How Logs Are Turned Into Boards, Part 1: Plainsawn — www.core77.c...
1 of 5
http://www.readability.com/articles/qlx746mc
core77.com
Still Time to Apply to SVA's MFA in Products of Design Program for 2013 Posted by Christie Nicholson | 5 Jun 2013 | Comments (0)
This is the first post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers and builders. Check back every Wednesday for the next installment. Here's a dangerous assumption we might make about wood: That is once it's cut it remains, more or less, the same. But this is not the case. Even after being cut it moves; it expands, contracts and warps. And this movement often results in dramatic alterations to (or completely ruins) the design of furniture or environments. Understanding the nature of wood is the first step to avoiding surprises, and it is crucial knowledge for a furniture designer to amass. Changes in humidity alone can remove or add as much as an inch from a board's size. Think about what that might do to your carefully-placed joints on either side of, say, a table. So to unpack wood, I'll start a few steps before the lumber finds its way into your shop. What goes into a sawmill is logs, and what comes out of it is boards. But turning the former into the latter is a more detailed process than one would think. Initially it's a geometry problem: We have one log and we want to cut it in
7/8/2013 3:47 PM
How Logs Are Turned Into Boards, Part 1: Plainsawn — www.core77.c...
2 of 5
http://www.readability.com/articles/qlx746mc
such a way as to yield the most useable boards. Over the years, sawyers have devised three main ways to cut a board: Plainsawn, quartersawn and riftsawn.
In this series I will describe each cut—its advantages and disadvantages—and its best applications. Let's start with plainsawn, also referred to as flatsawn or "throughand-through." Looking at the illustration above we see that plainsawn yields the most useable wood with the least waste. The boards are cut in parallel, one right after another, up to the pith of the log. Plainsawn is simple, very fast, and the most cost-efficient. And that's why this is the type of cut found in most mills. And the cut can be used for the most species of tree. In fact these days where serious custom milled wood is becoming more and more rare, plainsawn is typically the cut that people use for well, nearly everything. It is estimated that well over 90% of all logs cut into boards are cut plainsawn. "Quartersawn is used only in situations where the grain, or look of the wood, is very important or the quality of wood is very important," says Josh Vogel, a wood artist and owner of Black Creek Mercantile and Trading in Kingston, NY. "These days everything else is pretty much plainsawn." Plainsawn yields distinctive patterns on the face of the board. Because it is cut parallel through the log it results in a face with large graphic ovals and u-shaped patterns, also referred to as "the cathedral effect." You can see the long ring patterns, that lie almost parallel to the face of the board, here:
7/8/2013 3:47 PM
How Logs Are Turned Into Boards, Part 1: Plainsawn — www.core77.c...
3 of 5
http://www.readability.com/articles/qlx746mc
While the dimensions of the "cathedral" will vary depending on where in the log the board was cut--think of the variety of places where the tree's rings will intersect the cut line--you can look for it to quickly identify if a board has been plainsawn. Here are some examples of boards where the cathedral does not always come to a perfect point, but that you should still be able to identify as plainsawn:
7/8/2013 3:47 PM
How Logs Are Turned Into Boards, Part 1: Plainsawn — www.core77.c...
4 of 5
http://www.readability.com/articles/qlx746mc
It's worth noting that most may refer to this as the "grain" of wood when actually the correct term is "figure." So if you want to display your new expertise, say this instead: "The figure of that wood table is really beautiful." By the way, the word "grain," when used alone, technically refers to the different kinds of cells within a tree that form the characteristic growth rings. Of course, plainsawn wood is only attractive depending on what you are building. It might be great for a long dining table but not necessarily for kitchen cabinets. For a smoother, more refined look consider either quartersawn or riftsawn boards (more on those cuts in future posts.) The big drawback in using plainsawn boards is how much they change due to drying, aging and changes in humidity. Take a look at this shot:
This angle is known as "end grain" of the board. As a board ages and dries the tension of the grain can cause the boards to twist, cup and bow. While every type of cut yields boards that will absorb and release moisture, plainsawn boards are the most likely of all to suffer distortion during that ongoing process. In fact, the longer the length of tree ring that you see in the end grain the more
7/8/2013 3:47 PM
How Logs Are Turned Into Boards, Part 1: Plainsawn — www.core77.c...
5 of 5
http://www.readability.com/articles/qlx746mc
likely the board will cup or bow. Some final tips: If you are going to use plainsawn wood, have it kiln-dried so that any movement and shrinkage is minimized. Also, keep in mind the final destination for the wood. If you are building a piece in hot, dry Phoenix and shipping it to rainy, humid Seattle you may want to consider a more stable cut like riftsawn, which we'll discuss in a future post. The way you join the wood can also help accommodate movement. We will discuss some options for joinery in future posts. Also from a design perspective, if you are having the logs sawn at a mill, ask the lumber sawyer to stack the boards (for drying) in the same order as they were cut. This way it's much easier to match up the grain, figure and color of the boards. More: Materials • Comments (0)
Original URL: http://www.core77.com/blog/materials /how_logs_are_turned_into_boards_part_1_plainsawn_24890.asp
7/8/2013 3:47 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
1 of 8
http://www.readability.com/articles/u79xlaqv
core77.com
Posted by Christie Nicholson | 12 Jun 2013 | Comments (0)
This is the second post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers and builders. Check back every Wednesday for the next installment.
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
2 of 8
http://www.readability.com/articles/u79xlaqv
[image via Make Me Something Special]
For a furniture designer interested in working with wood, it's important to first understand how it's cut into boards. Because the different ways that wood is cut can have a huge impact in how it behaves once it's integrated into your final design. This information may be covered at fine furniture schools, but we were surprised to learn that a lot of industrial design programs skip this crucial wood information altogether. This series is intended to arm you with some basic information to help plug those knowledge gaps.
In my first post of this series I described one of the most common cuts, the plainsawn, it's advantages and disadvantages. Now we'll look at quartersawn and briefly touch on riftsawn, and try to clear up the confusion between the two. Quartersawn vs. Riftsawn Terminology Issues There are conflicting opinions--even in the woodworking community--between what constitutes "quartersawn" versus "riftsawn." You'll even see even professional companies interchanging the terms, with two different companies defining them in complete opposition to one another. To make it even more confusing, board-cutting processes referred to as "quarter sawing" can actually yield boards that are both quartersawn and what's called "riftsawn," which we'll get into later. To clear it up, we spoke with Randy Johnson, a lifelong woodworker and the former Editor-in-Chief of American Woodworker magazine. "The way I was always taught is that quartersawn boards are the ones where the grain, or the tree rings if you will, are 90 degrees to the face of the board," Johnson explains. This is what's referred to as "radial grain."
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
3 of 8
http://www.readability.com/articles/u79xlaqv
The bottom line: When viewed from the end, a board that is truly quartersawn will have the grain running straight up and down, like this:
Here's another shot where it's even more clear:
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
4 of 8
http://www.readability.com/articles/u79xlaqv
Contrast that with the endgrain in our plainsawn post. Quartersawn Properties In the last post we saw that plainsawn is the most efficient way to cut up a log--so why bother with quartersawn? Because a long, long time ago, woodworkers discovered a unique property to quartersawn boards: They exhibit very little wood movement--the way that wood can shrink, swell, warp, twist, cup, or bend with changes in humidity--compared to plainsawn boards. Needless to say, this stability is attractive. That's why in the old days, when resources were abundant, anyone working with wood always preferred to use quartersawn wood. If you look at antique furniture pieces online, even with a small, crappy Craigslist or eBay photo, you should be able to spot quartersawn oak a mile away by its distinctive look:
However, as part of nature's bargain, quartersawn is not only the most stable and desireable--it's also the least efficient to produce, yielding the most waste. As you can see from the photo below large triangles between each board are discarded to make this cut.
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
5 of 8
http://www.readability.com/articles/u79xlaqv
That inefficiency means quartersawn is the most expensive type of board to produce. Will you want to use it in your designs? Yes. Will you pay more for it? Yes again. Quartersawn Aesthetics With quartersawn wood, the cuts are made perpendicular to the tree's growth rings which creates a straight grain pattern that is much more uniform. The result is a board face that is more refined and might be suitable for cabinets or floors, or in any situation where the designer wants a uniform pattern. For instance, quartersawn will yield nice-looking table legs because all four legs will look the same (assuming you designers have taken the time to properly orient them!). Quartersawn is also useful in pieces where there is something you want to highlight, instead of detract attention from. Among woodworkers, the look of quartersawn is considered a desireable aesthetic due to their radial pattern of so-called Medullary rays, which produce an attractive wavy pattern also known as "ray fleck." You can see such patterns here:
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
6 of 8
http://www.readability.com/articles/u79xlaqv
Compare that to the more decorative cathedral patterns of plainsawn boards. Quartersawn Applications As mentioned earlier, a distinct advantage to using quartersawn boards is how the cut wood will handle one of its biggest enemies: moisture changes. Quartersawn boards tend to warp and expand the least, and I will explain why this happens in a future post on wood movement. So for things that cannot tolerate much movement, like floors, quartersawn is preferred. Some quartersawn flooring is shown below.
Quartersawn is also the preferred cut for most string instruments. Many high-end acoustic and electric guitar necks and fret boards are made from quartersawn wood. The major advantage of this cut is that it can produce extremely thin pieces of wood that do not warp or twist. This kind of stability is required to preserve consistent sound through the lifetime of the instrument. Looking at the figure and the Medullary rays that you learned about above, you should be able to identify the guitars in the photos below as having been made from quartersawn wood:
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
7 of 8
http://www.readability.com/articles/u79xlaqv
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 2: Quartersawn — www.core77...
8 of 8
http://www.readability.com/articles/u79xlaqv
Lastly, because of its look, quartersawn is often the preferred cut for those looking to design and construct high-end furniture or paneling. In the next entry in this series, we'll take a look at riftsawn, and show you a handy video that will clear up the confusion between quartersawn and riftsawn. More: Materials • Comments (0)
Original URL: http://www.core77.com/blog/materials /how_logs_are_turned_into_boards_part_2_quartersawn_24891.asp
7/8/2013 3:48 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
1 of 8
http://www.readability.com/articles/sqvgacqd
core77.com
Posted by Christie Nicholson | 19 Jun 2013 | Comments (0)
This is the third post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers. Check back every Wednesday for the next installment. The first two posts are here: » How Logs Are Turned Into Boards, Part 1: Plainsawn » How Logs Are Turned Into Boards, Part 2: Quartersawn What is Riftsawn? As we mentioned in the previous post, there is confusion about what constitutes riftsawn versus quartersawn, and this entry will help clear it up for you. To recap, a board is referred to as quartersawn when it is cut radially from a log. As you get further away from a radial cut on a log, the angle of the grain/tree rings will start to get further away from 90 degrees. And as that angle starts to become less and less perpendicular to the face of the board, the classification of the board goes from "quartersawn" to "riftsawn." These diagrams should make it clear:
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
2 of 8
http://www.readability.com/articles/sqvgacqd
"True quartersawn" means 90-degree, radial grain, but these days the "quartersawn" term has been sliding--it won't be unusual for you to spot a board labeled "quartersawn" where the grain angle is all the way down to 60 degrees. These two boards, for example, were listed by a supplier as quartersawn, even though we can see the angle of the grain seems to wander past that 60-degree boundary:
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
3 of 8
http://www.readability.com/articles/sqvgacqd
Endgrain Photo 1
A sawing diagram illustrates this more graphically. The drawing below left allows you to see how they're letting the modern-day definition of quartersawn slide a bit, while the sketch below right shows the ideal:
Riftsawn, then, is modernly used to refer to boards where the grain is between a 30- to 60-degree angle, like this:
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
4 of 8
http://www.readability.com/articles/sqvgacqd
Endgrain Photo 2
Now if you compare Endgrain Photo 1 with Endgrain Photo 2, you'll see the endgrain angles look identical. Yet the former is being sold as quartersawn, and the latter is being sold by a different supplier as riftsawn. This is why it's important for you to be able to distinguish different cuts with your own eyeballs, so you can spot issues like this. Adding to the confusion even further, the term "quartersawn" is also used to refer to the modern-day process of cutting logs, whereby they are quartered, then continuously tipped back and forth between cuts. I know that sounds bewildering, but luckily for you, we've located a video that will handily clear this up in a visual way you can easily remember:
As shown in the video above, modern-day mills have figured out it is most efficient to pull multiple types of board cuts out of one log, yielding both quartersawn and riftsawn. In fact, and this gets a bit confusing, you can have all three cuts--that's plainsawn included--in one log. "If cut a tree into boards by the time you get to middle there are two boards that are essentially quartersawn - the grain is perpendicular to the plane of the wood
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
5 of 8
http://www.readability.com/articles/sqvgacqd
- and the boards on top and bottom would be riftsawn," says Josh Vogel, a wood artist and owner of Black Creek Mercantile and Trading. "If you were grading the lumber basically you receive two quartersawn boards, four riftsawn boards, and the rest would be plainsawn." In a nutshell: If a board isn't plainsawn nor quartersawn, it's riftsawn. We don't often say this, but please ignore what a Google Image search will turn up for "riftsawn," or even what you'd read about it on Wikipedia; it will only confuse you, as many folks out there--including wood suppliers!--incorrectly transpose riftsawn and quartersawn. That's why it will be useful for you to learn to identify the three types of boards by sight, using the pictures we've provided in this series. Why Use Riftsawn? In terms of wood movement, riftsawn (as defined in this series of entries) is inferior to quartersawn--though not as bad as plainsawn--due to its angle of grain. For that reason, no one seeks out riftsawn for its stability, unless they don't have the money to cough up for the more expensive quartersawn. From a performance standpoint it is a middle ground; riftsawn boards are essentially a wood-processing byproduct of the quartersawing process, and it's a less-expensive alternative to quartersawn that people can live with. What Does Riftsawn Look Like? Aesthetically, the figure of riftsawn provides a middle ground between the visually busy face of a plainsawn board and the stark lines of a quartersawn board.
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
6 of 8
http://www.readability.com/articles/sqvgacqd
In fact, most of the wood you're going to see in your life that isn't plainsawn is going to be riftsawn. (Quartersawn only turns up in expensive stuff.) You'll recognize riftsawn right away because, absent the cathedral effect of a plainsawn board, it looks the most like what we think of as "wood":
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
7 of 8
http://www.readability.com/articles/sqvgacqd
We hope that this sheds some light on the process. In any case, by now you should easily be able to identify the difference between plainsawn, quartersawn and riftsawn by looking at a board's endgrain. For example, you should be able to look at this photo...
...and by examining end-grain, see that the bulk of these are riftsawn--but you'll also spot two boards that are quartersawn, which will provide the best stability. This should come in handy if you select your own wood for a project, and learning to visually identify the different types should overcome any misuse of
7/8/2013 3:49 PM
How Logs Are Turned Into Boards, Part 3: Riftsawn — www.core77.c...
8 of 8
http://www.readability.com/articles/sqvgacqd
terminology by a seller (assuming you don't buy your lumber sight unseen!). In the next entry, we'll give you an overview of why wood moves in the first place. With that knowledge handy, you'll better understand the techniques that designers and builders use to compensate for it. » How Logs Are Turned Into Boards, Part 1: Plainsawn » How Logs Are Turned Into Boards, Part 2: Quartersawn More: Materials • Comments (0)
Original URL: http://www.core77.com/blog/materials /how_logs_are_turned_into_boards_part_3_riftsawn_24918.asp
7/8/2013 3:49 PM
Wood Movement: Why Does Wood Move? — www.core77.com — Rea...
1 of 6
http://www.readability.com/articles/h7lemt17
core77.com
Presenting PENSOLE's 'Future of Footwear' Master Class Scholarship Winners Forthcoming Adidas Springblade Shows Benefits of Sneaker Design Industry Posted by Christie Nicholson | 26 Jun 2013 | Comments (0)
This is the fourth post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers. Check back every Wednesday for the next installment. The first three posts are here: » How Logs Are Turned Into Boards, Part 1: Plainsawn » How Logs Are Turned Into Boards, Part 2: Quartersawn » How Logs Are Turned Into Boards, Part 3: Riftsawn My front door that used to open easily now sticks and requires a massive shove. The heat of summer is here and with it comes the impact on wood. When we think of wood—constructed into a table, a bookshelf, a door—most of us assume it will remain more or less the same. After all it's dead, right? But as I mentioned in my first post of this series, wood moves. And in some cases it can move significantly.
7/8/2013 3:51 PM
Wood Movement: Why Does Wood Move? — www.core77.com — Rea...
2 of 6
http://www.readability.com/articles/h7lemt17
Understanding why it moves in the first place can help us know how to allow for such movement in design. So in this entry we're going to get a bit science-y (and not so much about the pretty pictures). I should point out that you could literally spend entire semesters studying this stuff, so reading this one entry is not going to make you an expert; what we are doing here is providing some basic information and terminology, so that you will know what to search for, should you decide to research further for your specific application. It is the cellular structure of the tree, and the living systems within the board, that influences its movement, long after it's been cut. Because the wood in living trees is very, very wet. Its cells are swollen with sap, which is mostly water, but also some minerals and sugars. Water within cells is called "free water" and it can be squeezed from the wood, but water can still remain within the cell walls. This is called "bound water" and it is only when the bound water is removed (by drying) can the wood shrink and become strong. Take a look at these images. Below left is a magnified photo of wood. You can see vessels that look like bundles of drinking straws running through the woodc. Below right, the graphic illustrates the difference between bound and free water. It's like the free water is sitting inside the straws, but the bound water is moisture trapped within the walls of the straws themselves.
How much bound water is lost, and so how much shrinking occurs, depends on the surrounding humidity of whatever environment you're in. You'll need to learn some terminology here, which I'll italicize in the next sentence. Put scientifically, wood is hygroscopic, which means its moisture content is directly related to the relative humidity of the immediate environment—and that varies, obviously. The following map shows some general differences within the United States:
7/8/2013 3:51 PM
Wood Movement: Why Does Wood Move? — www.core77.com — Rea...
3 of 6
http://www.readability.com/articles/h7lemt17
And luckily for you, the collective wisdom of wood experts have led people to create graphs to help you calculate what's known as EMC, or equilibrium moisture content, taking your local climate conditions into account:
Now let's explain EMC. You've dried your wood properly, worked it, and now shipped your finished wood product to an area with a different climate. Well, that wood may still move, screwing up your joints. Although proper drying can help limit the future moisture content of the wood, it is impossible to "lock" the wood into permanent stability. As the humidity of wherever your piece is rises,
7/8/2013 3:51 PM
Wood Movement: Why Does Wood Move? — www.core77.com — Rea...
4 of 6
http://www.readability.com/articles/h7lemt17
the moisture content of the wood rises and the wood expands. When the outside air becomes drier the moisture content decreases and the wood contracts. This process leads to the so-called equilibrium moisture content or EMC. The EMC is reached when wood accommodates the relative humidity of the environment around it. So ideally, you need to calculate the EMC of the exact wood you're using for whatever environment it will end up in—and figure out what the seasonal changes in that environment will be, too. It's probably becoming clear to you why Ikea uses so much particle board. In addition to it being cheap, there's no worries about wood movement. So yes, it's a lot of work to calculate these things. But if anything, we ought be thankful that modern science and technology have provided a way for us to calculate it at all. If you're going to use natural wood in your projects, you should be aware of the potential ramifications. To give you an idea of those ramifications: Most U.S. homes have levels of relative humidity that can range from 25% to 65% and this range can cause a 6% change in the moisture content of wood, which can cause a 12-inch wide maple board to move about a quarter inch! Note that plywood and other composite wood boards will move about one tenth of the rate of solid wood. Also, the wider the wood the more movement it has. Species of wood also affects movement. Red cedar tends to move less than maple, for instance. If you want to calculate how much your wood might move, you'll have to do some Googling using the specifics of your project: The type of wood used and even how the boards were cut. (Aren't you glad you now know the difference between plainsawn, quartersawn and riftsawn?) For example, one fact you can easily find online is that plainsawn wood will increase or decrease by 1% for every 4% change in its moisture content. You might be wondering, well how can I tell what the moisture content of my wood is? As you've probably guessed, simply looking at it or stroking the surface of a board isn't going to tell you anything. A board can feel dry on the outside and still have moisture on the inside. So to measure the moisture content of wood, pros use a tool called a moisture meter.
7/8/2013 3:51 PM
Wood Movement: Why Does Wood Move? — www.core77.com — Rea...
5 of 6
http://www.readability.com/articles/h7lemt17
The probes on this tool are inserted into the wood you're using, and you'll have to do a bit of Googling to learn how to use one (here's a good place to start). As with all tools, a good rule of thumb is to avoid the cheap stuff; we've read accounts of the probes snapping off of a cheap meter within the first few uses. In any case, it is highly recommended that you measure your wood's moisture content as soon as you receive it. All wood will eventually acclimate to its surroundings, so knowing the moisture content and relative humidity can really help you predict movement. And you'll find tons of resources online containing more detailed information than this entry. Like this handy online shrinkage calculator, for instance. Interesting trivia/tip: Wood continues to move even in freezing temperatures, because the moisture is protected within the cells of the tree and doesn't freeze. In the next two posts I will write about ways to deal with movement including drying and establishing safeguards within the design. Material Matters: Wood » How Logs Are Turned Into Boards, Part 1: Plainsawn » How Logs Are Turned Into Boards, Part 2: Quartersawn
7/8/2013 3:51 PM
Wood Movement: Why Does Wood Move? — www.core77.com — Rea...
6 of 6
http://www.readability.com/articles/h7lemt17
» How Logs Are Turned Into Boards, Part 3: Riftsawn » Wood Movement: Why Does Wood Move? More: Materials • Comments (0)
Original URL: http://www.core77.com/blog/materials /wood_movement_why_does_wood_move_25000.asp
7/8/2013 3:51 PM
Controlling Wood Movement: The Drying Process — www.core77.com...
2 of 8
http://www.readability.com/articles/bhllsxc0
This is the fifth post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers. Check back every Wednesday for the next installment. The previous posts are here: » How Logs Are Turned Into Boards, Part 1: Plainsawn » How Logs Are Turned Into Boards, Part 2: Quartersawn » How Logs Are Turned Into Boards, Part 3: Riftsawn » Why Does Wood Move? The number one enemy facing designers working with wood is water. Long after it's cut, a board will attempt to reach its equilibrium moisture content (or EMC), which will cause it to change shape, sometimes dramatically. The EMC
7/8/2013 3:52 PM
Controlling Wood Movement: The Drying Process — www.core77.com...
3 of 8
http://www.readability.com/articles/bhllsxc0
is the amount of water that the wood will hold in order to match the relative humidity of the surrounding environment. If the wood moves from Seattle to Phoenix there will be some shrinkage and change as the water is pulled from the wood cells and into the surrounding hot and dry air. There's no way to tell how dry your cut lumber is by just touching it. You will need a moisture meter, a tool that measures the percentage moisture content of the wood, based on its volume or weight. (You can learn more about moisture meters here.) An acceptable measure is about 6 to 7 percent. But remember that each species of wood has its appropriate EMC. The only way you can properly work with lumber is to have it dried. So how wood is dried becomes an important variable for anyone working with wood. One of the first things you need to know about drying wood is that drying begins in the outer layers of the log and then moves inward, the inner zones will only start to dry once the outer layers have reached appropriate dryness. This process can create its own issues. If the outer layers dry too much (below what is called the fiber saturation point) and the center layers remain wet then so-called "drying stresses" cause rupturing in the wood, which leads to checking. Checks are splits in the wood that often appear during the drying process.
There are two ways to dry wood: Air dry or kiln dry. The important difference between the two is time.
7/8/2013 3:52 PM
Controlling Wood Movement: The Drying Process — www.core77.com...
4 of 8
http://www.readability.com/articles/bhllsxc0
Kiln drying is perhaps the safest way to dry wood, because the process is controlled. Basically the wood is stacked in a kiln, where humidity and temperature is controlled using steam and drying is controlled with fans. It's fast, only taking about six to eight weeks. Because it is dried at high temperatures, drying wood in a kiln also takes care of any pockets of pitch (also known as resin) that can sometimes liquefy and seep out from the board later on if the wood is placed in a very hot area after being dried.
Air drying takes much longer, typically about a year for every inch of board thickness. Of course, this timeframe can vary greatly depending on the environmental conditions and species of wood. Here is a general tip: Low-density wood tends to be easier to dry than high-density wood. Low-density logs have thinner cell-walls so the moisture movement is faster and this of course leads to faster drying. Also, the weaker cell structure of lower density wood reacts more favorably to the stress of drying rather than resisting and checking. To air dry wood, lumber is stacked between "stickers," two by two pieces of wood, and then either strapped or weighted down by a plank of particle board and cinder blocks. (Note that the stickers are placed directly above and below each other, to avoid the uneven weight transfer that would lead to sagging.) Then, as long as there is sufficient air flow between and around the individual
7/8/2013 3:52 PM
Controlling Wood Movement: The Drying Process — www.core77.com...
5 of 8
http://www.readability.com/articles/bhllsxc0
boards, it is simply left to dry.
Air drying is trickier since it's up to you to consistently check the rate of drying and to monitor the wood for moisture, fungus and any developing checks. But overcoming the challenges involved in air-drying wood often rewards you with more stable wood, as the wood has more time to work through the process and develop any inevitable twists and bows it might have. An important thing to remember: Boards that are dried outside must be brought indoors to the environment where they will ultimately stay, because they will continue to absorb or release moisture depending on the surrounding humidity. And this should happen a few weeks or months before you plan to work on the wood. Here's a video from the Woodworkers Guild of America explaining the air-drying process:
7/8/2013 3:52 PM
Controlling Wood Movement: The Drying Process — www.core77.com...
7 of 8
http://www.readability.com/articles/bhllsxc0
Ultimately neither method of drying has any advantage over the other as far as moisture content is concerned. Dried wood will always re-absorb moisture in the same ways. Stay tuned next week for how design itself can safeguard against any further wood movement that can happen even after the drying process. Material Matters: Wood » How Logs Are Turned Into Boards, Part 1: Plainsawn » How Logs Are Turned Into Boards, Part 2: Quartersawn » How Logs Are Turned Into Boards, Part 3: Riftsawn » Wood Movement: Why Does Wood Move? » Crontrolling Wood Movement: The Drying Process More: Materials • Comments (0)
Original URL: http://www.core77.com/blog/materials
7/8/2013 3:52 PM