If you’ve been following the blog for a few weeks, you probably already know the pattern: I tend to post something substantive (nearly) every weekday, but weekends and holidays are generally met with either an abbreviated post or more likely radio silence. I spend 10-12 hours of each weekday in front of a computer screen, so I relish my days away from electronics and prefer to spend that time with my family or in the workshop.
Even better is when I get to spend time with my family in the workshop. My daughter Ellery just turned four a couple of weeks ago, and she has taken to the workshop like a pig to mud. I generally just had her a hammer, some wood scraps, and a handful of nails and let her pound away in the corner while I work on some other project, but she has enjoyed it so much that I decided to take a page from Joshua Klein’s book blog and spend one night a week working together on a project with her.
Since Christmas is coming up, we decided to work on a special project for Mommy – a birdhouse. I gave her some posterboard and a pencil and let her sketch out what the birdhouse should look like (in case you have trouble interpreting a 4-year-old’s architectural plans, the birdhouse is the steep-roofed structure, and it’s sitting on the end of an apparently truncated branch).
We searched through my wood stacks for some suitable stock and pulled out some sassafras scraps and some 3/8″ Douglas-fir plywood. I sawed out the shapes and Ellery planed them to size (with a bit of help. I should really build her own workbench so she can get some more practice with planing, but I’d have nowhere to put it in my tiny shop!)
She needs no assistance at all with the hammering, though. She has practiced diligently for the last couple of months, and she can easily sink 4d nails to the head without any pre-drilling. I did pre-drill the holes for the birdhouse, though, to make sure things were aligned properly.
Ellery was rightfully proud of the end result, but it’s been a bit of a struggle keeping Mommy’s Christmas present a secret. Next shop night will include painting the birdhouse, drilling the doorway, and turning a peg for the residents to land on.
I had to take a video as she was driving the nails. There was one brief pause to kiss a boo-boo, but she immediately gets back to work afterwards.
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:
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:
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.
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.
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…
My friend Jessica won last week’s spoon drawing. She is from just across the state line in St. Mary’s, GA, so rather than shipping the spoon, she offered to come pick it up. She and her husband Josh are cool people and we don’t hang out with them often enough, so we just made plans to spend the whole day with them instead.
Saturday morning, we went to “Pioneer Day” at the Okefenokee Swamp. They had a blacksmith, a spinner, a bowyer, a cane-grinder, and a cane syrup boil. My daughter especially enjoyed feeding sugar cane into the cane grinder. What kid wouldn’t love that? I enjoyed talking to the bowyer. His bows are made from Osage-orange, which wouldn’t have been the traditional wood for natives in Southeast Georgia, but he was from Arkansas, so it made sense for him. He even had a quiver of arrows made from riven white ash and turkey feather fletching. I’m not much of a hunter, but it was cool stuff.
I enjoyed the old Chesser Island Homestead. I remember going there as a child and admiring the log cabins and the roughsawn planks of the farmhouse. This time, I took notice of the furniture in the house. There were scads of ladderbacks, many of them in various stages of disrepair. A couple of them were quite nice, though. The chair on the left (below) had a seat of white oak splints, a material which – along with hickory bark seats – seems to get plenty of attention these days from books and modern chairmakers.
Honestly, though, the buckskin seat (in the middle) is as typical a seating material as any for old ladderbacks. Those things were all over antique shops in South Mississippi, and there were at least two of them in the Chesser farmhouse. Never seems to get mentioned much in woodworking books, though, presumably because it doesn’t come from a tree.
I also couldn’t help but snap a photo of the one Windsor chair in the house, a factory-made chair with a shapeless seat and stocky bamboo turnings. Yeah, it’s ugly. Much easier to make an ugly Windsor chair than a pretty one. Helpful to look at the bad ones too, though, if you aim to make a good one.
After we left the Pioneer Day festival, we all headed back to our home for some barbecued chicken (cooked over live oak and maple scraps, of course). I gave Josh and Jessica a tour of the workshop and we talked at length about the Windsor chair build. I know most people probably don’t even know what “Windsor chair” is, and even fewer (okay, many, many fewer) are as obsessed about them as I am. I must have gotten a little starry-eyed, because at one point Jessica asked, “So, why Windsor chairs? What’s so special about them?”
There are a lot of good answers to that question. Indeed, I’ve written about it before. but that night my answer was this: Windsor chairs are the only furniture form that I know of that can’t be improved in any way with power tools.
Sure, there are points in the process where you could introduce a power tool to gain some speed, but almost without exception, you will be giving up quality. The legs cannot be replicated by a machine with the same crispness as a hand-turned leg. A machine can make them all identical, but they will all be identically inferior. The seat cannot be shaped to the same organic look with a router or even a CNC machine. The spindles cannot not be shaved to precisely follow the grain – and therefore to perfectly preserve their strength – without the wedge, the froe, and the drawknife. Even the snugness and precision of the joints cannot be replicated without the continuous measurement and correction of the hand-made process. Joints are made to fit one another, not a plan a set forth by an industrial designer.
Last night, Peter Follansbee shared a link where master chairmaker Curtis Buchanan says it better than I ever could. You might as well listen to the guy who has made a few thousand chairs, rather than some dude who is halfway through his first:
Nothing’s like using the tools. Nothing. If I couldn’t make the chairs using these tools then I would find something else to make with these tools…
I could spend the next 30 years making continuous-arms and comb-backs and being completely content. And I think contentedness is very underrated. To me it’s a goal to be content, to be very content, just doing that same thing over and over and over again, which might sound boring to some people, but not to me at all. It’s just a lovely thing to come down here and do, day in and day out.
Click here to listen to the whole interview. It might just be the best 10 minutes of your day:
I’m ready to carve the seat for my chair – one of the parts of the build that I’ve been looking forward to the most – but first, I need to get the legs fitted up. I start by reaming the holes to a 6° taper, using the tapered reamer that I built a few weeks ago. The tapered mortise make a stronger joint than a cylindrical mortise, plus it makes the chair easier to to assemble, so it’s the perfect joint for the leg-to-seat connection. I clamped the seat to my shavehorse so I could work on it at a comfortable height and give room for the reamer to poke through.
I set my bevel gauge to the desired angle, minus 3°. Since the angle of the reamer is 6°, cutting that number in half and sighting with the bevel gauge will result in the appropriate angle. Sight the angle every few turns. Once I have it nailed, I can keep reaming until the hole is tapered all the way through.
The angle is just a bit off here…
…but now it’s corrected.
When the mortises are reamed, I can set my sight on the matching tenons:
After getting my feet wet with the baluster leg turnings, I proceeded on to the most difficult turning on the whole chair – the posts. In case you need a refresher on the chair parts:
The posts are the two turnings that frame the spindles above the seat. They are 22″ long and 1/2″ in diameter at their slightest dimension. If you’ve never turned wood before, then let be just say that your tools must be razor-freaking-sharp and your concentration must rival that of a Buddhist monk lest the spindles start vibrating like a coin-operated motel bed. This was the most challenging turning that I’ve ever attempted. Made the legs feel like I was turning rolling pins.
Straight-grained wood is a prerequisite for these parts. Riven wood would be ideal, but I have air-dried wood that was sawn. The grain isn’t perfect, so I’ll have to make it perfect. I start by knocking off some of the ugly with a hand plane so I can see the grain lines better.
The board I selected is water-stained…
so I knocked off a few shavings with a hand plane to see the grain lines better.
Then I strike a line parallel to the face grain and lay out all of the cuts.
Then some quick work with a Skil saw, and I have a stack of turning blanks.
Now I can examine the edge grain. Most of the blanks have very straight grain, but a couple of them have some defects that will need to be addressed.
The blank on the left has a small pin knot. I’ll make sure to locate the knot in a wide section of the turning. If it were located in a narrow part, like a cove, it could weaken the turning too much. The blank on the right does not have straight edge grain. It curves midway through and runs out to the right. I won’t be able to get a 22″ post out of it, but I can cut off the end and get a 16″ stretcher, making sure to located the lathe centers such that the grain runs straight.
I select the best blanks for the posts. These are turned down to 1/2″ in their narrowest dimension, so if the grain isn’t perfectly straight and free of defects, they simply won’t be strong enough to stand up to the rigorous life of a chair.
The stretchers seemed like child’s play after completing the posts. I turned them out in short order from the remaining blanks. I also turned one additional leg to replace the one that I messed up.
The turnings are now complete, and the underside of my lathe is ready to be cleaned out!
When I started my Windsor chair, I had resigned myself to the fact that I would not be able to get suitable spindle stock here on the island. My plan was to get as much done as I could before Thanksgiving, and when I visited my family in central Georgia on the fourth Thursday in November, I would snag some straight-grained red oak from my grandparent’s firewood stash.
After finishing up my shavehorse, though, I was eager to put it to use, so I decided to see what I could do with the wood on hand. Ideally, spindles are riven while green and fresh from a ring-porous hardwood like red oak, white oak, ash, or hickory. None of which grow with decent form or any sort of abundance on our coastal Florida island. I do, however, have a small stash of air-dried, quartersawn post oak (a species of white oak). I decided to rive out a few pieces to see if it could be worked dry.
It was an unmitigated disaster. The dry stock ran out while splitting. The splits could not be controlled like they can in green stock. I did manage to get two blanks that had promise, but the wood was rock-hard and even my well-sharpened drawknife would barely budge in the tenacious grain. Finally, I tried popping the blanks on my lathe, which yielded even worse results. Disgusted, I threw the stock into my scrap pile and once again accepted that the spindles would have to wait.
Today, though, I had a bit of a revelation. My wife is down to three weeks until her due date, so each week I take her to her doctor’s appointment and watch the kids for a half hour or so. Right across the street from the doctor’s office is a big sign for “FIREWOOD”.
I figured that they would mostly have live oak, which doesn’t interest me for spindle stock – too hard, and rarely straight-grained. But today I decided it was worth a closer inspection. We walked across the road and passed a mountain of split oak.
I noticed that much of the oak was actually laurel oak, a species of red oak, which should be a fine choice…if only there was some wood long enough and straight enough.
I walked up to two young fellows and introduced myself.
“I have a bit of an odd request,” I said. “I’m a woodworker, and I’m building a chair. I need some straight-grained oak, and I was wondering if you would mind if I looked through your piles to see if I could find something I can use?”
One of them laughed and told me that he’s had much stranger requests. “Last week I had a fellow come by and say that he needed a piece of wood that couldn’t be split. Said he was trying to toughen his daughter up for wrestlin’ and he need something she could just pound on.” We shared a laugh and he welcomed to have a look around.
With that invitation, I spent the next 20 minutes searching the piles for stock that might be suitable. The main problem I found was that the vast majority of the wood was cut too short for spindles. I would prefer to start with 24″ stock, but I can probably make do with 20-21″. Most of the firewood was cut to 16-18″. Of the few pieces that were long enough for my purposes, even fewer had grain that was even remotely straight enough to use. But I picked around and finally had a small armload of wood that I hope I’ll be able to use. I even picked up a short length of hickory for a hatchet handle.
I fished a crumpled $10 bill from my pocket and handed it to the fellow. He shook my hand and thanked me and welcomed me back any time if I needed more wood. I might do just that.
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.
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.