Wood hardness testing

Looking around on the web, there is the well established Janka hardness test. This test involves pushing a steel sphere of 11.28 millimeters half way into a piece of wood. Doing this requires a specialized machine that can apply several tons of force while measuring that force precisely.

Test procedure

Because the size of the tip of a Robertson screwdriver is standardized, to reproduce the test results, all you need is a Robertson screwdriver of the same style, or at least one of the same weight. I weighed mine at 92 grams (3.25 ounces). The tip of a #2 Robertson is nominally supposed to be .125" (3.17 mm), though they typically are a little bit less than that to go into the screws more easily. Mine measured .118" x .118"

I ended up putting a small hook in the back of my screwdriver to allow me to drop it more precisely. I also put a mark on the wall for where to hold the screwdriver, and a mark on the baseboard for where it lands if dropped carefully.

It's important to make sure the sample being tested is on a firm base, especially for small pieces of wood. I rested the wood on some pieces of solid steel, and then weighed the piece of wood down with another piece of steel to make sure that it was making full contact with the piece under it. Otherwise, for a light piece not well supported, the screwdriver's momentum might get absorbed by moving the piece of wood instead of damaging the surface.

It's more important for the base to be heavy than firm. As long as the piece of wood and the base it's resting on weigh 20 times as much as the screwdriver, it should be ok.

For furniture and floors, the main advantage of hardwoods is that they dent less easily. Characterizing the size of divot from a standardized size impact is a more realistic measure of how the wood will stand up to daily abuse than it is to force a much larger steel sphere half way into the wood as is done with the Janka hardness test. So how well the wood stands up in these tests should be at least as good a measure for durability than Janka hardness test scores.

Most of the divots I made were deep enough that I could get a good measure of them with a digital caliper. I usually did four drop tests, just to average out variations that might occur from hitting different parts of the wood's grain.

My test does give a small advantage to more resilient materials compared to the Janka hardness tests, because I measure the depth of the divot after the impact. Rubber would do well in my test because it bounces back after impact. But for furniture, and for flooring, material that indents and bounces back also has an advantage, so the fact that my test rates resilient materials higher only makes it a better indicator of how well a material will stand up to daily abuse.

For some African hardwoods, and non-wood materials, I had to resort to using a dial indicator, and slowly moving the wood back and forth to figure out the depth of indentation. For example, for this piece of Lignum vitae at left, the divots were only .002" to .004" in depth. Less deep than the divots I got when I tested my method on brass (.005") and aluminium (.005"). Lignum vitae is incredible stuff. It's no wonder that wooden bearings from this wood would outlast ones made of brass or steel for some applications.

Test results

Material	Divot depths (multiple of .001")					Average depth
Spruce (very light)	155	130	140	148		143.3
Spruce (light piece)	124	127	125	129		126.3
Spruce	116	104	107	108		108.8
Pine shelving (white pine)	105	95	90	104		98.5
Ikea shelf (pine)	88	84	97	84		88.3
Spruce (many rings)	98	90	83	79		87.5
White cedar (Tuja)	94	94	71	86		86.3
Mahogany	72	83	85	92		83.0
"Pine" shelving from brazil	79	70	85	66		75.0
Spruce - tight growth rings	86	65	67			72.7
Mahogany (light red)	63	71	71	79		71.0
Teak	83	66	57	64		67.5
Spruce (heavy piece)	60	68	65	73		66.5
Red oak	68	72	53	48		60.3
Walnut	79	57	51	50		59.3
Spruce (heavy piece)	50	63	58	64		58.8
White oak	56	64	46	60		56.5
Cherry	54	53	61	56		56.0
Dark mahogany	71	56	49	39		53.8
Particle board	52	45	53	65		53.8
Yellow birch	47	52	57	58		53.5
Ash	53	52	48	60		53.3
Russian birch plywood	50	49	48	48		48.8
Cork flooring	54	49	43			48.7
Cherry	47	46	47	52		48.0
Maple	39	49	53	48		47.3
Beech	47	46	34	55		45.5
Maple	44	46	44	41		43.8
Ash	46	44	44	32		41.5
White oak	33	39	40	41		38.3
Cherry	40	37	32	40		37.3
Crabapple	35	27	36	37		33.8
Oak flooring	32	32	31	32		31.8
MDF	26	27	31	35		29.8
OSB	24	40	21	19		26.0
Hard maple	21	30	35	17		25.8
White ash (very dense)	26	25	20	28		24.8
Coated MDF (plaque)	26	22	14			20.7
Melamine counter top	17	22	23			20.7
"Ironwood"	14					14.0
Brass	5					5.0
Cast aluminium	5					5.0
Cocobolo	4	5	5			4.7
Lignum vitae	4	3	2			3.0
Mild steel	2	2				2.0

Some notes       For many woods I tested more than one piece. The depth of divot on individual pieces usually was quite consistent, but from piece to piece, I got quite a bit of variation. Species of wood that I tested more than once are listed multiple times at left.

Results

My softest piece of spruce divoted .143", while the hardest divoted only .058". What's interesting is that my heaviest pieces of spruce beat out several hardwoods for hardness, including some samples of walnut and red oak.

The hardest piece of spruce is the blue stained piece at the top, labeled '5'. My softest and lightest is the one below right.

I had already started using heavy pieces of spruce in some jigs where hardness was called for, but I didn't want to splurge on hardwoods. Now I feel justified in doing this. Some pieces of spruce don't just feel hard and heavy - they really are as hard and heavy as hardwoods.

But it wasn't just spruce that showed a lot of variability. In general, any species that I tested more than one sample of showed quite a bit of variation. For example, the "Red oak" and the "Oak flooring" pieces were both red oak, but varied a lot in hardness.

What also surprised me is how well plain old MDF and particle board performed in the test. I personally rather dislike particle board and MDF for making jigs and such, but the MDF did better than any non-exotic or rare species of wood that I tested.

Conclusions

So when you need to select lumber for hardness, it may be wise to just test it on the spot by dropping a screwdriver on it. It's a quick and easy test, and it only does relatively minor damage to the sample.

You can also download my data as an excel spreadsheet

There is also a German version of this page.

See also:

wood grain macro photos

Which wood is strongest, hardest, stiffest

Seasonal wood shrinkage