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What tree has the hardest lumber? Is it harder than aluminium or even steel?

Posted September 4, 2019

The hardest lumber in the world is, of course, very hard. People call it “axe breakers” and lumber specialists don’t like working with it, because it is so hard that tools become dull very quickly. But is it really harder than commonly used metals, such as aluminum? Could it be harder than steel that woodworking tools are made of?

Allocasuarina luehmannii trees rarely grow up to be very old and big, but their lumber is exceptionally hard. Image credit: MargaretRDonald via Wikimedia (CC BY-SA 4.0)

Meet Allocasuarina luehmannii – an ironwood tree native to Australia. It mostly grows in south-eastern part of the country and is relatively rare. These trees rarely grow old and nowadays they are being driven away by clearing land for farming and harvesting lumber for construction or production of various goods. Allocasuarina luehmannii is very important for the environment. For example, red-tailed black cockatoo often stay close to these ironwood trees because they are important sources of food for these endangered birds. For us Allocasuarina luehmannii is interesting as the hardest wood in the world.

The hardness of Allocasuarina luehmannii on Janka hardness scale (named after its inventor Austrian-born emigrant Gabriel Janka) reaches 22.5 thousand Newtons. And that very hard. For comparison, white oak is 6 and red maple ir 4.2 thousand Newtons hard. That is why it is generally thought that  Allocasuarina luehmannii is the hardest commercially available lumber. There may be some harder species, but we are not cultivating them and they may not even be tested. Allocasuarina luehmannii is plenty hard enough – although its lumber is used in various applications, it is very expensive and rare. One reason is that it is so hard that the equipment of sawmills and lumber yards becomes dull very quickly. But does that mean that it is harder than steel?

Allocasuarina luehmannii is actually slowly decreasing due to clearing land for farming, bushfires and other human activities. Image credit: Mark Marathon via Wikimedia (CC BY-SA 4.0)

Well, no. It is abrasive in a way, but results of that kind of hardness only show up after some time. Think of it as your kitchen knives. You never cut anything as hard as the stainless steel that blades are made of, but knives become dull anyway. In fact, that Janka scale may help us a comparison with other metals.

While Allocasuarina luehmannii is nowhere near as hard as the steel, it is significantly harder than aluminium. While ironwood’s hardness is  22.5 thousand Newtons, aluminium is only 15 thousand Newtons hard. Of course, it depends on the grade of aluminium, but generally speaking, Allocasuarina luehmannii is harder. But that’s not really that big of a surprise – aluminium is rather soft. Lignum vitae, another very hard wood, reaching 20 thousand Newtons in a not very scientific test proved to be harder than aluminium and brass.

So no, no lumber is too hard even for basic steel woodworking tools. However, there are a lot of interesting species of trees that have interesting characteristics. Ironwood is actually so dense that it doesn’t float on water.

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