Wednesday, Woody Wednesday: Get to know redbay while you still can.

Now that the fantastic insanity of the holidays has passed, my son has recorded three weeks outside the womb, and I finally have a phone again, I’m happy to resume my regularly scheduled Wednesday programming. In this week’s edition of Woody Wednesday, we’ll take an in-depth look at yet another southern forest resident that has been decimated by a foreign invader. Not as romantic as the stately chestnut (already gone), the ubiquitous and eminently useful ash (on the way out), the picturesque hemlock (give it a decade, at least in the South), it may be one you’ve never heard of: the redbay (Persea borbonia).

Redbay is the wallflower of the southern coastal evergreen hammocks. It does not grow to the impressive architectural proportions of live oak and southern magnolia. It does not have the fragrant white flowers of sweetbay and loblolly-bay. Nor does it have the bright red berries of American holly or yaupon. If you weren’t specifically looking for it, you’d scarcely even know it was there. However, if you are on the margins of a coastal plain wetland anywhere from Texas to North Carolina, it’s likely that you’re not too far away from one of these diminutive evergreen trees.

Redbay is an occasional tree in the coastal plain from Texas to North Carolina, but it seems to be most common on the Atlantic coast of Florida to South Carolina.

What redbay does have is a powerful and pleasant spicy aroma in the leaves, bark, and wood. It shares this trait with practically all other members of the Lauraceae family – bay laurel (a European species from which we get bay leaves); sassafras (sassafras tea, anyone?); spicebush (a native bush that lives up to its name); camphor-laurel (an Asian species from which we get camphor); and cinnamon-tree (another Asian species that produces cinnamon). I use redbay as a substitute for bay leaves when cooking beans and chicken stock. In fact, it’s a required ingredient in authentic gumbo.

So what is the pest that is killing off this little-known tree? It’s a one-two punch, actually. The Asian ambrosia beetle Xyleborus glabratus bores into the stems and carries with it its symbiotic fungus, Raffaelea lauricola, commonly know as laurel wilt. The fungus grows and spreads in the tree’s xylem, eventually cutting off the flow of water between the roots and the leaves. According to the USDA’s Recovery Plan for  laurel wilt, the disease “is now well established in the southeastern Atlantic Coastal Plain region of the U.S. and eradication of the vector and pathogen in this region is not feasible. Continued dramatic reductions in redbay populations are anticipated, although survival of redbay regeneration in the aftermath of laurel wilt epidemics suggests that redbay will not go extinct.” Super. So we can probably expect to redbays to exist as short-lived seedlings and sprouts, only to be ravaged by the foreign intruders once they reach a reasonable size.

Dead redbays, an all-too-common sight.

I suppose the best thing we can do as woodworkers to preserve the heritage of this tree is to make useful things out of it that will last longer than the trees themselves. And since we’re all woodworkers here, what we really want to know about is the wood. There’s not much information about the wood in the public domain, and frankly most of the information that is out there is generic, misleading, or flat-out wrong. Hopefully I can correct some of that nonsense today. The USDA silvics manual says that redbay wood “is heavy, hard, strong, and bright red, with a thin, lighter colored sapwood.”

So does that mean that redbay could be a domestic substitute for bloodwood? Hardly. In fact, redbay is neither hard nor heavy nor strong. Though the heartwood is indeed reddish (similar in color to cherry or mahogany), it does not begin to approach “bright red” in color, nor is the sapwood band particularly thin. Was the technician who wrote this stuff just making shit up? If you want the straight dope regarding a tree species, you can do no better than to listen to urban forestry professor extraordinaire Kim Coder [from his publication Redbay (Persea borbonia): Drifting Toward Oblivion]:

Redbay wood is difficult to find in the commercial lumber or hobbyist marketplace, and then only in small pieces. As such, redbay has only limited local use as a wood material. Heartwood is redcolored, fine-grained, brittle, water resistant, works moderately well and polishes very well. It was traditionally used for tableware (like spoons), furniture pieces, boat and interior trim, and cabinets. It was gathered for boat trim in the live oak maritime forests during the live oak gathering days of early sailing vessels.

Alright, that’s a little more informative, and it’s definitely more accurate. Redbay is pretty common in the woods here on Amelia Island, so I’ve been carving spoons with it for the last couple of months, and I’ll share a bit of my personal experience with it as well. (By the way, how cool is it that Dr. Coder mentions that the wood was traditionally used for spoons? Score! It is indeed a fine spooncarving wood).

The wood is very much unlike most temperate hardwoods in that it combines large pores with a diffuse-porous wood structure. Typically, large-pored temperate hardwoods tend to be ring-porous or semi-ring porous. It definitely has the appearance of a tropical hardwood. In fact, it would easily pass as African mahogany (Khaya) to the untrained eye. Or shoot, even the well-trained eye.

Reddish heartwood, large pores, and diffuse-porous structure give the appearance of a tropical hardwood, like Khaya.

It’s easy to see why this wood was favored for the interior trim of boats. Not only does it have an attractive appearance, it works quite well also. The density and hardness remind of Honduran mahogany or butternut. Which is to say, it is not very hard or dense at all. It carves very easily and takes a nice polish straight from the tool – as long as you’re cutting with the grain. It does tend to tear out around grain reversals, and the grain can be quite wavy.

Check out this curly grain. Beautiful stuff, but tricky to work. The straight-grained wood is dreamy, though.

Probably the most unfortunate thing about redbay wood is the fact that it does not have a significant amount of heartwood until the tree gets to be quite large in size. A 12″-diameter tree is likely to have a hardwood core only 8-9″ wide. Since larger trees have mostly been killed off by the laurel wilt in many parts of the South, this means that you’ll likely be relegated to working with the sapwood.

Not all is lost, however. As long as you are working with fresh wood, the sapwood is creamy white and attractive, not unlike walnut sapwood.

This ladle was carved from fresh sapwood; there is just a bit of pinkish heartwood in the center of the bowl.

If the wood sits around for a couple of weeks or more, however, the pores begin to turn brown, which gives the wood a grayish pallor when viewed from more than a few inches away. I waited a bit too long to carve these eating spoons, and it shows:

You can really get an idea of how large the pores are in this photo. They are quite prominent for a diffuse-porous wood.

Luckily, all eating spoons begin to take on the same brownish tint with age, so all I have to do is use these and eventually the color will improve.

They are nicely shaped spoons, after all.

So there you have it. Redbay is a disappearing tree, but it’s a fine tree for woodworking. I can personally vouch for its pleasant nature for carving, and historically, it was commonly used for cabinetry and trim (especially in boats and ships). If you happen upon some, why not try to make something from it? It’s one way to preserve this stuff for the next generations, who may not be able to enjoy the trees in the same way that we do. I’d certainly love to get my hands on some trees that are large enough to mill for lumber, since a well-built piece of furniture is likely to last longer than my wooden spoons, but my window of opportunity seems to be rapidly closing.

I’ll leave you with a few more words from Dr. Coder:

Redbay is a biological, ecological, and cultural treasure of deep woods on the edge of an ecological precipice. People of the Southern and Southeastern coasts of the United States have been blessed with redbay along wetland edges. Coastal development, forest changes, and new pests are placing redbay under more stressful conditions. This burial tree of Native Americans, this historic wood of polished trim for captain’s cabins on Yankee clippers, and this special food and home for several rare butterflies is being pushed farther into oblivion.

This unique tree species is now under attack from new pests which could destroy this old flavor of Southern gumbo. Understanding how redbay grows and how to identify the tree may help to combat threats as well as appreciate what we have always had but may have overlooked. Care is needed to sustain our redbays for the next generation.

Wednesday, Woody Wednesday: How Does Steam-Bending Work?

Last night I set about building the steam-bending rig for the crest rail for my Windsor chair. Steam-bending is one of my favorite processes in all of woodworking. There is something that feels heroic about taking a piece of 3/4″-thick white oak and bending it as though it were a popsicle stick. The simplicity and integrity of creating graceful curves by steam bending fall very much in line with my philosophy of woodcraft. The alternatives, for lack of a better word, suck.

Sawing the Curve: Requires thick wood and results in a weak piece and lots of waste.

Laminating Thin Plies: Requires a lot of machining, a lot of glue, and a lot of clamps.

Splitting from a Bent Log: Strong piece with continuous grain, but a lot of work to find the log and hew the piece.

Steam-Bending: Minimal waste, few tools, and strong, straight grain. It’s a winner.

I could have sawed out the curve for my crest rail from straight wood, but that would require a huge chuck of oak – 3 1/2″ thick – and a comparably huge saw. And worse, it would result in a piece with inferior strength due to short grain weakness near the ends.

I could have laminated the bend from multiple thin plies, but that would require a lot of glue, a lot of clamps, and (the worst part) a lot of machining to make those thin plies. After all that work, piece would be ugly because of all the glue lines. On the plus side, it would result in a piece with comparable strength to a steam-bent piece.

Or I could have split the crest out from a piece of wood with the appropriate bend already in it (like I did with my ladle) but then I’d have the trouble of finding the right piece of wood. And good luck finding the right tree if you decide to build a balloon-back.

See the back? Trees don’t grow like that.

With steam-bending, I can procure my stock from a straight-grained log with a wedge and a froe, shape it with a drawknife, and bend it with a steaming rig that I built using a few simple items from the hardware store (look for a post on my steam-bending rig before too long, though there’s plenty of information on the web. Nothing special about mine). Essentially, I can re-write the tree’s history and make it believe that it did, in fact, grow in the shape of a ballon-back (or almost any other shape I choose).

If you weren’t convinced of (or aware of) the simple beauty of steam-bending curved pieces before, I hope that you are now. But just how does steam-bending work? Glad you asked.

For this discussion, it will be critical to understand that wood is not a homogenous material. It is a composite material. Wikipedia defines a composite material as “a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components.”

Composite materials most often comprise fibers to impart strength and stiffness with a matrix that binds the fibers together and provides form and additional strength to the material. Some familiar examples of composite materials are adobe (grass/straw fibers in an earthen matrix), fiberglass (glass fibers in a plastic resin matrix), and steel-reinforced concrete.

Adobe: the earliest manmade composite material.

The primary constituents of wood are cellulose (40-50%) and lignin (25-30%), and hemicellulose (25-35%). Cellulose is a strong linear polymer that forms the majority of the cell walls of wood and all other plant parts. It forms the “fiber” of the composite material. Lignin is an amorphous polymer that fills the spaces between the cells walls, binding them together, which strengthens the wood. It forms the “matrix” of the composite.

Going back to an earlier example of a composite material, let’s visualize wood as steel-reinforced concrete. (Not because it’s the most accurate analogy, but because I assume almost everyone is familiar with it). The steel (like the cellulose) has more tensile strength than the concrete (like lignin). Suppose you have a straight steel-reinforced concrete post, but you need a bent one. What will happen when you bend it? The concrete will fracture, causing the post to fail. I’m assuming you have a method of actually bending steel-reinforced concrete, of course.

Sorry, earthquake. Concrete has low tensile strength. You’ll need to find another way to get a bent post.

Same thing with wood. Most cross-grain failure are the result of ruptures in the lignin that then propagate through the wood. But what if we could plasticize our concrete, bend it while it’s soft, and then allow it to re-harden? The steel, with its high tensile strength, would be happy to comply. Unfortunately, concrete can’t be plasticized. But lignin can.

At normal temperatures, lignin locks the cellulose in place, preventing the strands from sliding past one another. This gives wood some of its characteristic stiffness. However, if you heat it to about 200°F, a very useful thing start to happen: it gets very soft and pliable, just like a plastic. If you cool the lignin back down to room temperature and dry it out, it will tend to retain the bent shape.

This, at its heart, is the basic gist of steam-bending: Heat wood to about 200°F to plasticize the lignin, bend the wood to a desired shape, and let it cool and dry out. When it is completely dry, the wood can be removed from the form and it will hold its shape.

Unfortunately, wood is a complex material and it doesn’t necessarily make things easy on us. Lignin tends to lose its plasticity when it has been dried beyond a certain point. There is no specific point at which wood is no longer useable for steam-bending, but if the moisture content drops below about 12%, your odds of bending it successfully will be severely diminished.

Kiln-dried wood – wood that has been heated to a temperature of 140-160°F and dried to 6-8% moisture content – is next to useless for steam-bending. Air-dried wood is ideal, and the bend is most likely to be successful if the wood is near the fiber saturation point (25-30%).

In addition to loss of plasticity, another reason for unsuccesful bending in drier wood is its lack of conductivity. You probably remember from middle school science class that wood is a poor conductor (i.e., a carrier of heat and/or electricity) while water is a good conductor. When wood loses its water, it also loses the primary medium by which it transfers heat from the outside of the piece to the inside. Green wood is far more conductive than dry wood. If the center of the workpiece is not heated to the appropriate temperature, the bend is likely to either splinter while bending or fail to hold the desired shape.

Not surprisingly, then, thinner pieces of wood are easier to bend successfully than thick pieces. It’s simply much easier to thoroughly heat a 1/4″-thick board than a 2″-thick board. In fact, if your pieces are thin enough, you don’t need steam at all. I think this is a rather common misconception. It is the heat, not the moisture, that plasticizes the lignin. The steam is simply a very efficient carrier of heat, and it has the added benefit of being the right temperature.

Guitar sides are traditionally heated and bent around a hot iron pipe, and crooked chair spindles may be heated and straightened with a heat gun. The downside to these methods is that it can be much easier to scorch your pieces if you aren’t careful.

Guitar sides are bent around a hot iron. (photo used with permission from Haze Guitars)

A heat gun can straighten crooked spindles (or arrows, if you’re a bowyer).

Not all woods are created equally when it comes to steam-bending. Far from it, in fact. Generally speaking, softwoods are rated poorly for steam-bending. That’s not to say that they won’t bend – simply that you must take more steps to ensure success – thinner pieces, straighter grain, longer steaming times, and/or milder bends will improve your chances of success. Most steam-bending is done with hardwoods.

I wish I could offer you some scientific explanation as to why softwoods don’t bend as well as hardwoods (since that’s kind of my thing) but unfortunately I simply don’t know. Just a wild guess: It might have something to do with tensile strength, since the outer circumference of a bend will be under severe tensile stress until the piece sets.

In the 1962 classic “Machining and Related Characteristics of United States Hardwoods” (E.M. Davis, USFS Forest Products Laboratory), the author tested 25 species of hardwoods for steam-bending characteristics and concluded: “Specific gravity influenced bending, in that the heavy woods bent better than the light woods. In table 21, for instance, all the heavy woods (those with a specific gravity of 0.50 or over) except hard maple are in the upper half, whereas all the light woods (those with a specific gravity of less than 0.40) except willow are in the lower or the poor half. No consistent differences were noted in breakage be tween light and heavy pieces of the same wood.”

As we’ve previously discussed, there is a strong correlation between modulus of rupture and density, so perhaps the correlation that Davis noted is partially related to tensile strength? Feel free to send me any resources on this topic if you know of any.

Davis also noted “Ring-porous woods as a class gave better results than did diffuse-porous. The best 4 woods are all ring-porous, and 8 of the 10 ring-porous woods were among the best 12 woods.” Again, this may be an example of correlation and not causation, since the strongest/densest woods are mostly ring-porous.

But who knows? Perhaps there is a logical link between porosity and bending that I am unaware of. I’m including the entire publication on the Wood Properties Resources Page for perusal at your leisure, for anyone interested.

I also made this handy-dandy table. The left two columns (species and successful bend %) are from Davis’ report. The additional data (pore structure, specific gravity, and modulus of rupture) are from various other sources. Mostly the USFS Forest Products Laboratory. Where a range is given, it is because the wood type covers multiple species.

There is plenty more to write on this topic, but that’s all for now. Time for me to soak my aching typing fingers. Thanks for joining me for another steamy edition of “Wednesday, Woody Wednesday”.

Wednesday, Woody Wednesday – How Does Growth Rate Affect Strength and Density?

I’ve written before about the strength of various different wood species. One shortcoming of any discussion of strength by species is the unavoidable fact that the same species can produce wood with very different strength properties, dependent upon a number of factors. Today, we’ll discuss an important factor affecting wood strength within a species: growth rate.

It is commonly – and incorrectly – assumed that faster growth inherently results in weaker wood. As we’ll see, this is hardly the case. The reason that this myth has become so popular is primarily because it is true for certain species that also happen to be among the most common woods that a layman is likely to use. Let’s start by breaking up some common domestics species into groups based on their anatomical similarities.

The first division we can make is into hardwoods and softwoods. Or more accurately, conifers and broadleafs. Or most accurately, gymnosperms and angiosperms. But I digress. We’re woodworkers, and we tend to refer to gymnosperms (trees that don’t have flowers) as “softwoods” and angiosperms (trees that do have flowers) as “hardwoods”. That’s because common softwoods, like white pine, spruce, fir, and cedar, do indeed have softer wood than common hardwoods, like oak, beech, maple, cherry, and walnut. But there are confusingly some hardwoods, like basswood, willow, and balsa, that have wood much softer than some softwoods, like southern yellow pine and yew.

So we have established that the anatomical difference between hardwoods and softwoods is not hardness. What is the difference? Pores. Hardwoods have them, softwoods don’t. Softwood xylem tissue is composed primarily of narrow, short cells known as tracheids. Tracheids are not open on the ends, so water moves from cell to cell up the tree via small perforations known as pits. Hardwood xylem also contains tracheids, but they evolved a different cell type for transporting water known as pores. Pores are larger in diameter than tracheids, and they are connected end-to-end like a straw. As we will see, the pattern and size of these pores have a profound impact on the strength of hardwood lumber.

Softwoods have only narrow tracheids.

Hardwoods have tracheids plus wide pores.

We can further divide the hardwood group by how the pores are arranged in the xylem. Hardwoods that have an even arrangement of pores throughout the wood, regardless of the time of year that the wood was produced, are known as “diffuse-porous hardwoods”. The other group of hardwoods produce a continuous band of pores around the circumference of the trunk each spring when growth begins anew. We call these “ring-porous hardwoods”. There is also an intermediate group that we call “semi-ring-porous hardwoods” whose pore arrangement is intermediate between the two.

Red oak – ring-porous

Persimmon – Semi-Ring-Porous

Sugar maple – Diffuse-Porous

The softwoods can be divided by seasonal growth characteristics as well. In softwoods, it is the thickness of the cell walls of the tracheids that may vary with the seasons. Woods that have cell walls of consistent thickness throughout the growing season we refer to as “even-grained softwoods”. If the cell walls start out relatively thin in the spring, but become abruptly thicker later in the season, we refer to these as “uneven-grained softwoods”. This is a more subjective measure than the pore arrangement in hardwoods, and there is a wide range of overlap, sometimes even within a species.

Yellow Pine – Uneven-grained

Red Spruce – Intermediate

White Pine – Even-grained

Let’s have a look at some familiar species and see where they are classified:

Now, to the heart of the matter: How does growth rate of a particular tree affect the strength and density of the wood from that tree? I mentioned previously that conventional wisdom states that slower growth = stronger wood. In fact, this is only true for a very small group: the uneven-grained softwoods, a tiny group that includes yellow pine, red pine, and Douglas-fir, as well as a few less common species. So how did this myth become so prevalent? Well, if you were framing a house or a barn somewhere in the U.S. in the last 150 years, chances are very good that the lumber you were using was yellow pine or Douglas-fir.

The reason for the increase in strength with a decrease in growth rate is simple: the cell walls of the tracheids get thicker (and thus, denser and stronger) as the season goes on. The light wood in the two photos below is known as “earlywood” since it is produced in the spring and early summer. The darker wood is “latewood”, produced in the late summer and fall. Notice that the width of the latewood in these two specimens of yellow pine is about the same, despite the fact that the earlywood is much wider in the second sample. Since the majority of wood in the sample on the right is earlywood, the whole piece will be weaker as a result.

Slow-grown yellow pine still has a wide band of dense latewood…

But fast-grown yellow pine has a wide band of earlywood and a comparatively narrow band of latewood.

There is a limit to this correlation, however. When the rings get spaced very tightly, the latewood bands begin to get considerably narrower and the wood actually begins to get somewhat weaker and less dense. This is especially true in Douglas-fir and to a lesser extent in yellow pine. So the next time you’re picking out some yellow pine or Douglas-fir framing lumber, keep two things in mind: you want a slower growth rate, and wide bands of latewood. These two factors will be the two most important in determining the strength of the lumber (aside from knots and defects). As an added bonus, slower-grown wood tends to be more stable and workable than fast-grown wood.

What about the rest of the softwoods? Well, for the vast majority, including white pine, spruces, cedars, hemlocks, baldcypress, and true firs, there is absolutely no correlation between growth rate and density or strength. That’s not to say that density doesn’t vary widely within these species. Some of them – hemlock and baldcypress are two that come immediately to mind – can be as light as white pine or nearly as dense as yellow pine, but the dependent variable is simply not growth rate. It can still be advantageous to use slower-grown wood. It will be more stable and consistent, but it will not be stronger.

White pine density does not vary much throughout the year…

So fast-grown wood is as dense as slow-grown.

And what of the hardwoods? Diffuse-porous hardwoods and semi-ring-porous hardwoods, like even-grained softwoods, display no correlation between growth rate and strength or density. So don’t worry about how fast your maple, cherry, birch, or walnut was growing. It might change the appearance of the wood, but it won’t affect the strength.

The ring-porous hardwoods, on the other hand, pose an interesting conundrum. They actually get stronger as the growth rate increases. The reason for this is intuitive once you understand how ring-porous hardwoods actually grow. The earlywood layer consists mostly of pores – big, hollow tubes that mostly contain air. Unlike conifers, the earlywood in ring-porous hardwoods tends to be about the same width from tree to tree regardless of the growth rate. The strong, dense latewood band that is produced in the summer and fall  tends to get much wider in fast-growing trees. So we can think of ring-porous hardwoods as sort of the opposite of uneven-grained softwoods.

Slow–grown red oak has a wide band of earlywood pores and a narrow band of latewood…

But fast-grown red oak has a much wider band of dense latewood.

So should you look for fast-grown wood if you’re using a ring-porous hardwood? Well, it depends what what you’re using it for. If you’re making a baseball bat from ash, an axe handle from hickory, or a steam-bent oak rail for a Windsor chair, then absolutely, your preference should be for faster-grown wood. But keep in mind that oak and ash and hickory are already quite strong and dense, so if you’re building a normal furniture piece where strength is not a critical issue, the slower-grown wood will be easier to work and still plenty strong (and possibly more attractive, depending on your definition of attractive).

Thanks to The Wood Database for the excellent pictures. Go visit the website for a tremendous collection of pictures and wood property information.

 

Wednesday, Woody Wednesday – Understanding Shrinkage

Well, that title sounded a bit more salacious that I intended. No really, I’m writing about wood properties here, not the unfortunate results of skinny-dipping in a cold pool.

In this, our second edition of Woody Wednesday, we will discuss the one wood property that causes more ruined projects and gnashing of teeth than practically any other – its propensity to expand and contract with  changes in moisture content.

Wood is hygroscopic and anistropic. Hygroscopic means that it has an affinity for water – very helpful for a living tree, which must conduct thousands of gallons of water tens to hundreds of feet above ground through the pores of its wood to reach its leaves. Let’s pause for a moment to consider what a marvel this is. This video will help:

Those water-conducting pores are what makes wood anistropic – meaning that its properties are different depending upon direction. You can think of wood like a bundle of straws. The properties at the top of the straw bundle are very different from the properties along its sides. The opposite of anistropic is isotropic – materials that have the same properties in all directions. Examples of isotropic materials are metal and glass. On a microscopic level, you could correlate them to a jar of sand, rather than a bundle of straws.

Wood vs. Steel

Wood is like a bundle of straws – different in every direction.

Steel is like a jar of sand – the same properties in every direction.

It is this combination of hygroscopic and anistropic properties that causes wood to shrink and swell with seasonal variations in moisture content. When we saw or split wood from a log, it contains water. Lots of water. Some of this water is what we call “free” water. This is the water that was moving freely up through the pores, from the roots to the leaves. The wood also contains “bound” water – water that is chemically bonded with the cellulose and lignin that make up the cell walls.

When the wood begins to dry, the free water tends to exit the wood rather quickly. Since it isn’t chemically bound, it is free to evaporate. The point at which no more free water remains in the wood is called the fiber saturation point. Only bound water remains. This occurs at 25-30% moisture content.

What exactly is moisture content, by the way? You’ve probably heard it mentioned, have you ever wondered what it means? It’s pretty simple: moisture content, or M.C., is simply the weight of the water divided by the weight of the woody, fibrous material, expressed as a percentage. So a 100 g sample of wood at 26% M.C. contains 20.63 g of water and 79.37 g of actual wood (20.63/79.37 * 100 = 26%).

As the free water dries, the wood changes very little in size and shape. After the bound water begins to dry, however, funny things start to happen. When water that was previously bound to the cell walls evaporates, the cell walls contract. This wouldn’t be problematic if the wood shrank evenly, but wood is anistropic, so the shrinkage occurs differently depending on what plane we’re looking at.

When scientists talk about wood, we discuss three different planes: transverse, tangential, and radial. As woodworkers, we tend to refer to these planes as end grain, quartersawn grain, and flatsawn grain. There’s also a term for a plane that is in between quartersawn and flatsawn, which we call riftsawn. I’m not an artist, but here’s a drawing to wrap your mind around it:

Three wood planes: transverse, radial, and tangential.

Wood reacts very differently along these three planes. It shrinks very little or not at all along the grain. It shrinks 2-6% in the radial plane (from pith to bark), and about twice as much (5-10%) in the tangential plane (along the growth rings) from the fiber saturation point to air-dry. This calls for more custom artwork:

This differential shrinkage has a profound impact on the stability of the wood as it dries. Quartersawn wood tends to be the most stable, but it will shrink in thickness more so than flatsawn wood. Flatsawn lumber has a definite propensity to “cup” as it dries due to the differential shrinkage between the tangential surface in the middle of the board and the riftsawn grain on the edges. Something that is turned round from green wood will end up egg-shaped as it dries. Again, a picture should help to visualize these effects:

Credit: CSWoods.com

Wood is considered “air-dry” when the moisture content of the wood reaches equilibrium with the relative humidity of the air. Scientist call this the equilibrium moisture content (EMC). It can be as low as 5-6% if you live in the desert, or as high as 15-16% in a rainforest. A more common range for air-dried wood in temperate climates is 10-12%.

Now, wouldn’t it just be dandy if we could dry our wood down to the equilibrium moisture content, and then have a stable, predictable material that we could glue and screw to our hearts’ desire, without any consideration for dimensional changes over time? That would make everything so easy!

There’s just one problem. Wood will always, always, reach equilibrium with the air surrounding it. So unless you live in a temperature-and-humidity-controlled laboratory, you can expect weather patterns, seasonal changes, and modern heating and air-conditioning to cause the moisture content of your wood to vary by 3-4% in normal use. This might not sound like much, but if you don’t incorporate allowances for wood movement in the design of your furniture, the results can be catastrophic wood failure.

Putting It Into Practice

Consider the sassafras table that I built a few weeks ago. The top of the table is 48″ wide. Sassafras is a pretty typical domestic hardwood in regards to dimensional stability. It changes by about .003″ (three thousandths) per inch of width for each percent of change in moisture content. Three thousandths sounds like we’re picking nits, but the numbers multiply rapidly. If we assume that the MC starts out as 12% and drops to a minimum of 8% in the dead of winter with the heater running at full blast, that’s a 4% change in MC. Multiply .003 x 4 (% MC change) x 48′ (width of the panel in inches) and we get 0.576″ in shrinkage along the width of the table. That’s over a half-inch!

So, since the wood does not shrink along it’s length, if I had screwed down the battens without making allowances for wood movement, it’s obvious that the result would be, at best, a severely bent tabletop, or more likely an ugly split right down the middle. Wood will swell as it absorbs moisture and shrink as it releases it. It is as certain as the sun rising in the east and time slowing down as you approach the speed of light.

Since I am fully aware that my tabletop will shrink, I was able to design the battens with this in mind. I drilled elongated holes into the battens and screwed the top on using washers, which will allow the top to shrink and swell with the seasons, all the while sliding as it wishes along the battens without bending or splitting.

This is but one of many examples of designing furniture to allow for seasonal wood movement. The most famous example is the ubiquitous frame-and-panel door. The narrow members of the door experience very little movement, but they house a wide panel in a groove that is free to shrink and swell as need be without affecting the dimensions of the door. If doors were designed from solid wood, they wood stick shut as they swelled in the summer or leave ugly gaps as they shrunk in the winter.

Credit: canadianwoodworking.com

Okay, so now you have a basic understanding of why it’s necessary to design your furniture with wood movement in mind. It’s beyond the scope of this (already long) article to discuss any more specific techniques for addressing wood movement, but a basic understanding of wood movement and a little common sense can go a long way to avoiding furniture failures.

I will tell you that I typically take a very unscientific approach to addressing wood movement myself. I know that my wood will shrink when I move my furniture from my un-heated, un-cooled workshop to inside my house, so I plan for it. I typically figure that the wood that comes out of my shop will be around 11-12% MC, but I never measure it. I have probably used wood as high as 14% MC, but since my furniture is designed with wood movement in mind, this is never problematic in practice.

The woodwork inside my house is generally around 9-10% MC. Maybe a percent drier in the midst of winter, or a bit wetter when the windows are open during the spring. Figuring on a maximum change of 5-6% is generally pretty safe. But again, I don’t bring any numbers into my figuring. Wide panels get more allowance than narrow members. Experience is my guide. If you are uncomfortable with this seat-of-the-pants design, then Popular Woodworking has a great online resource for calculating wood movement. Give it a shot.

Also understand that the numbers for EMC in my neck of the woods may be vastly different in your part of the world. Check out this page for typical EMC in different cities in the United States.

Finally, if discussions of equilibrium moisture content, hygroscopicity, and transverse planes has really whet your appetite, the Wikipedia page on Wood Drying is actually quite excellent.

 

 

Extra Credit

It’s one thing to design your furniture such that we avoid problems with wood movement. But what if we could design furniture that actually takes advantage of wood movement? Wouldn’t  that be something?

In fact, woodworkers have been doing exactly that – for centuries. But that’s a topic for another day…

Wednesday, Woody Wednesday – What’s Up with Maple?

Every good blogger should have their schtick – something that they do better than anyone else to differentiate themselves from the crowd. I’m not a good blogger, but in my ongoing quest to pretend to be one, I’ve decided to leverage my wood properties skills as an ongoing feature. Surely that will catapult me to the big-time, right? Everyone loves wood properties. Thus, without further ado (and with apologies to Bono) I hereby decree that Wednesdays shall henceforth be designated “Wednesday, Woody Wednesday“.

My plan is to discuss a different wood-centric topic each Wednesday. Some days we might cover a specific wood property that is relevant to woodworking. Other days we might examine a particular species of wood in scrutinous detail. In any case, I hope to keep the posts interesting, useful, and super-geeky. (Yes, I did just use all three of those words together.)

What’s Up with Maple?

In our inaugural edition, we’re going to be talking about maple. Most woodworkers – at least, American woodworkers – will recognize only two varieties of maple: hard maple and soft maple. It’s a simple classification, but I’ll argue that it is not just overly simplistic, but flat-out wrong. I believe that there is value in knowing your maple down to the species, and I’ll do my best to prove why. Now, I’m aware that it probably isn’t possible if you’re simply buying boards in the form of lumber (unless you have a good sawyer), but it would certainly be prudent for any green woodworkers out there to make sure your dendrology skills are up to snuff.

So, what exactly are hard and soft maple? Let’s refer to a publication on the maple genus from the venerable Center for Wood Anatomy Research at the U.S. Forest Service:

The Maples can be separated into two groups based on the ray widths of their microscopic anatomy, the soft maple group and the hard maple group. Species within each group look alike microscopically.

Specifically, the microscopic difference between hard and soft maples is this: If you take a clean cross-section of end grain and examine it with a microscope or a 10x loupe, the rays of a soft maple will all be narrow and of uniform widths. If you examine a sample of hard maple, the rays will be of two different widths: some will be narrow, but some will be quite wide and prominent.

Get a clean cross-section of the end grain to see the difference between hard maple and soft maple. Click on the photo for a larger version. End grain photos are from http://www.wood-database.com

So what, exactly, does ray width have to do with the relative hardness or softness of the wood? Absolutely nothing. The terms “hard maple” and “soft maple” are American construct that were simply meant as a shorthand for differentiating the most common timber-sized maple species in the Eastern U.S. Referring to the same USFS publication, we see that they only classify five species, all of them native to the eastern U.S., as hard or soft maple:

The wood of sugar maple and black maple is known as hard maple; that of silver maple, red maple, and boxelder as soft maple.

And yes, in general, sugar maple and black maple are quite a bit harder than red maple, silver maple, or boxelder. But it’s a big world out there, and those five species are hardly the only ones out there (Wikipedia says there are about 128 maple species, in fact). The waters get considerably muddier once you venture out to western North America or across the pond to Eurasia.

We’ll only discuss eight species today, but they are the most common maple species that English-speaking folk (and therefore, people who are likely to be reading this blog) will encounter in lumber-sized trees.

Three additions to the aforementioned five are: 1) Norway maple, which has a huge native range that extends from Scandinavia eastward into Russia, and as far south as northern Iran. Norway maple is a familiar (and invasive) ornamental species in the northeastern U.S. 2) Sycamore maple, more likely familiar to the Brits as simply “sycamore”, although that’s confusing to Americans, since we refer to an entirely different genus as sycamore. Sycamore maple is native throughout Europe and naturalized in Great Britain. And 3) Bigleaf maple, which grows along the Pacific coast from the southern tip of Alaska to the Sierra Nevada of California. It is the only commercially important maple of the western U.S.

Okay, then. We have a list of important maple species and their corresponding wood properties¹. What should we care about? Well, since we’re talking about hard vs. soft maple, let’s start by ranking the species according to their hardness: 

Well, dang. It would appear that the “hard” vs. “soft” distinction is vindicated by this graph. Sugar maple is head-and-shoulders above the pack, a full 1200 Newtons (almost 25%) harder than black maple – the other hard maple. The soft maples (red, silver, and boxelder) comprise 3 of the bottom 4. But notice the degree of separation between red maple and its fellow soft maples. The gap between red maple and boxelder is as large as the gap between red maple and black maple. Red maple, along with the European species, seems to be in more of an intermediate territory between the hardest and softest maple species.

Now, hardness is all well and good. It’s the ultimate wood property of concern, if you’re building a bowling alley or a basketball court. But how many of us are actually doing that kind of work? For most woodworkers, hardness is a mixed blessing. Sure, sugar maple is less likely to dent, but it’s also much harder on your tools. What we as furniture makers (and particularly chairmakers) tend to be more concerned with is the strength of the wood – and that gets to the heart of my complaint with the whole “hard” vs. “soft” distinction. Hardness seems to get conflated with strength, but is that really appropriate?

Well, no. It isn’t. Let’s look at how these species rank with two common measures of strength: modulus of elasticity (MOE) and modulus of rupture (MOR) [here’s a link that includes an explanation of these properties if you need it].

 

Intersting, no? Yes, sugar maple is still the king of MOE, but look who’s sitting at number 2: red maple. Ahead of the European maples, and even slightly ahead of black maple. And what about MOR? The king has been displaced by a European interloper. Sugar maple sits somewhere between Norway maple and sycamore maple in ultimate breaking strength, and not far behind are red and black maple. Moreover, look how poorly silver maple and boxelder perform on both of these tests. Does it make any sense at all to include red maple in a group with these impostors? I would argue that it does not.

So, to wrap up my thoughts, let me just say that, from now on, I will be silently cringing any time a woodworker or wood peddler refers to their maple as “hard” or “soft”. (If you catch me on a bad day, there may be less silent cringing and more vocal argumentation.) Yes, it’s clear to me that sugar maple is superior – with regard to strength – to red maple, but does black maple deserve it’s lofty elevation, together with sugar maple, to be collectively referred to as the “hard maples”? Nope. Black maple is the equal of red maple, and not its superior. And red maple has certainly done nothing that is worthy of condemnation as a “soft” maple, together with boxelder and silver maple (both fine woods, mind you, but not stalwarts of strength and not to be confused/used as such).

I realize that I speak from a bit of a position of privilege here. I can readily identify any maple that I’m likely to encounter, and I process almost all of my own wood from tree to finished piece. Most woodworkers aren’t afforded that advantage. BUT, if you do have that option, then I would suggest you take advantage of it. Learn how to tell black maple from sugar maple, and red maple from silver maple². Not just by the leaves, but also by the bark. If you know what wood you’re using, you should be more confident in pushing it to its limits. And feel free to consider Norway maple or sycamore maple as a substitute for sugar maple. Those European species acquit themselves well when multiple properties are considered.

This is important to me is because, in my quest to find suitable substitutes for sugar maple for my Windsor chair legs, I’ve been forced to turn over every rock and examine every option. I’m satisfied at this point that red maple is a credible substitute, so I decided to give it a spin. More on that in this post from earlier today.

Footnotes

1. Wood property data comes mostly from The USFS Wood Properties Handbook. Data that was unavailable in that publication was sourced from The Wood Database.
2. I recommend a region-specific field guide. For those of you in the Southeast, you can’t beat Trees of the Southeast. A good, free resource is Dendology at Virginia Tech.