The Hardness of Wood

What’s the hardest wood in the world? It turns out to be an Australian timber called waddy wood. Its name comes from the fact it was used by aborigines for waddies or nulla nullas, that is, war clubs.

Waddy-wood (Acacia peuce), also known as Birdsville wattle, grows in Central Australia and is an endangered species. Four of the world’s five hardest woods are Australian although the one ranked 13th in hardness, Gidgee, is the only one that most woodworkers will ever see for sale.

There are many lists on the Internet claiming to show the world’s hardest woods but most are from unreliable sources. Eric Maier, a wood-obsessed American, created a great website www.wood-database.com which is highly regarded for its careful, scientific evaluations of all things to do with wood.

Meier has recently published a poster called Worldwide Woods Ranked by Hardness. It presents 550 wood species grouped by region and ranked using the Janka hardness test. Each species has a specimen photo and its ranking for the region and for the world, together with the Janka rating in pounds force. This poster has been mounted and is on view in our workshop.

The Janka hardness test was developed in the early 1900s by Gabriel Janka and adapted in the 1920s by the US Dept of Agriculture to determine the suitability of various timbers for flooring. The test measures the force required to push a steel ball with a diameter of 11.28 mm (0.444 inches) into the wood to a depth of half the ball’s diameter.

And if you were wondering why that diameter was chosen, it’s so the area of the indentation will be 100 square millimetres. But if you’re wondering why they wanted that exact area you will have to keep on wondering. The force to push the ball into the wood is usually measured in pounds force but in Australia this is usually converted to Newtons (one pound force equals 4.45 Newtons). Various parameters need to be agreed on in the test such as the size of the sample, moisture levels and whether it is tested on tangential or radial grain. Several samples need to be tested and

 

The world’s 10 hardest woods (Newtons)
1 Waddy-wood Australia 20,604
2 Quebracho South America 20,381
3 Boonaree Australia 20,292
4 Belah Australia 20,025
5 Yarran Australia 19,892
6 Mangkono Pacific Islands19,803
7 Red Bauhinia Africa 19,714
8 Mgurure Australia 19,580
9 Lignam Vitae Central America 19,536
10 Surinam Ironwood South America 19,491

Buloke, an Australian timber often cited as the world’s hardest, actually ranks 21st at 16,732 Newtons. One of our hardest commercially available woods is coolibah at 16,063 N. turpentine is rated at 12,238 N and spotted gum rates only 10,368N.

In the northern hemisphere there’s hickory at 9,523 N, European Ash at 6,586 N, rock maple at 6,452 N and European beech also at 6452 N. American white oak looks a bit of a softy at only 6,007 N. Among the softer woods there is huon pine at 4,094 N, hoop pine at 3,338 N, Australian red cedar at 3,115 N, Douglass fir at 2,759 N, and western red cedar at 1,558 N . Botanically a hardwood, balsa shows its true colours at the bottom of the list with only 298 N.

It’s interesting to note that the Janka scale is most often referred to by the flooring industry. That’s where hardness really matters. Imagine how western red cedar floorboards would look after a year of use. But hardness can also indicate other properties of wood.

Density is an obvious case. It makes sense that the denser a piece of wood is, the harder it is likely to be. Comparing several species bears this out. This gives a good power relationship between the two qualities expressed by the equation: D = 7.7 H0.52 . Or, as a rough approximation, the density of a wood is equal to eight times the square root of the hardness. 

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