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Make it special with spalted wood

Alog on the forest floor, sprouting mushrooms and missing its bark, doesn’t look like a source of premium woodworking stock. But it might be a treasure. It might contain spalted wood, marked with a network of random lines that look terrific on projects like those shown above.

Spalting occurs in sapwood during the normal decay process, but produces dramatic patterns only in light-colored wood. Birch, beech, and maple are prime candidates for spalting, with hard maple ranking as the most commonly used spalted wood because of its color and its high proportion of sapwood. If your local specialty lumber outlet doesn’t carry spalted stock, check mail-order suppliers. You can find several options on the internet by searching for “spalted wood lumber.” Best of all, you might find your own source if you live in a northern region, and have access to timber with permission to harvest it.

Sugar-maple leaves
Sugar-maple leaves

When hunting for spalted maple, for instance, look for standing maple trees. (See the drawing above to identify the sugar-maple leaf.) Chances are good that any nearby fallen trees also are maples. When you find one, cut into the log with a chainsaw to check for spalted areas. Save the best portions, as shown in Photos A and B, below.

Photo A&B.jpg

What causes spalting?

When a tree dies, nature’s abundant fungus spores begin the decay process. White-rot fungi do much of the work, feeding on wood and moisture.  As they move through the wood, they create spalting’s dark “zone lines,” shown in Photo C, below.

Photo C.jpg

You’re most likely to find good spalted wood in cold climates. Decay pauses when temperatures drop below about 35°F, creating a longer window of opportunity.

Can you induce spalting in a newly fallen tree? Yes, but it takes patience, and the results are unpredictable. Cut the log into suitably sized pieces, store them on a bed of leaves in a shady spot, and cover them with more leaves. Moisten the pile occasionally if the weather turns dry. Check the wood every so often, but expect to wait at least a year for the desired result.

Or, you can cut small pieces, moisten them, and store them in unsealed plastic bags. Either way, the key is to stop the rotting process before the wood turns soft, or “punky,” as shown in Photo D, below. You can call a halt to spalting by placing the wood in a dry place. Kiln drying stops it quickly.

Photo Dx.jpg

Work it with care

You should avoid breathing all sawdust, of course, but the fungal material in spalted wood creates an extra risk of an allergic-type reaction. “The dust has been known to cause severe respiratory or skin problems in a few cases,” says Dr. Eugene Wengert, professor emeritus of wood processing at the University of Wisconsin-Madison.

Use a dust-collection system, and wear a properly fitted dust mask. Those measures should keep you comfortable and safe when you work with spalted wood. Here are some other details to keep in mind while cutting and turning:

In some situations, you can save a marginal piece by applying cyanoacrylate glue or epoxy to punky areas. Remember, however, that adhesive on the surface can show up as a light-colored or shiny spot after you apply finish. An alternative method for treating very small soft spots: Coat them with shellac or sanding sealer.

• If you cut stock thinner than 12 ", it might crack along a zone line if sufficient pressure is applied.

• The noticeable variation between hard and soft areas—an extreme example is visible in Photo E, below—can result in a pitted surface when you turn spalted wood. Take extra care to keep your tools sharp, make light cuts, and plan to do more sanding than usual.

Photo Ex.jpg

• Some spalted wood tends to expand and contract more than unspalted wood. To be safe, if you use spalted wood for floating panels, allow 116 " more space on each side than normal.

Finishing thoughts

You took pains to find great-looking spalted wood and put its beauty on display. Now, be sure to finish it in a way that will emphasize the striking contrast between light wood and dark lines and preserve it.

We recommend applying lacquer or water-based varnish, two finishes that add little color to wood. However, if you prefer a warmer, more amber look, you can use oil-based varnish or shellac. You might choose polyurethane for items that will receive a lot of use.

Avoid applying oil, such as boiled linseed oil. A good oil finish depends on even absorption into the wood, and spalted wood’s extreme variations in density make that unlikely.

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