Knowing our Raw Material: Wood
We have a unique material for our craft as hand-turners -- we use wood (yes, there are some metal-spinners, and turners of alabaster, bone, ivory and other materials out there). And we use it not as a commodity, but as the very heart of our craft. We need to love it, know its strengths and weaknesses and how to use them, where to find it, and how to anticipate and make best use of the unique beauty of each piece.
I will show you several qualities of wood that you may find helpful, and describe how we need to treat it to make best use of it. Spindle turners are most often content with kiln-dried, store-bought lumber, though most often in plank form. Some swear by air-dried as less “brittle.” But bowl turners tend to like the raw wood, green or semi-green, and often have to find it themselves.
Types of Wood
Straight wood- sound structure, interesting grain, natural edge can make more interesting in a bowl, bark held on (winter cut). Quarter-sawn or flat-sawn figure? Shape is more important than ever when the grain is not spectacular.
Color - there are lots of beautiful colors in wood; think of walnut and ash, purple heart, padauk, zebrawood. Color is usually based on extractives in the wood. Remember, all colors fade.
Tropical woods are most vivid and they tend to have toxic extractives in the wood that give protection against “bugs.” Color tends to fade with exposure to ultraviolet light. Try ArmorAll for slower fading. Contrast of wood is the very heart of segmented “polychromatic” bowls.
Contrast of heartwood and sapwood works great with a natural edge bowl, as in black ash, walnut, and Russian olive. The contrast between heartwood and sapwood in a green log disappears within about a year of cutting as the extractives migrate into the sapwood and make the whole log the same color.
Match the figure to the object- too large a grain figure on a small object is not attractive.
Crotch wood- The feather figure is thin and surrounds the pith in the plane of the Y. It is most useful if you do a natural-edge bowl and put the feather figure on the bottom of the bowl.
Curly grain, also called fiddleback, tiger, or rippled. Curly grain is the result of stress and you will need to plan for the warp factor. You can often identify it even before y ou cut down the tree. It is Visible as a horizontal (at right angles to the bark ridges) rippled effect in the outside bark of the tree.
Birdpeck is most often a result of a yellow bellied sapsucker. Single or multiple years of attack. It’s probably most evident in maple. I have also found sugar tap stains in the same tree.
Spalted is the old Anglo-Saxon word for "spoiled." Bacterial and fungal organisms invade the wood, break down the wood fibers and also add color and figure to the wood. The colors are distinctive- crimson for box elder, lavender and shades of brown in elm, yellow and black zone lines on birch etc. A wood technologist recently told me that the dark zone lines are battle lines, where the living wood marshals its defenses of antibodies and other disease fighters to combat the encroaching infections. Salted wood is fussy to work with and walls of the bowl need to be left thicker for strength. Sanding is essential for a good finish. Stock up on Superglue and oil.
Wormy wood is experienced another of nature’s composting processes. Can be very interesting. Most people don’t like holes in the bottom of a bowl, but are OK with holes high on the sides.
Burl wood comes in two varieties, onion or eyed.
Onion or layered burl is often the result of injury that the tree tries to heal over. This is often very curly figured grain but not nearly so prized as an eyed burl. I have a picture of an oak burl I recently brought home and here is a piece of that burl, sliced into slabs for stool tops and other projects.
Eyed burl is actually a tumor. These are buds that can never follow through to become branches. The effect is like bird’s eye figure on steroids- larger, more dense, and colorful. The wood itself is also more dense (boxelder turns like its cousin, sugar maple) and has no grain direction, so they tend to turn beautifully, even in usually punky woods. Caused by virus, injury or heredity? Very rare, but most common in box elder and black ash.
How to best use burl wood? Take advantage of the conical orientation of the eyes. If the burl is shallow, place the figure where it will show most. Perhaps a hollow vessel for a shallow burl.
Drying wood without cracking
The best way to keep a bowl from cracking is to turn it thin as appropriate for its final use, and uniform. Thickness in the bottom is most likely to cause a bowl to crack.
Wood takes about a year per inch to air dry. Carefully treat the vulnerable end grain.
You almost always need to avoid the pith, the center of the tree, because it is juvenile wood and very unstable and prone to cracking. The outside of tree shrinks more than the center. If you break that rule, then turn the bowl thin so the pith can “dimple out” when it shrinks.
My ideal is that they would dry for a month or two outdoors so they lose most of their moisture but are still at little risk of cracking. (free moisture gone, bound moisture still in wood) In the winter, you need to dry them inside but be very careful because with the low humidity there is great risk of end grain cracking.
I like to harvest wood in the longest pieces that I can carry since the inside pieces of a longer log have virtually no risk of cracking, at least for a year. Store out of rain and direct sun.
Plan for the shrinking (5-10 percent). If you need a truly round object, like a box and cover, you need to turn it thick enough to still get a circle out of the dried oval, wax to prevent cracking, especially the end grain. Then remount on the lathe and finish off the bottom. You can keep track of the weight to track the drying or use a moisture meter.
You can try the microwave to speed this up but there is a lot of stress in a thicker blank and your chances of cracking the wood go up fast. If you microwave, do it in short bursts with careful monitoring of how hot it gets.
The fruitwoods are beautiful to cut but crack much more easily than the nut woods, so be careful with them.
Some woods have a bad reputation for being “brittle” and for splitting, especially the family of mulberry, black and honey locust and Osage orange. Pecan too.
People enjoy the flashier woods, like burls and box elder. However, remember that the shape of the bowl, and that means mostly the curve, is what counts in the long haul.