What is Spalted Wood and How to Turn it

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spalted pecan with zone linesWhat is spalted wood or what does spalted mean?  Not surprisingly most people (outside of woodworkers) have no idea what spalting is, even autocorrect doesn’t have a clue.  Spalting is a common woodworking term used to describe the colorations and patterning created by the growth of fungus in wood.  From “Spalted Wood” (Sara Robinson) “spalting is a process that occurs on wood when fungi colonize the substrate and extract various nutrients”.  By the way, “Spalted Wood” is an excellent book and goes a lot more in depth than this post will.  Generally, spalting results in the creation of zone lines and staining, the color of which will be influence by the type of fungus present. 

Most commonly (in my area) blues, grays, browns, and blacks are found, however yellows, oranges, reds, purples, whites, and greens are also possible.  In actuality the whole spectrum of colors is possible, these are just the most prevalent in my climate.  These stains are a lot more uv resistant than commercially developed colorants today. 

The species of tree also plays a role as some types of fungus prefer certain species of tree.  Thus, some colors are more common in cooresponding species, i.e. bluegreen stain in holly.  Likewise, I usually only find red stain in magnolia, sweet gum, tulip poplar, and pecan. People have been using spalted wood in woodwork, art, and sculpture for hundreds of years. 

spalted wood - fine line spalted dogwoodZone lines are the black (most common), gray, brown, etc. patterns of random lines created in the wood.  They are the visual representation of the interface boundary created when two opposing fungi grow and come in contact with one another.  Zones lines are extremely diverse in their thickness and arrangement.  With zone lines, no two pieces of wood are identical.  Horsehair pottery creates an effect that is similar to zone line patterning. 

…how to find spalted wood…

Perhaps you are wondering how to find spalted wood?  The easiest way is to look for flushes of mushrooms on the wood surface.  Spalting is a by-product of the decay process in trees, so the wetter the climate the greater the degree of spalting.  Moisture is critical for the decay process so arid climates are naturally going to have limited to no spalting.  Where I live everything spalts if left outside. 

Spalting in wood is a double-edged sword for the woodworker in that the attractive patterns are created by a decay process.  The downside is that it is a decay process so the integrity of the wood is being compromised.  The longer the wood is left to spalt the greater the risk that the wood is no longer usable as it will break apart too easily.  When wood has progressed to this condition it is referred as being “punky”.  There is a bit of an art in knowing when to stop letting the wood spalt before it is no longer usable.

To stop the wood from spalting it just needs to be covered up, and let dry out.  Other factors that influence spalting rates are the age of the tree and the species of the tree.  Sapwood spalts much more quickly than heartwood, the fungi feed more rapidly on the sugars in the sapwood.  Older trees tend to have less sapwood than younger ones and generally decay less quickly.  Oaks are a good example here, as the sapwood will spalt in a few months and the heartwood is basically untouched. 

Some species of tree just rot more quickly than others.  Walnut and Black Cherry for example basically do not spalt, other than the sapwood.  I have found that softer spalted wood - spalted persimmon with insect gallerieswood trees, i.e. softwoods and less dense hardwoods usually spalt more quickly.  Persimmon is an exception to this as it rots very quickly by comparison to other hard dense woods.  I suspect this is due to the high degree of sugars in the wood itself.  Freshly cut persimmon oozes a sap that is sticky and gelatinous.  That may promote more fungal/insect activity. 

Insects go hand in hand with fungi growth.  Some insects feed on the fungi in the wood and other times the fungi piggy back on the insects.  “Ambrosia Maple” is caused by the fungi that trails with the Ambrosia beetle that bores through the maple tree.  Either way insects are always going to be around when wood is decaying.  Implement pest control methods if spalted wood without insect galleries is desired.

Another benefit of spalting, aside from the nice visual qualities, is the softening of the wood.  Some species, pecan comes to mind, can be extremely hard.  Some areas of pecan are so hard that they will dull a turning tool almost instantly.  I like to spalt all hard dense species as it softens the wood and makes it much easier to turn even when dry. 

Turning with Spalted Wood…

What is Spalted Wood - Turning With Spalted WoodFor the most part the turning of spalted wood is not a problem (click here or the pic for the video).  An awl is a good tool to have around to check on the condition of the wood before mounting it on the lathe.  If the wood is too “spongy” then I won’t use it in the first place.  Just poke the suspect areas with the awl to see if there is wood that still has sufficient structural integrity.

In practice, if the awl pokes into the wood too far then its most likely not worth wasting time on.  I would say if it sticks in more than 1/8” then its probably not that good to use.  Ideally the awl just scratches the surface of the spalted areas. 

Also keep in mind that wet spalted wood is going to feel more spongy than if it were completely dry.  This stuff is going to flex if you poke it with your finger.  On spalted wood that I think might be too far gone I make sure that it is completely dry, as some wet spalted wood may become usable if it is completely dry.  The wet spongy stuff is going to pose too many complications with turning so its best to let it dry or not use it at all. 

Sanding spalted wood poses the greatest challenge.  Areas that have degraded abrade and cut more quickly.  Sanding those areas typically lead to depressions resulting in irregular surfaces.  This may or may not be an issue, personal preference comes into play here.  Either way use sandpaper that cuts well to avoid applying excessive pressure.  Also avoid sanding the spalted areas for too long, the idea is to minimize its exposure.

Hollowing spalted wood may result in areas that have cut too aggressively from the inside of the vessel.  This happens less frequently but keep it in mind so as to avoid thin spots.  Sanding depressions also contribute to thin spots in a vessel wall. 

Salted wood is also hyper absorbent, so it soaks up finishing oils very aggressively.  Usually, this results in a bit more yellowing if using oils and a few extra coats to get the wood stabilized.  I coat my hollows with Danish oil before beginning hollowing.  On spalted areas I keep applying oil until the oil rests on the surface.  Otherwise the surface of the spalted wood will be very flat.

Splated Wood - Spalted Sweet Gum with glue marksSpalted wood is going to be weaker comparatively, as such cracks in those regions are not surprising.  When making repairs be sure that spalted wood areas are adequately saturated with the final finish before using glues.  The finish creates a barrier that help prevent glue marks.  Just like the finish, spalted wood absorbs glue aggressively.  So much so that glue marks will not be removed except with difficulty.  The wood glue marks extremely easily and its very easy to ruin your work while attempting repairs or stabilization.