Woodsense: Spotlight on Spalted Wood

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This article is from Issue 59 of Woodcraft Magazine.

Spalted African tamarind

The rotten truth behind the decay

By Dr. Sara C. Robinson
Technical consultant: Larry Osborn

Many woodworkers misunderstand spalted wood–that is, wood that displays specific physical characteristic associated with decay. Some claim that bacteria cause those fantastic winding black lines that run both with and across the grain. Others point to “bugs” and “worms” for the peculiar markings and patches of bright color. Still others hold up pieces of flaming red boxelder and cry, “fungus!” Then, there’s the sizable number of woodworkers who hear the word “spalt” and quickly back away, convinced that death spores will come for them and their families from a contaminated piece brought into the shop.

So what is real, and what is myth? It turns out that wood scientists understand spalting quite well, with many having spent years researching the causal mechanisms behind wood decay. In fact, they have developed ways to induce spalting in sound, clear wood. Below is a concise guide to spalting and what woodworkers need to know to put this attractive element in wood to best use.

Peeling back the bark

Spalting is caused by very specific fungi with very specific goals. It’s not caused by bacteria or by staining, chemical or otherwise. It’s not caused by the tree as a host response.  A wide range of basidiomycete (wood decay, specifically, white rot) fungi and a very small number of ascomycete fungi are responsible for spalting. Keep in mind, however, that most airborne molds don’t spalt.

Below the bark, you’ll find three categories of spalt: The first, “bleaching,” lightens the wood and causes strength loss (referred to as punk). The second signature category includes zone lines, often black but sometimes red, green, pink, yellow, brown. These scroll across wood and serve as a boundary for one fungus to protect its resources from another. The third, pigmentation, displays bright colors inside wood, as shown bottom left. These are commonly pink, green, or blue, although other colors exist.

Curly maple bowl displaying zone lines, bleaching, pink stain, green stain, orange stain, and yellow stain.
Spalted sycamore and walnut box by Paul Anthony, featured in Woodcraft Magazine No.26 (Dec/Jan 2009).

History in woodworking

The earliest uses of spalted wood trace back to 14th-century Italy where green spalted wood showed up in inlaid intarsia works. Today, however, woodworkers choose spalted wood (regardless of the species, since all woods spalt, some more dramatically than others) for door panels, box tops, knife scales, and turnings. In other words, it’s sought for its decorative–not structural–qualities.

The top lodgepole pine board exhibits blue and yellow stains; the bottom lodgepole pine example includes blue and pink stains.

Where to find spalted wood

For starters, check out any woodpile or logs lying on the forest floor, since spalting is part of the natural decay process of wood. The most common type to find is zone lined, about which most woodworkers are familiar.

Pigmented wood can be more difficult to locate and very rare to find in a useable condition.  Rummaging around a forest near very decayed logs will probably lead you to the green stained wood referred to as Elf’s Cup (Chlorociboria spp.) Unfortunately, the wood will likely be so soft and decayed that you can only use it for inlay work, unless you want to impregnate it with a thin cyanoacrylate or shellac to stabilize it. Most of the other colors you’ll only find by walking around the woods and hacking into downed logs with a hatchet or chainsaw.  If you’re lucky, you may see bright colors–pinks or yellows–several inches under the bark. These tend to be in more sound wood, and might work for turnings or panels.

Blue stain is an easy color to find. Blue stain spores tend to constantly fill the air, and any fresh-sawn green wood left outside for a day or two readily picks up blue stain.  Alternatively, you can go to any big box store and ask for blue pine. They’ll probably sell it to you at a discount, because most people see it as “defective.”

Of course, you can purchase spalted wood, usually maple (but sometimes other species) from woodworking supply retailers and online. Here, you’ll find boards in random lengths and widths and turning blanks in a variety of standard sizes.

Fruiting Fungi

Dried fruiting bodies of Elf’s Cup, Chlorociboria ssp.
Fruiting bodies of Dead Man’s Finger (Xylaria polymorpha)

Fruiting bodies of Turkey Tail (Trametes versicolor)

Inject cracks and soft punky wood with thin CA to stabilize it.

What you’ll pay

Spalted maple can be found for as little as $4.25/board foot. Small spalted (stabilized) turning blanks sell for around $5. 

The level of spalting impacts price. A highly spalted 2 × 2 × 8" turning blank may run $30. When available, different species, including exotics such as tamarind, may command an even higher price. That’s why hunting down your own spalted wood (or spalting wood yourself as explained on page 72) makes sense by saving cents. Plus, it’s fun.

Selecting the best stock

Aside from commercially available spalted species, which tend to be limited, it’s important to note that when hunting down spalted wood some species serve better than others. These are the lighter-colored woods, which provide good contrasts. Smart choices in North American hardwoods include hard maple, birches, buckeye, elm, sycamore, persimmon, red oak, apple, basswood, pecan, holly, and hackberry.

When you encounter a candidate during your “walk in the woods,” cut into a downed log to inspect its suitability using a handsaw or chainsaw. Cut out a piece, and examine it for telltale signs of spalting.  Does the piece delaminate in rings or layers? Are there hairline cracks? Can you scratch it with your fingernail, or do chunks pop out easily when pried with a penknife? Does the wood feel too punky or soft when you press your finger against it? If so, than the piece or log may be too far gone. You want wood that won’t come apart when machining or turning, creating a safety hazard, not to mention a waste of time. In some cases, you can stabilize the wood if it’s not too far gone with thin CA (cyanoacrylate) glue as shown at left or shellac. Once you bring the wood inside for use, dry it out slowly. Storing it in a 3-mil trash bag contains the moisture. Sealing the ends with Anchorseal or Pentacryl will help prevent checking.

Working spalted wood in the shop

Safety concerns abound in the literature about working with spalted wood. But are they valid? It is highly unlikely that there are fungal spores inside wood. Spores live on the surface, if they exist at all, and not all fungi produce spores all the time. If you have a spore fear, spray your wood with 91% isopropyl alcohol (available at drug stores).

Beyond that, careful woodworkers know they need to wear respiratory protection when working with any wood. That’s because wood dust can cause allergic sensitivity and even cancer. If you’re wearing (as you should be) a NIOSH 95 or better mask, you’re blocking whatever spores may exist and protecting against mold as well. Standard cutters should work fine when machining spalted wood, but avoid running suspect spalted boards through the jointer, planer, or tablesaw.

Go Spalt: A Beginner’s Guide

A small, dedicated team of researchers has studied spalting. Members can get wood of any size and species, in general, to spalt in about 12 weeks or less. (See their site online at http://www.northernspalting.com.) To try your hand at spalting, consider their following lab-proven process.

To get started, gather the needed materials: 91% isopropyl alcohol, paper towels, wood to inoculate, and fruiting bodies off hardwood logs (at least two different types of fruiting bodies, if not more). In addition, you’ll want water and a dark-colored plastic bin or tub with a snap-on lid (size is up to you). See the photo at left. If your wood is dry, soak it underwater overnight in a tub to get moisture in it.

Next, spray down the outside of your wood with the alcohol to clean it. Do it first by spraying and wiping down the wood with paper towels; then spray and let the moisture evaporate. Spray down the inside of your tub. Place the wood in the tub, and pack the pieces together tightly. The more the surface areas touch, the better. Break apart the mushrooms and rub them over the surfaces of the wood. When done, leave the remains in the bottom of the tub. Put the lid on and store the tub somewhere warm (above 70° F is best). Every so often, to keep the contents moist, fill a pint glass with water, and pour it into the tub and over the wood. Wait about 12 weeks, and open up the tub. Take the top piece out.  Plane or turn it down to check on the spalting. If you like the results, take out the rest of the wood. If not, wait another few weeks.

About Our Contributor

In addition to being a woodworker, Dr. Sara C. Robinson is currently an assistant professor of anatomy of renewable materials in the Department of Wood Science and Engineering at Oregon State University in Corvallis, Oregon. She has researched spalting for ten years.

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