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Saturday, June 16, 2012

Wood Science 101(5) - Soft hardwood, hard softwood, and vice versa

Softwood is soft...hardwood is hard. Right?

No, not really. This is by far the most common misconception non-woodites have about wood when they browse the aisles at the Big Box. And you have to ask "Well, why do they call them that, anyway?"

The most likely reason has to do with logging back in the old days. Farmers clearing their land in the east back in the 18th and 19th centuries would have encountered a great range of deciduous trees, scientifically categorized as angiosperms, those that have broad leaves, true flowers, have their seeds enclosed in a fruit, and shed their leaves in the fall (they are deciduous). The soil of the northeastern part of North America was typically thick and rich in the valleys, because of the ancient age of the Appalachian mountains and the temperate climate that inhibited frequent and large wildfires. The result was a widely ranging deciduous forest, and the varied species that made them up consisted of a large percentage of oak, hickory, and maple. The oaks and hickories were spread far and wide by animals that loved the mast (nuts) produced by them, and recycled them periodically in their ramblings.

Seed cases, or samaras, of Norway Maple (Acer platanoides).
Source: http://en.wikipedia.org/wiki/Maple
Maples, on the other hand, are prodigious self-seeders; their seeds are encompassed in a light fruit sack that has wings which take the seeds on a flight of the wind's fancy.  They also are powerful stump sprouters, and reproduce themselves easily even when the farmers cut them down for timber or firewood. So, maple trees are everywhere in the Northeast, and provide it with lots of syrup in the early spring and color in the fall.

Now, the folks that were out there clearing all these oak, hickories, and maples with axes and two-man saws, and shaping them for utensils with draw knives, found them pretty tough customers. The oaks and hickories, in particular, are heavy woods, going from 60 to 70 pounds per cubic foot (960 to 1120 kilograms per cubic meter). And the folks down south, who were harvesting live oaks for ship timbers and bows, really had a chore...live oak is the heaviest hardwood in North America, running well over seventy pounds per cubic foot (1120 kg/m3) when green.

Ahhhh...cottonwood cool.
Source: http://www.turkeycountrymagazine.com/
Apparently, these "heavy" species (technically, the ones with the highest "density") left enough of an impression on these early pioneers that they generally thought of these deciduous angiosperms as "hard" wood, even though other species, such as cottonwood, aspen, American elm, and American chestnut, which were common back in those days, were quite a bit lighter. The aptly named cottonwood weighs less than 60 pounds per cubic foot (960 kg/m3) when green, and whittles easily with a dull pocket knife, as I found out a long time ago. For this reason, cottonwood has always been one of my favorite trees...its leaves fan the air on hot Texas summer days when there is no air, and thereby help perpetuate the state of mind of a twelve-year-old boy that "it ain't so hot out here..."

Now, those old-timers generally didn't talk about different woods like scientists. They didn't have time or mental energy to waste thinking about the relative variability of wood properties expressed in different angiosperms at different moisture contents or growth rates. They just knew that the deciduous trees really wore out their saws and axes, and their muscles...so they got in the habit of calling them "hardwoods".

Yes, white pines (Pinus strobus) get big.
Source: http://en.wikipedia.org/wiki/Pinus_strobus 
As opposed to the gymnosperms, which are those cone-bearing (coniferous) trees that have needles and retain them in the winter; that is, they stay green when the other trees drop their leaves. Most of the coniferous trees in the Northeast are fairly light species; the famous Eastern white pine, which was the favorite of the King's navy back in colonial days for its straight, light, yet strong wood, and made perfect masts for their ships, is slightly lighter than cottonwood at about 50-55 pounds per cubic foot (800 – 880 kg/m3) when green. Eastern hemlock (the state tree of Pennsylvania), is just a tad lighter, at 48 -50 pounds per cubic foot (775 – 800 kg/m3), as is Eastern redcedar, at about 45-52 pounds per cubic foot (700 – 830 kg/m3). Naturally, then, when they compared the heaviest Northeastern conifers with hickories or oaks, the conifers seemed light by comparison, and so became "softwoods" in the local vernacular.

But as we spread out into the rest of the country, our common man's classification system started to break down. As we harvested the Lake States to build Chicago, Milwaukee, and Detroit, we found that aspen was a pretty "soft" hardwood. And early settlers out west found abundant red alder, a light-weight hardwood that has somewhat the look, weight, and feel of Western redcedar.

But the folks harvesting the southern U.S. were really confused, because not only did they find the super-light "hardwood" species basswood and cottonwood, but they found some of the continent's heaviest softwood, of which four species, longleaf, slash, shortleaf, and loblolly, are now marketed under the unifying moniker of Southern Yellow Pine. Southern pine not only has a relatively high density when dry (try driving a nail into a southern pine stud with ten or more growth rings per inch, and you'll bend a few nails) but it also has a resinous "pine tar" that served navies well in the wooden ship days (naval stores were buckets of pine tar and turpentine that were used to caulk seams and cracks in hulls, and seal wood from moisture), and this pine tar, or "pitch" retained moisture in the stem and added even more weight to the wood. Resultantly, old southern pine trees could yield pitch-filled logs that could weigh as much as the lighter oaks even though the specific gravity, the weight of a wood species relative to the weight of water, is quite a bit lower than those tough old oaks and hickories.

Microscopic view of southern pine. Koch, P. 1972a. Utilization of the southern pines. I. The raw material. USDA Forest Service Agricultural Handbook No. 420. 733 pp. 
As you can see in the above picture of southern pine cell structure, softwoods are comprised of long, thin tubular cells, and it is these that carry the water through the stem of the tree. It is this uniformity, in addition to the density of the wood, which makes softwoods seem relatively soft when being sawn or machined.

On the other hand, the moisture is transported in hardwoods through larger diameter pores, or vessels. These come in different shapes, sizes, and locations in the different hardwood species, and this variation contributes to the woodworker's sense that certain hardwoods are rough, or "hard" to machine.

Microscopic view of white oak.  http://classes.mst.edu/ide120/lessons/wood/cell_structure/index.html 

Nowadays, wood "hardness" is complicated (or, depending on your point of view, simplified) further by hardness standards developed and adopted for wood grading for different products. The most commonly used hardness metric used in the various wood industries is the "Janka-ball" hardness test, which is the amount of pounds-force (lbf) or newtons (N) required to imbed a .444″ (11.28 mm) diameter steel ball into the wood to half the ball’s diameter, as specifed in ASTM Standard D143. These standardized results are then used as a relative measure of the hardness of a wood, the results of which are fairly easy to find, such as at this Wikipedia page. If you browse the Janka hardness table, you'll see that the hardest woods are tropical hardwood species, but then below that, softwoods and hardwoods are relatively randomly mixed.

So, now you know the rest of the story...that hardwoods aren't necessarily hard, and softwoods aren't necessarily soft, and why. So next time, don't go wandering into Home Depot and start asking questions that make you look like a wood neophyte; do what I do, and pretend to actually know what you're talking about. :-)


2 comments:

Anonymous said...

Great additional information to the basic hard/softwood question. The microscopic picture are helpful in understanding how wood reacts when cut or hammered

Reamerstraff said...

A great article, but a picture of native North American maple samaras would have been more appropriate.