two is insignificant.

The Janka test, a quick way to evaluate and compare the hardness of wood used in flooring

Janka hardness is very useful because it helps to choose the right species when it comes to difficult uses or machining. The higher the hardness of the wood, the more resistant it will be to dents, impressions and wear. In a very hard wood nails and screws go in harder, but they also tear out very hard. Hard wood can also be difficult to sand and machine. A carpenter can tell you how hard it is to work with hard wood. acacia. It's so tough it can break tools.

One area where knowing the hardness of wood is very useful is in flooring. Shoe heels, heavy furniture, heavy traffic are all tests that a good floor must withstand. The harder the wood, the stronger the floor will be. Beech or ash flooring or stair treads will be much stronger than oak, walnut or fir.

Carpenters are accustomed to testing the hardness of wood with a fingernail. If they manage to scratch it, it's no good for floors or table tops.

I hope you find the information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.