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Spalted Wood, Part 1

This is the first of three articles on techniques for working with spalted and degraded wood.  It covers, among other things, the methods for mounting problem wood on the lathe.  Part 2 deals with roughing out the form and getting a clean cut.  Part 3 covers finish turning and applying a finish.

Fungal growth in some woods during the initial stages of decay produces regions of discoloration and intricate patterns of dark lines. The process is known as spalting.  Spalted wood is favored by many turners because the spalting adds another dimension to the figure of the wood.  

If you wish to learn more about the scientific aspects of spalting, do a web search for “spalted wood” and you will find a ton of information.  

There is a downside to the spalting process; it weakens the wood because the spalting occurs as the wood is actually rotting.  If the decay is well advanced, the wood may be degraded to the extent that it is useless.  

Spalted wood may have parts that are very hard right next to regions that are very soft.  This lack of homogeniety complicates matters for the turner.  Also, the wood may be crumbly. or stringy, and very difficult to cut cleanly.

When is wood too degraded to work?  There is no simple answer to this question.  It depends, largely, upon the amount of time and effort you are willing to expend to salvage something from the wood. As you begin to work with spalted wood, you will soon be able to judge what is practical to try to salvage, and what is not.  

Wood that has decayed to the point where most of its strength is gone is known as punky wood.  It may be so soft that a nail can be easily forced into it by hand, or it may be possible to tear sections of the wood away with the point of a knife.  In this case, it is not likely that the wood can be worked; the decay has simply gone too far.

Frass.  This fine powdery material occurs tightly packed in pockets or channels in wood that has been infested by insects; it is bug poop.  It may be compacted to the extent that it appears to be part of the wood, but it will erode away during turning and sanding and leave either a depressed area or an outright hole.  In my opinion, it should be removed and the resulting hole filled in some manner.  

If a blank has numerous pockets of frass, I will reject it. I have had little success in firming up frass, leaving it in the wood, and having it look like anything except what it really is.  

Special consideration is due in several areas when turning spalted or degraded wood.  These include mounting the piece on the lathe, getting a clean cut with the tools, sanding, and applying a finish. That is, almost the entire project will involve techniques that go beyond what is required for sound wood.  

Hazards in Working Spalted Wood  

The immediate danger in turning degraded wood is having it come apart and fly off the lathe.  Precautions include keeping the RPM at or below the minimum sound-wood speed suggested for the size piece you are working. (See the article on Lathe Speeds.)

The most dangerous defect in wood, spalted or not, is the presence of ring shakes.  In this case, cracks develop parallel to the growth rings.  Shakes can be produced by a lightening strike, damage to the tree as it is felled, or simply by the drying process.  They are fairly common in degraded wood.  My advice is simply not to turn wood that has ring shakes.  

Beware of voids and bark inclusions.  In regard to a bowl or hollow vessel, a void in the wood weakens the overall structure.  The same is true for sections of bark that are included in the wood.  Make sure enough sound wood surrounds the void or inclusion to provide the necessary strength to hold it together.  

Avoid breathing wood dust.  Turning and sanding generates a lot of dust.  If you work in an enclosed space, some sort of dust collection equipment is almost mandatory even if it’s only a makeshift system with the hose of a shop vacuum mounted near the origin of the dust.  Wearing a good dust mask is advised.  

Whether working with spalted wood presents a greater hazard is still a subject of considerable debate.  Some say that dormant fungal spores associated with spalted wood can become active when ingested into the lungs, but yet, documented cases of respiratory problems clearly attributable to spalted wood are rare, if they exist at all.  

Nevertheless, play it safe and protect yourself from the dust. Even if spalted wood presents no greater danger as far as toxicity is concerned, turning and sanding it does produce a lot of dust because of the degraded nature of the wood.  

Mounting Spalted Wood on the Lathe

Not everything described in this section has to be applied to every piece you attempt to turn.  What is required depends on the degree of spalting and the amount the wood is degraded.  You play it by ear.  

Sometimes prominent spalting can be present while the wood is basically sound.  In this case, little if anything extra is required.  However, some woods may degrade with no spalting at all so that spalting, by itself, is not a good indicator of the amount the wood is degraded. For the purposes of this discussion, the assumption is that the wood is well advanced in the decay process and requires special treatment.

Mounting the piece between centers.  First of all, plan to use the largest-diameter drive center you have that is practical, relative to the size of the piece.  

To make the installation more secure, use a spade or Forstner bit to drill a hole about 3/8” deep that matches the diameter of the drive center, and then insert the drive center into the hole.  Drill a similar hole for the tail center.  

If the wood is soft or tends to crumble badly, apply a few drops of thin CA to the bottom and side of the hole for the drive center.   At the other end, put a drop or two into the hole where the point of the tail center will go.  This will reduce the amount the point penetrates into the wood.

If you get severe tearout when drilling the holes for the centers, repair the damage by packing fine sawdust onto the bottom of the hole; then firm it up with thin CA.  

Mounting the piece on a faceplate.  Use the largest faceplate you have that will fit the piece.  Consider making a faceplate extension if all your faceplates are small.  

Use screws with course, deep threads as large as you have that will pass through the holes in the faceplate. Drill pilot holes for the screws if they tend to crush the wood as you drive them in.  

If the ability of the wood to hold the screws appears marginal, mount the faceplate but do not fully tighten the screws.  Then, remove the screws and faceplate.  Drip a little thin CA into each screw hole and give it a couple of minutes to cure.  Then, reinstall the faceplate. Do not overtighten the screws; barely snug is good.

Use tailstock support whenever it is possible to do so.  With tailstock support, most of the stress is taken off the faceplate screws.  In fact, the faceplate becomes little more than a jam chuck with friction supplying most of the torque to turn the piece.  

Mounting the piece in a scroll chuck.  First of all, with degraded wood, the use of a scroll chuck in expansion mode (internal chucking, where you expand the jaws in a recess) is not recommended.  It is generally accepted that the expansion mode does not provide as much “holding power” as having the jaws compress the wood by gripping a tenon, even for solid wood.

Form as large a tenon as the design of the piece will conveniently allow because a larger tenon distributes the force over a greater volume of wood.  Then, if you think it is warranted, apply thin CA to firm up the gripping surface and the outer 1/4” on the end of the tenon.  

Don’t overtighten the chuck jaws; you can crush the tenon or cause cracking where the tenon joins the shoulder.

A red flag should go up if you have trouble getting a clean cut for the tenon.  At some point, you will have to either remove the tenon or incorporate it into the foot of the vessel, and chances are you will have similar problems during that phase of the turning.  

Mounting the piece on a waste block.  If you have problems with the tenon, you might consider attaching a waste block to the piece.  What this means is to glue a disk of sound wood onto the flattened bottom of the vessel and then cut the tenon in this sound wood.  The glue surface between the waste block and the piece will distribute the stresses of turning over a larger area and avoid the risk of having the jaws of a chuck either crush the tenon or simply tear it off the piece.

A related option is to use a contrasting wood for the glued-on disk and make the vessel into one with an attached foot.  A tenon is formed on the disk for use during turning, but later, the tenon is removed and the remaining part of the disk shaped into a foot for the vessel.

Using a screw chuck.  The use of a screw chuck (basically, a faceplate with a single screw at the center) is not recommended unless you can use tailstock support for the entire procedure.  You simply cannot depend upon the single thread to hold in degraded wood.