Continuing our series, "Getting Technical," this week, I'm going back to Oregon State University's Wood Innovation Center to talk to Scott Leavengood, about wood hardness. Scott has agreed to offer up a multi-part miniseries on wood hardness. This week we're going to start by discussing Janka generally and then we're going to go in depth, really "hit the point" into the wood. (That word play that makes more sense when you consider what the Janka test really is…)
The tip of a Janka test ball.
When it comes to flooring, there's one physical characteristic that seems to be the first question many people ask: What's the hardness? Most folks seem to feel that's going to indicate the performance of the product. Certainly, most of us understand that the quality of manufacturing, particularly the quality and type of finish, will be more important in the long term than the wood itself, but we still need to understand hardness basics. So how is it measured? And how reliable and valid are the test results? Scott and I take a shot at addressing those questions in this and subsequent posts. And to the readers, please know while we may get a bit technical at times, you're going to want to pay attention to this series-I think some of the test results that Scott and his team did specifically for this series will really interest you!
Scott, thanks for doing this. So let's start by just looking at "Janka." Can you give me something of the history of the test? For example, what does the name mean? And how is it pronounced properly? Hard J or soft? "jan-ka or" "yank-a?"
Well, everybody I know calls it the Janka test with a hard J, however, given that it's named for an Austrian, I wonder if the Y isn't more accurate. Maybe it's like "rodeo"- ask a Texan how to pronounce it, then ask a native Spanish speaker... And according to everyone's very fast reference source, it's related to the name "John," which would imply the softer version, as well.
The Austrian in question is Gabriel Janka, who worked for the Forest Products Lab of the U.S. Department of Agriculture (USDA). He was asked to scientifically measure the hardness for U.S. hardwoods. He developed this test that we use to this day (since formalized by the American Society for Testing and Materials (ASTM)).
OK, so how are we going to test for this?
In many tests of material properties, we measure one property to serve as an indicator of some other property of interest. For example, veneer mills often measure how long it takes for a sound wave to travel from end-to-end on a sheet of veneer; in simple terms, the faster the travel time, the higher the density of the wood and therefore, in general, the greater the strength. This technique is used to grade veneer for engineered wood products, like the flanges on wood I-beams used for floor joists.
Hardness, however, is one of those properties where we essentially directly measure the property of interest. With a wood floor, one critical performance criteria is the likelihood of the wood denting due to pressure from a stiletto heel, the tip of a small chair leg, a rock stuck in the sole of a shoe, etc. And so the standard test methods essentially replicate that process by embedding a metal ball into the wood and then reporting the force required.
So I can take any ball bearing and try to push it into wood?
As with every good scientific test, you want to follow an established standard so that your test results can be duplicated by others. We use the ASTM's standard D143 for measuring the mechanical and physical properties of small clear specimens of wood.
The standard specifies using "clear specimens" to avoid the variability that can occur due to features such as knots, pitch streaks/pockets, decay, etc. While we rarely use "small clear specimens" in practice, we have greater assurance that we can compare species since the published test values were from tests conducted on clear materials. That is, it would make little sense to compare the hardness of red oak measured on a knot vs. the hardness of hard maple measured on clear wood.
So what are the specifications for a Janka test?
ASTM D143 measures hardness by measuring the weight force required to embed a 0.444 inch diameter ball to half its diameter into a test specimen. (See the image at the top of the blog? Note, the collar ensures the technician is able to stop the test when the ball is embedded to half its depth.) Of course we want to be more specific: The test standard further states that the load shall be applied continuously at a rate of ¼ inch per minute; test specimens should be 2" thick x 2" wide x 6" long and the test is to be conducted twice on a radial surface, twice on a tangential surface, and once on each end; the reported value is then the average of the six values (four on surface/edge grain plus two on end grain).
Got it. What kind of results might we get?
The USDA Wood Handbook: Wood as an Engineering Material lists the side hardness (average of radial and tangential-but not end grain) values for dozens of wood species. For example, at 12% moisture content, the hardness for northern red oak is 1290 lbs., for sugar maple it's 1450 lbs., and for basswood it's 410 lbs.-so now you can stop wondering why we don't see more basswood floors!
Do a Google search on "Janka floor rating" and you'll see hundreds of colorful charts comparing species. Remember though, these are averages only and that many other factors are going to go into the final flooring performance.
Thanks, Scott. Next week, let's look at some actual tests.