Saturday, 20 March 2010

What is the Janka Hardness Scale?

Even the briefest examination of the topic of bamboo flooring will bring you across the mysterious thing called 'the janka hardness scale'. As with many things on the internet it is accepted as an article of faith.

The Janka Hardness Scale is a way of measuring the strength of wood. The number refers to the pressure needed to push a 11.28mm steel ball into a piece of wood so it makes an indentation which is 100mm. The force is applied perpendicular to the grain. Unfortunately, there is no standard way of expressing the Janka Scale. Americans, who are still hung up with imperial measurements, express Janka as pounds force (lbf or PSI), the Swedes measure Janka in Kilograms force and the Australians in newtons or kilonewtons force. This is irritating because a lot of web pages just give a Janka rating without telling you what system of measurement it refers to.

Another odd thing is that nobody can seem to decide on the exact Janka Hardness of strand woven bamboo. I've read a hubpage which gives strand woven bamboo a Janka Hardness of 3,000 pounds per square inch. Ambient makes the boast that its strand woven bamboo flooring is 3,944 PSI and harder than Brazilian Walnut and Brazilian Teak. On Youtube Bob Banks from Build Direct claims strand woven bamboo flooring is between 2,500 and 2,600 PSI. Who is lying? 

I think that a quick step back from the Janka Hardness Scale will be most useful at this point. It seems to me that one of the major reasons why people don't buy strand woven bamboo flooring is because they believe it is not strong and hard enough for their needs. science is one of the best ways to dispel people's fears is to use science. Thus, the Janka Hardness Scale is always pulled out by marketers to show how very hard strand woven bamboo is. It doesn't matter that people don't know what the scale refers to, they are still impressed.

I don't know the exact hardness of strand woven bamboo but I do know that it is:
  • Harder than vertical or horizontal bamboo flooring
  • Hard enough to compete with most hardwoods; and definitely hard enough to be used as a material for flooring.
Wikipedia (which is not an infallible authority) gives the following ratings in pounds force:
  • Lignum Vitae 4,500
  • Brazilian Ebony 3,692
  • Bolivian Cherry 3,650
  • Brazilian Teak 3,540
  • Ebony 3,220
  • Red Mahogany 2,697
  • Maple 1,450
  • Natural Bamboo 1,380
  • Oak 1,360
  • Ash 1,320
  • Carbonised bamboo 1,180
  • Brazilian Eucalyptus 1,125
  • Cedar 900
  • Chestnut 540
A few things are worth noting. Firstly, a strand woven bamboo plank is nearly twice as dense as a normal bamboo plank. This might not double the Janka Hardness rating but it will certainly make strand woven bamboo harder than maple. Secondly, the carbonising process weakens bamboo. And thirdly, even before strands of bamboo are compressed together to make strand woven bamboo it is still harder than oak or ash.

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