Pine Case Hardening

[busenitzcww]

I have a Nyle L200 and dry a lot of Hardwood. I had a customer who messes around with building guitars and he likes to experiment, so he went to the big box store and got 3 semi qtr sawn boards with 2 of the containing the pith. I didn't measure them when I put them in the kiln but I'm assuming they were around 20% or lower being construction lumber. Left them in the kiln a couple days and they measured around 7-9% mc. He took them home and resawed them into guitar tops but said they were "case hardened" as they cupped after he resawed. My question is did this happen when going from 20% mc down to 8% mc or earlier, or is it just from having the pith as part of the board?

[Southside]

Case hardening would have meant there was a moisture difference between the core and the jacket wood.  You say pine and box store, so I am guessing SYP, and plantation grown.  Wide growth rings, juvenile wood around the pith, enough internal stress that banding is needed to keep the lumber from twisting.  It's possible that was not even KD lumber to begin with as you can have air dried construction lumber as long at it's 19% or less before final surfacing, so your kiln brought out the final movement just due to the drying process.  

He bought the wrong lumber to make into an instrument, it's on him, not you. 

[busenitzcww]

I should say I just put it in with the rest of my hardwood load @ 115° and wet bulb at 70 or something as I air dry all my lumber down to 20% first and figured that the pine would just need finished off the last few percentage and would fit in alright. Haven't seen pics yet but I'm guess the pith is where it's cupping the worst.

[Don P]

Pine construction lumber probably went through a hi temp kiln, above boiling point, and exited in a day or two. If it was case hardened, that is more likely when the tension set occurred rather than late in the cycle. 

This is a paper on hardwoods but read the stress relief paragraph on pg 2;
Review of High-Temperature Kiln-Drying of Hardwoods (usda.gov)

[Ianab]

Pith in the board is the problem. With pine the first few growth rings is called "juvenile" wood, as it's laid down when the tree was just a sapling. It has a physically different structure, and actually shrinks in length a bit as the board dries. So one area of the board has shrunk, and other bits havent, and the board is naturally going to move.

Like Southside says, it's the wrong wood for making instruments, and isn't even that great for construction.

[K-Guy]

It was the mill that sent it to the big box that damaged it.

[busenitzcww]

Pith in the board is the problem. With pine the first few growth rings is called "juvenile" wood, as it's laid down when the tree was just a sapling. It has a physically different structure, and actually shrinks in length a bit as the board dries. So one area of the board has shrunk, and other bits havent, and the board is naturally going to move.

Like Southside says, it's the wrong wood for making instruments, and isn't even that great for construction.

yeah I figured he was going to cut the pith out but yeah it's hard to make something of any quality out of construction lumber

[jimF]

Casehardening has nothing to do with a moisture difference between layers.  The presence of a moisture difference can give the appearance of casehardening being present. The degree of casehardening can only be measured when there is no moisture difference.
All lumber whether airdried, or kiln dried will have casehardening if it is not conditioned properly.
The presence of juvenal wood will cause problems with dimensional stability. One needs to see the distorted wood and the casehardening test samples and the moisture gradient to make a definitive  judgement.
If further dried correctly and completely construction grade lumber can be used for instruments.

[Ianab]

f further dried correctly and completely construction grade lumber can be used for instruments.

Yes, but you have to be picky about the individual boards you choose, and avoid that pith and surrounding juvenile wood. Even if the pith isn't in the board, you can judge how close it was by the curve of the growth rings. If they are fairly flat, then the board was sawed well away from the pith, and should be fairly stable. 

Last log I cut was a ~36" cypress, pretty nice log apart from a couple of punky spots.  Got some nice material from it, and some not so nice. A couple of 4x2s from around the pith were just a collection of knots with a heart check up the middle. Might be good enough to hold up a chicken shed, but that's about it. 

So the fact that some of the boards contained the pith is an explanation for them misbehaving. Other boards cut from 4" away from the pith might be perfectly fine.  How close you can get to the pith depends on the species, and how it dries and shrinks. But it's always looking to mess up your day. 

[GeneWengert-WoodDoc]

We need to be sure that we are using the word "casehardening" correctly.  Casehardening is created at very high MCs (roughly above 70% in pine) when the ting shell dries and tries to shrink, but the wet core prohibits shrinkage.  So, the shell dries to a larger size than if it had been to unds like the cup showed up a day or so later, so we are dealing with Dr shrink normally.  Here is the key...casehardening will normally show up immediately when machining dried wood.  A MC gradient can confound the results, but casehardening causes immediate warp when machining.  It seems like thee Fe was a day delay n cupping, so the issue is moisture along with juvenile wood mentioned in another posting.