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AW Extra 1/24/13 - The Way Wood Works: Curly Wood

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The Way Wood Works: Curly Wood

How to buy, machine and finish this amazing wood.

By Tom Caspar


Hidden within a few trees in every forest lies a mysteriously distorted wood that has always fascinated woodworkers.

You can’t spot curly wood from the outside, but inside the tree a peculiar switch has been flipped, turning straight tree cells into wavy cells. Cut the tree open, plane the wood and you get spectacular, three-dimensional rippling wood grain.

But beauty comes at a price.You can tear your hair out trying to tame this unruly wood.Here are some practical tips on how to buy,machine and finish curly wood.

 

What Is Curly Wood?

The stripes you see in a finished board of curly wood come from the play of light on grain that waves from side to side (Photo 1). The troughs and crests of the waves reflect light in different directions. As you turn a curly board around in your hands, its surface actually shimmers. Light areas turn dark and dark areas turn light.

The biology of curly wood is as mysterious as its appearance. No one really understands why some trees have this wavy grain. It’s not genetic.You can take seeds from a curly tree,plant them near their parent and get nothing but straight-grained timber. The best guess is that stressful growing conditions, such as cold and drought, turn on the curly switch in a few trees,but no one has figured out how to duplicate these conditions in order to grow curly trees.

Curly grain can appear and disappear within a single tree. One side of a tree can be curly, and the other side straight. Young outer layers may be curly, but not older inner layers. It’s totally baffling!

 

Buying Curly Wood

Any kind of tree can become curly,but some species that grow in tough northern climates produce a greater percentage of curly wood than others (Photo 2). Flame birch and tiger maple are well-known examples, but you can uncover curly walnut, curly cherry and curly oak, just to name a few.

If you’re lucky, you can find curly wood in any pile of lumber for the same price as a straight-grained board.Many lumber mills process logs so fast they don’t stop and cull the unusual curly ones. If you search for curly boards in the rough, look for a striped barberpole surface or alternating areas of smooth and fuzzy grain.

A few folks in the lumber trade make it their business to find curly logs.Wood prospectors mine for curly gold in the hundreds of average trees felled by a lumber mill.The prospectors peel back the bark of some logs before they’re sawn.If they hit pay dirt,they’ll purchase the log and gamble that most of it is truly curly.No one will really know how spectacular or faint the curl is until it’s sawn.

Each dealer has their own system of grading curly wood, based both on general figure and the number of curls per inch.They’ll be the first to tell you that curly wood is so unusual that it defies classification. Your best bet for consistency is to stick with a dealer who has a large stockpile and familiarity with this enigmatic wood.

 

The Benefits of Wide Boards

Many dealers in curly wood hoard wide stock.Their logs are custom sawn to maximize the width of each board.

Why go to so much trouble? Imagine a drawer front made of three curly boards glued together. Individually, each board looks fantastic, but they don’t work together (Photo 3). One wide board for the drawer front would look much better. Experienced builders of reproduction furniture look high and low for wide boards.

 

Bookmatching

If you resaw and bookmatch curly wood, light can play tricks on you (Photo 4). Sure, you’ve made a wide board with mirror-image grain (the physical structure of the cells),but look what happens to the figure (the surface appearance of the cells).The grain runs uphill on one side of the board and downhill on the other.

What does that do to the figure? One side of the board can be light, the other side dark. Shift your viewpoint and the brightness shifts the other way. Again, a wide board may be a better choice. Musical instrument makers routinely bookmatch the curly wood they call fiddleback maple, but they’re awfully picky about selecting just the right boards.

 

Reducing Tear-Out

Curly boards are notoriously difficult to joint and plane,but armed with some woodworking savvy you can usually produce a blemish-free surface.The problem is the grain,which changes direction with every ripple (Photo 5). It runs downhill on one side of a wave and uphill on the other side. So no matter which way you feed a board, whole hunks of wood can be yanked off the surface by a machine’s knives, leaving an ugly pit behind. If you’ve just spent a pile of money on some special wood, this can be heartbreaking.Here’s how to minimize tear-out:

- Change your knives. Dull knives on a jointer or planer pull on wood grain; sharp knives cut it cleanly.

- Take a light cut. Set your machine to remove 1/64 to 1/32 in. at a time. Sure, you’ll take many more passes, but you’ll minimize the depth of any tear-out.

- Wet the wood. Green wood is easier to cut than dried wood, because wet cells are easier to separate and less likely to pry out their neighbors.You can temporarily achieve the same effect on kiln-dried wood by lightly sponging the surface of a board before you joint or plane (Photo 6).Give your jointer bed a good waxing and the board won’t drag. Rest assured, you won’t rust your cutter heads as long as you clean and dry them right away.

- Feed slowly. Go slow on the jointer, about half the speed you normally use.

- Scrape, don’t plane. If you’re working with hand tools, use a No. 80 scraper plane (#03.12.05; $33 from Highland Hardware, 800-241-6748). Its steep cutting angle allows you to quickly remove milling marks without any fear of tear-out. A card scraper is the perfect tool for smoothing small areas of shallow tear-out.

Despite your best efforts, a little tear-out when working curly wood is inevitable. Don’t get too discouraged. Some curly boards are so wild that even the finest woodworkers turn to two more tools: drum sanders and putty. A drum sander is a surefire (and expensive) way to surface curly wood perfectly smooth. It’s slower than a jointer or planer, but you’re guaranteed a clean shave. For tiny pits of tear-out that run pretty deep,use putty. If you scrape or sand down to the bottom of the tear-out you might end up creating a whole new problem: a shallow, dished out divot.You may not see it right away, but it’ll show up under a finish.

 

Finishing Tips

The amount of color in a finish can make a big difference in bringing out the curl. The explanation lies in looking once more at curly wood’s grain structure.Remember how the surface of a flat board cuts right through the rising and falling grain? When the grain rises up to the surface it exposes the ends of many cells. These end-grain cells are thirsty to absorb a finish, just like the end grain of any board. But the side-grain cells on the crests and troughs of each wave don’t absorb as much finish. The result is that some areas of a board soak up more finish (and color) than others.

Finishes such as shellac, oil and varnish are slightly colored. The thirsty parts of a board soak up an extra amount of this color. This extra color really makes the curl pop. Orange or amber shellac has more color than blond shellac, making the effect particularly striking (Photo 7).

Other finishes such as lacquer and waterborne polyurethane don’t have much color in them.You can still see the curl under them,but it’s not as dramatic. However, if you apply these finishes over a coat of dewaxed shellac you can have the best of both worlds.

 

Dyeing Curly Figure

Here’s a neat trick using dyes instead of stains to emphasize the dark figure of curls. This is a shop-tested recipe to simulate the look of antique curly maple furniture:

1. Sand your maple to 220 grit and apply one coat of Dark Mission Brown water-soluble dye,mixed 1/8 tsp.powder to 1 cup water. Let dry.

2. Re-sand with 220 grit paper. Sand until the wood surrounding the dark-colored curls is light.

3. Apply Early American water-soluble dye mixed 1/2 tsp. powder to 1 cup water. Let dry.

4. Apply a liberal amount of boiled linseed oil.Wipe off the excess after 30 minutes. Let dry at least three days.

5. To further warm the color of the wood and give it an aged look, apply a burnt umber glaze.Visit the Web site listed below for more on using glazes.

6. Apply your favorite topcoat. If you’re using a waterborne finish, apply dewaxed shellac first.


Sources

(Note: Source information may have changed since the original publication date.)

Homestead Finishing Company, homesteadfinishing.com, 216-631-5309, Water-soluble dyes: Dark Mission Brown, #3274, and Early American Maple, #3273, $7.25 for 1 oz.; Japan color: Burnt Umber, #7002, $6.25 for 8 oz.


This story originally appeared in American Woodworker February 2002, issue #92.

February 2002, issue #92

Purchase this back issue.

Click on any image to view a larger version.


1. Split open a curly log and you’ll find grain that bends back and forth like a wave. Planing the wood cuts through the waves, producing a shimmering effect like light bouncing off the ripples on a pond.This chunk of white oak is spectacular-looking wood!


2. Curly wood is found in every kind of tree under the sun. Environmental factors probably turn normal trees into curly ones, but nobody knows for sure how it happens. Soft maple and birch are the most common domestic curly woods.



3. Glued-up curly boards may not go well together. Curly figure is unusual because it runs at right angles to the edge of a board. It’s difficult to align figure like this in a group of narrow boards. Instead, look for wide boards.


4. Resawing a curly board into two bookmatched pieces makes a wide panel, but you may not like what you see. Although the grain structure is the same in both boards, the figure doesn’t look the same. Bright areas on one side may look like dull areas on the other side.


5. The planed surface of a curly board slices right through the waves of grain. Making a curly surface smooth and free of tear-out is pretty tricky, because the grain constantly changes direction. Half the time you’re actually cutting against the grain!


6. Wetting the surface of curly wood before jointing or planing virtually eliminates tear-out.This may be hard to believe, but temporarily softening the fibers really works!


7. Finishes that are slightly colored emphasize the curls more than perfectly clear finishes.Amber shellac has an extra amount of color, enhancing the effect.


8. Dye penetrates deeply into the thirsty end-grain areas of the curls. It makes them stand out from the surrounding wood. For even more contrast, sand the board.The curls remain dark while the surrounding wood stays light.Then, apply a second coat of dye.