Compression wood

[Ned Milburn]

Originally Posted by John Arnold

The reason why you rarely see compression wood all the way around a log is simple.....with a leaning trunk, one side of the tree is under compression, and the other side is under tension.

Prevailing winds would likely be another factor...

[Alan Carruth]

"Instead of relying on general information about the stiffness and density of compression wood, I like to evaluate each top individually. I have found that just like normal wood, there is a wide range of stiffness in compressed tops."

I'm basing my remarks on careful measurements of the properties of (probably) a couple of hundred tops. I've talked with several other folks who've made similar measurements. In general, there is a fairly close relationship between the density of softwoods and the Young's modulus along the grain. If you graph out Elong against density, about 2/3 of the samples will fall within 10% plus or minus of the same line for all species of softwoods I've looked at. Thus you can to some extent predict the Young's modulus along the grain if you know the density. Samples with 'compression wood' tend to be outliers, with significantly lower Young's modulus than the density would predict. As you say, you can see tops with a wide range of density and stiffness, but they most often are less stiff than they 'should' be for their density. If you work to a stiffness target, these tops will end up heavier.

[John Arnold]

As you say, you can see tops with a wide range of density and stiffness, but they most often are less stiff than they 'should' be for their density. If you work to a stiffness target, these tops will end up heavier.

Do you also measure the cross-grain stiffness? when I evaluate a top, I am more interested in that, as opposed to long grain stiffness.

Prevailing winds would likely be another factor...

IMHO, the wind would have to be consistent to have a meaningful effect.
Instead of speculating about the causes, I think it is better to actually look at the tree before cutting it.....then mark the location of each block that comes from the trunk.
For example, the statement that trees tend to have more limbs on the south side may generally be true, but only if the tree grows by itself, with no trees around it competing for light. The truth is, limbs will stay on the trunk until they are shaded out. A spruce grown with no competition is useless for instruments, because the limbs will tend to extend all the way to the ground.
On some of the red spruce trees we cut this year, the east-facing slope was in excess of 50 degrees. Those trees all had more limbs on the east side, where the slope naturally allowed more light. Most (not all) of those trees had compression wood on the downhill side, and if the trunk was curved, it was convex on that side. In that case, the leaning trunk was the primary cause of the compression wood, not the limbs (which tended to be small). The reason I say this is that the non-leaning trees with much less compression wood still had similar limb structure.

[Sam VanLaningham]

Ok so my summary looks like this:

The terms are:
Compression wood
Compression grain
Tension wood
Interlocked grain
Reaction wood


Compression wood: occurs in softwoods to adjust for asymmetric loads to encourage a straighter growing tree (assuming that the load asymmetry pulls a tree away from growing straight).

Compression grain: a less commonly used and understood term that refers to relatively more symmetric banding of compression-style material near the trunk of a large tree.

Tension wood: the hardwood equivalent of compression wood. Found on the upper side of a leaning tree or branch as opposed to compression wood which is found on the lower side of a leaning branch/tree.

Interlocked grain: grain that grows at an angle to typical growth bands, to offset shear stress on a tree. Not easy to work with.

Reaction wood: CONFUSION!

Arnold = synonymous with compression wood.

Carruth = synonymous with interlocked grain.

How'd I do?

Sam

[printer2]

Originally Posted by John Arnold

Do you also measure the cross-grain stiffness? when I evaluate a top, I am more interested in that, as opposed to long grain stiffness.

I hate to go off on a tangent but I just have to ask, why are you more interested in the cross grain stiffness? I always read about long grain stiffness being important but not a lot on cross grain.

[Ned Milburn]

Originally Posted by printer2

I hate to go off on a tangent but I just have to ask, why are you more interested in the cross grain stiffness? I always read about long grain stiffness being important but not a lot on cross grain.

Long grain stiffness has a greater deal of uniformity across a piece. Cross grain stiffness has greater variability and can be much more flexible than long-grain. A soft cross grain coupled with a stiff long-grain IMHO is not a good wood for a top. All stiffness/elasticity measurements of a soundboard are important, in my mind.

[Alan Carruth]

I test for both long grain and cross grain stiffness.

Long grain stiffness is more important structurally, since it sets the limit on how thin you can make the top to avoid it folding up under the torque load. In theory cross grain stiffness can contribute to that as well, but it seems that over time the cold creep in the wood negates any advantage that might confer.

Cross grain stiffness still contributes to the way the top vibrates, even when it's not structurally so important. Mark Blanchard, who, as I do, uses 'free' plate tuning methods to fine tune the bracing, found that the actual shapes of the patterns are far more affected by the long to cross stiffness ratio than they were by the bracing. Basically, to get things to work well in this system you tend to use wood with high cross stiffness for wider guitars, such as Jumbos and Dreads, and the stuff with lower cross stiffness on narrower bodies, like 00s and 0s.

Left of Sam wrote:
"Reaction wood: CONFUSION!"

Actually, there's no real disagreement about what reaction wood is. I'ts characterized by thick late wood lines with an interlocked fiber structure. It occurs on the compression side of softwoods and the tension side of hardwoods. As far as I can tell it's caused by the fact that wood tends to 'cold creep' when loaded in bending. This is due tot he fact that the lignin 'glue' that holds the structure of the wood together is somewhat plastic, and flows in shear, while the cellulose fibers don't flow, and can withstand fairly large tension and compression loads. Bending causes shearing forces in the wood, and the tree reacts by building in fibers that run at an angle, causing the interlocked structure. These will be more in tension or compression.

The confusion comes in because somebody proposed the name 'compression grain' to refer to wood that has heavy latewood lines that don't show the usual interlocked fiber structure of reaction wood. This is enough like 'compression WOOD', which is normally used to refer to the reaction wood in softwoods, to cause confusion.