Spalting: A Fungus Amongus

Nature's beauty hidden in the wood

Spalted Sycamore Bowl

What it is and isn't

WooDecay and rot is all around us. Fungus is everywhere and just awaits the opportunity to take advantage of the right conditions to grow and affect us in many ways.

No, I'm not starting a great novel about the social and political conditions of our times, although that could be my next article if this one doesn't work out. What I am talking about is what happens to wood. Wood decays. Wood gets colored. One of the outstanding effects, sometimes, of this wood decay and coloration is what we call Spalt.

Spalt doesn't always occur when a tree dies and neither does it always strike all specie of tree equally either.  When it does occur, it can be a beautiful and wonderful thing for those able to use or at least appreciate such a thing.  Spalted wood is always unique. No two pieces are exactly the same. Now, I grant that you might not want a nicely spalted piece of maple making up your kitchen cutting board. There's a time and place for spalted wood.

Here's how I will be structuring this article. First, I'll discuss what Spalt is and what causes it. Second, I'll mention a few things about how to work with Spalted wood especially from a woodturning perspective. Third, I'll give some tips and tricks on how to spalt your own wood. On with the show...


Spalt, What Is It?


As I alluded to above, spalt is a a result of wood decay.  That's partly true. Spalt is the combination of a wood decay and a wood coloration mechanism.  Brown Rot is something different with it's own fungus's and mechanisms.  With that said, let's make this really simple at this point.  Spalted wood is caused by a specific fungus. We can separate the two major groups of fungal-created wood coloration associated with Spalt into the White Rot fungus' and the Blue Stain fungus'.  I'll keep things easier and not mess with the fungal scientific names. If you really want them, look them up.. We'll just call the fungi by their types of decay and coloration. I'll also not go into the other types of decay and coloration. These are out of the scope of spalting but are very interesting in their own right.  Often, mineral stain and reactionary stain are confused with the coloration defined as spalting.  Did I mention that there are lots of different wood colorations?  Ain't nature COOL ?!

White Rot is a decay mechanism and gives those great thin, quite distinct lines with broad lighter colored areas between them.  Blue Stain does not contribute to the decay of the wood but does give, contrary to the single-color name, anywhere from yellow, orange, red, blue and some blacks and brown coloration.  These colors are often found near, and most often just underneath, the bark of the tree as opposed to nearer the heartwood. They are also not usually very widespread but tend to be blotchy and quite vibrant. This contrasts nicely with the White Rot type of coloration.


White Rot is caused by the wood decaying. Left to continue too long, the White Rot decay will turn your wood into a nicely colored piece of mush and be basically useless for woodworking much less woodturning.  Sure, you can sometimes beef up those "punky" areas that are spongy with some thin epoxy, CA glue, common White PVA glue, or other wood stabilizers / hardeners and make them usable.  Many times, though, large enough pieces just flake off of the too-decayed wood and doesn't even make good firewood. The key is to catch that White Rot at that stage where it gives the greatest amount of coloring but yet isn't too soft to work well.  And how do we know when that is?  See the next section for that.


So, what makes those fine lines?  The White Rot fungi groups do that.  Those are called "zone lines" and are caused by incompatible colonies of White Rot fungi coming together and, I guess, not liking the togetherness. Hey, it's not just us humans that are like that! Those zone lines are laid down by each colony as a barrier to surrounding colonies. The slightly different coloration on either side of those zone lines are the results of those fungi colonies moving through the wood. White rot attacks the cellulose, lignin (although not actually "eating" it), and other chemicals in the wood giving the wood an off-white appearance.


The cause of the Blue Stain coloration is not exactly known.  We do know that it is caused by a fungal invasion but we're not sure what the one, if there is just one, mechanism of that invasion.  Some think that the fungus enters the wood at or near an injury area such as a bird pecking or driven nail or whatever else that causes injury to the tree.  The tree reacts to this injury and it's that reaction along with the presence of the specific fungi that causes this coloration.  Others see the Blue Stain fungi invading simply via a convenient entrance point, probably along with bugs or other animals, and waiting for the right environmental conditions to begin affecting the wood.

Blue Stain generally affects the wood nearest the bark (sapwood and never the heartwood) and spreads out much more than it runs deeper into the wood.  This means that, as a woodturner, you're likely to loose this great color in your shavings if you're not careful.  This Blue Stain color is usually a far more vibrant color when compared to the White Rot color. Letting this Blue Stain type of fungi grow is just fine.  It's not a decay fungi and won't harm the wood. It'll only grow larger although it often doesn't get very widespread in one large clump.


Spalted Sycamore 1.jpg (47435 bytes)
Sycamore. Showing both White Rot and Blue Stain (the orange & red blotch)
Spalted Sycamore Endgrain.jpg (66353 bytes)
Same piece but showing endgrain


Now we know that Spalt is caused by fungus.  We also know that what we call Spalt is actually the combination of a decay fungus called White Rot and a coloration-only fungus called Blue Stain that can produce more colors than just blue. So, why do we want all of this fungus amongus?  How should I work it?  What do I do with it?


Spalt: Natures Artist


Part of the appeal with artistic woodturners of this craft (errr ...Art?) is what Nature provides us with our major material of choice ... wood.  Every piece of wood is different and thankfully so.  This is no more apparent than in the spalted wood with it's greatly unique patterning. We enjoy the beauty and unexpected showing of burls, knots, figures, bark inclusions, and other treasures coming from the wood that we, as woodturners, can't control.  We can control, to some degree, how those Nature's features are displayed and used in our woodturnings though.  The same holds for wood spalt as well.  We can get some small idea that spalt is contained in our blanks and can take steps to bring out it's best effect in our turnings.  Spalted wood, by the vary nature of the decay part of it especially, behaves differently in all phases of woodturning.  Let's explore these..


The only reliable way to determine if a piece of wood is spalted is to actually cut into it.  Looking for mold and mildew growth on the outside of the log isn't a reliable method although the presence of molds give a good indication that the conditions are appropriate to support the growth of the decay fungi.  Mold and mildew do not cause spalting.  They are a form of fungi but typically just grow on the surface of the wood and produce a terrible, circular, faded surface stain. You don't want to encourage mold or mildew growth.  See below...


Mold Bowl.jpg (21415 bytes) Mold Box.jpg (16942 bytes)
Mold coloration after finish turning and a mineral oil/beeswax finish. Yuck!


It must be noted that a tree need not be laying on the ground to be spalted.  I've found many standing trees that are spalted.  Look for trees with their bark still on.  Those trees are more likely to have the Blue Stain coloration.  You needn't cut too far into the log to determine if it's spalted. Start near the base of the tree, or where the base of the tree would have been depending on how you've gotten your log, and cut up just a couple of inches. If there's spalting in that piece, then it'll start near the bottom and work its way up the tree.  Often, it'll continue up into the primary canopy several dozens of feet up.


Now that we've got a piece of spalted wood, how do we make the best use of it? This is the hard part!  Unfortunately, or should I say "fortunately" for the beauty of it, spalting isn't consistent throughout the diameter or the length of the tree.  The sporadic patterns created by the fungi can dive deep into the piece or spread out wide and seem to almost disappear but reappear further up the piece.  About the best you can do is look at both ends of the piece you're working and make a guess as to whether it's going to continue through it and about where it'll run.  Trying to cut away the surface of the wood continually deeper will just waste what color is available especially the Blue Stain coloration present, if any.


The White Rot coloration runs parallel to the grain of the wood and is not always very deep into the heart of the tree. Therefore, spalted woodturned pieces look better on spindles and bowls, platters, and other faceplate turnings that run with the grain rather than against it. Say you're making a natural edged bowl with the top of the bowl facing the bark side of the tree. Most spalting will be running through the sides of the bowl and not along the length of it.  You won't get the most effect from the spalt that way. The spalt will often be broken and not continuous lines that the eye can follow. Often, a bowl with the bark side of the tree near the base of the bowl will look better because of the way spalt tends to run from the outside of the diameter of the log to the inside. It'll appear as if the "flames" of the spalt lines are running up the sides of your bowl. You'll capture more of the Blue Stain coloration this way too.  The bottom of the bowls often end up with more of the spalting than the sides. When cutting your spalted logs down the middle (taking the pith out), keep on cutting closer and closer to the bark until you get a good indication of spalt on that side too.  Many times, you'll see woodturned pieces with beautiful spalting on one side and then absolutely nothing on the other side.  This can be nice for the contrast but I personally think that that piece of wood wasn't used to the best degree as it could have been. Either something smaller should have been made from it or a repositioning needed to be made to get that spalt as uniformly present on that piece as possible. Again, you don't know exactly what you're going to get until you get down to when you're nearly done. That's part of the fun of woodturning, right?


Spalted wood machines differently than unspalted wood.  Mainly, I'm talking about the White Rot decay right now.  The Blue Stain doesn't affect much in the way of machining or finishing. White Rot, being a decay process, causes changes in the wood fibers and these can cause difficulties in working it.  Be aware that the normal working characteristics of a given specie of wood gets changed by this decay process. There will be intermitent areas that are softer and harder than normal and doesn't always follow throughout the entire piece. You must constantly adjust to the changing conditions even more so than normal.  For woodturners, this is represented by terrible tearout and fuzzy cuts on otherwise fine working woods. Extra care in using a very sharp tool and light cuts helps as does wetting/filling those areas with waxes, oils, glues, or a thinned amount of the final finish.  This will help to stabilize and stiffen those partially decayed areas enough to cut it cleanly.  Still, on very large zone lines, the woodturning gouge will tend to bump on that much softer area and this will cause problems with uneven and chattering cuts. Instead of "rubbing the bevel" as is the usual process for light cuts, try "floating the bevel" instead. This will keep the bevel of the tool from dipping slightly into those zone lines and causing the jumping of the tool.


Sanding spalted wood has it's own problems.  Much like pine and softwoods behave when sanding, the alternating soft and hard spots caused by the decay process especially at the zone lines cause uneven sanding.  You'll notice that there will be slight lines standing above the other areas because of this. Those zone lines will easily wear away while sanding and leave an uneven surface. Power sanding or hand sanding with the grain and along the length of the zone lines will help with this. Bleeding can occur on the larger zone lines as well. Sanding the dark lines will cause the dark color to streak across the surface. There's not much you can do about this other than sanding by hand parallel to the lines in those areas where it has bled to wear away the dark streaks.


Drying of spalted pieces tend to be generally easier but those zone lines like to crack. Because of the decay process, much of the moisture is either already released or is easily released as you turn it. This makes the drying process easier because you don't have to worry as much about a great many cracks or warping. This isn't to say that those issues are completely gone ... just tend to be better with spalted wood. The one area where drying causes more problems is with the zone lines (do we see a pattern here with the troublesome zone lines?).  Cracks, if they are going to develop, like to run along those zone lines. It makes sense, though. Those lines are weak and are easist to crack. Regardless of whether those lines are in the endgrain areas, seal them with whatever method you use (wax, emulsions, paper, etc.) too.


Finishes behave a little differently with spalted wood too. Oils are great here!  What might appear as fairly bland spalting (how can spalting be considered bland in any sense?) as you are working it, really pops out with the use of oils.  Of course, the use of oil will darken the spalted areas greatly and can be good or bad depending on what you want. I especially like pure Tung Oil for spalted woods. Not only does it greatly define the color and fine lines of the spalt, it also is a drying oil (albeit slowly!) that gives the decayed areas some strength once dried. Care must be used when using any kind of water-based finish.  Remember the bleeding caused by sanding mentioned above?  It's even worse with water-based finishes. Grain raising is also more of a problem with spalted woods too. Because of the uneven hardness and dryness factors of spalted woods, finishes penetrate or soak differently. Blotchiness is the result.  The use of a sanding sealer or a thinned final finish helps a lot here.  Take time to let this really soak in and then wipe off the excess so you'll get a good base in the wood before your final finish. This is always a good idea with any wood but especially for spalted wood.


At this point, let me say that there's been some concern over the health effects of working spalted wood.  There's not been overwhelming evidence of people getting sick but there has been a little.  The problems seem to arise from the spores of the fungi causing respiratory problems especially for those with preexisting conditions such as asthma or certain allergies. This is very much like the sickness encountered by farmers working their hay and the fungal spores present in them.  Proper drying of the wood before working it will stop the growth and kill the fungi present in the wood BUT the spores will still remain and could cause health problems.  I strongly suggest that you discuss the possible exposure to fungal spores with a suitable health professional and determine if you are at unusual risk before working with spalted wood.  Regardless, I would recommend taking precautions such as using adequate dust collection equipment and wearing a personal respirator or mask. Protect yourself as you see fit.


Come to me, my fungal friend


Now, we know what spalting is and how to work it. How do we MAKE it?  Give it up. It's not about you this time.  It's about Nature.  This is one of those rare moments the we, as woodturners, craftspeople, artists or just wood-butchers must defer to the natural processes of the environment, microorganisms, soil, and just plain fate.  You can't "make" wood spalt but you can "encourage" the formation of it.  I know, it's just a difference in what I think two different words mean. Let's cut to the chase, eh?


There are lots of "recipes" for spalting wood that can be found all over the internet; in a few books and even from magazines touting their own secret (how secret can it be!) recipe. Using every imaginable ingredient they can think of that makes their daffodils grow like bamboo, these recipes appear, at least to their creators, to take a wood that is un-spalted and make it spalt.  Why would you waste perfectly good beer on this?  How much effort do you want to give to pressure treating oak with 500 pounds of sugar?  Got some leftover MiracleGrow? Sprinkle that in!  It MUST be the Argentinian Oak leaves that they covered the logs with that made the difference. I'm sure of it.  <g>  Note the sarcasm in my writing?


Spalting occurs in many species but most commonly in the birches, beeches and maples. Buckeye, elm, basswood, sycamore, apple and the hickories spalt too, but it is relatively unknown in red and white oak although it does occur and can be quite dramatic (see below). Walnut spalts too but you can't see it too well because of the dark color.  Look at the light-colored sapwood in Walnut to find it there.


Turned Box Spalted White Oak.JPG (30525 bytes) Spalted Whiteoak Candle Holders.JPG (18791 bytes)
Spalted White Oak


I'm not going to give you a recipe for spalting wood.  There's no such reliable method of producing spalted wood on a large scale. It's either that or I don't want to give my commercial secrets away!   The only thing I can do is to help you "encourage" the natural formation of spalt (White Rot and Blue Stain) in acceptable woods.


The overriding factor to help you spalt wood is to make your friend, fungus, happy.  Remember, fungus, specific fungi, cause the decay and coloration that we call spalt.  Encourage the growth of that fungus and you'll be more apt to get spalting.  Actually, the hardest part of spalting wood is not getting it to start in most cases but it's getting it to stop before the wood is a bunch of mush.


So, what does fungus want?  Pretty much what we all want ... a nice place to live and grow; an environment not too cold or hot, too wet or dry; enough friends so that we can tear the place down with the party and then move on.  Oh, wait, I've gotten off track ... back to the fungus. Here are the best growing conditions for the fungus we're trying to help grow.


White Rot:  The ideal conditions for this fungi include temperatures from 70 to 90 F; moisture content around 30%; lots of oxygen; plenty of good wood to live in and chemicals IN that wood to eat.

Blue Stain:   This fungi type needs temperatures over 60 and under 150 F to thrive; moisture content between 20 - 30%; a good oxygen supply; nutrient rich food supply; access to bugs and other critters to bring the Blue Stain fungi in.


The main factor for the formation of any fungi is moisture.  Keep it moist, but not waterlogged, and there will be fungal infection.  The key is to get the right kind of fungus to grow.  Some areas simply won't support spalting. Fungal spores are found almost everywhere but it's the specific type of fungus for spalting that we're concerned with and there must be a threshold of fungus present to kickstart the spalting process. If one area doesn't seem to produce spalting, move the wood to another area and try again.  Some areas are better than others as well. The best I've found, without building a special structure, is in a valley where water frequently runs in heavy rains but yet is covered with lots of organic matter (leaves, manure, bark, etc.).  Old horse corals are great too although I suspect about any animal coral will work!  Keep the logs with the bark still on and in contact with the ground.  Keep the logs with the ends in contact with the ground.  The tree's natural vascular system will transport the fungus up and throughout the entire log section up to several dozens of feet high. It can do this quite rapidly too so that there's little difference between the amount of decay between the bottom (next to the soil) and the top of the log.  I regularly (several hundred logs a year) do this on logs up to 16' in length!  Don't bother with storing logs with the bark next to the ground. You'll just end up destroying the protective bark on those sides and letting the log dry out too quickly through the ends. It's tough to get enough moisture into the log without introducing Brown Rot (see the beginning of the article) and destroying it all.  You can seal the top of the log to help keep moisture loss to a minimum if you want.  Keep air movement to a minimum as this decreases moisture content. The fungus isn't going to come from the air. It'll come from the ground matter and the bugs/animals/birds but the specific fungus has to be there in the first place in order to cause the spalting process.


So, how will you know when the spalting is "done" ?   Take very small slices off the bottom of the log and look at it.  Simple as that.  That's how I get most of my turning stock.  If you do get some good spalting on the bottom, try cutting a bowl blank out.  If you have good spalting at the top of that blank then keep going.  If you notice that the spalting tapers off or just isn't present at that level, you can simply put the rest of the log back down onto the soil and let it continue.  The time it takes depends on the environmental conditions.  In tropical climates, it can take just a few weeks with some woods. I've found that, around the Ozarks area, it takes about 6 weeks to spalt hard maple in optimal conditions.  Sometimes it takes 3 months or more.  You just never know and must check it every so often.


The long view (or, effects of long-term spalted wood use)

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The absolute best place to find spalted wood is at a woodturning store that stocks wood (especially by us!). <grin>  You'll know what you're getting and not have to wait around for months to maybe ... perhaps ... get something useful and pretty.  Best reason of all is that I make money selling to places like that with my own spalted woods. <bigger grin>


I hope this has helped clarify what spalting is and given some ideas about how to use it and help it develop.  There is a lot more information on wood decay and coloration out there. I'll add some useful links below as I find them.


Helpful Links

Etiology of Red Stain in Boxelder

General --Wood rotters

Forest Industries Publications

White-rot Fungus

Blue Stain

Wood Rot