what causes case hardening???

[IMERC]

what causes case hardening when drying lumber???
is there anything or any other reason this could happen???

[beenthere]

What is happening to lead up to the question?

Generally, the surface dries too fast, causing shrinkage that leads to wood failure at the surface. The wood at the surface takes on a permanent "set".

When the interior wood dries and shrinks, it cannot compress the surface wood back in place, so the interior wood cracks open. Often the surface checks are then closed and cannot be "seen". The interior checks show up when processing the wood.  Collapse and honeycomb can happen as well.

Different phenomena can lead up to case hardening.

The wood drying handbook available in pdf form will have a more detailed explanation.

[IMERC]

green wood was put in an attic that gets pretty hot (120*+F) and it suffered through some radical temperature swings over the years...

a debate (if you want to call it that) arose over the usable condition of the wood....

so I came here to try and understand case hardening of wood and and it how it happens...

[IMERC]

The wood drying handbook available in pdf form will have a more detailed explanation.

can you point me to the handbook....

[beenthere]

Imerc

Here is a pdf of a chapter in the Dry kiln operators handbook  on drying defects.

http://www.fpl.fs.fed.us/documnts/usda/ah188/chapter09.pdf

This is one of several chapters found here.

http://www.fpl.fs.fed.us/products/publications/pubs_specified_topic.php?groupings_id=113&sort_criteria=title

[Ianab]

In an attic situation it's probably OK. It's really just an air drying situation, more like a solar kiln.

Case hardening usually happens when wood is dried at too aggressive a schedule in a conventional kiln and like Beenthere says, the outside of the wood dries faster than moisture can migrate out from the core.  This leads to all sorts of problems with stability, internal stress and checking.

It's mostly an issue with kiln drying. Easy to dry wood like pine, the kiln operator can crank up the temp, drop the humidity and get the load though quick. With a load of white Oak, if the try and push things to get loads through faster they can end up case hardening and ruining a load.

Once the wood has air dried it's very difficult to mess up with any natural temperature or humidity swings. Temp can go from zero to 100F, humidity from 0 to 100%, the wood might shrink or expand a little, but it wont be harmed.

Ian

[IMERC]

Thanks Beenthere...

lotta reading but it gave me answers...

[IMERC]

In an attic situation it's probably OK. It's really just an air drying situation, more like a solar kiln.

Case hardening usually happens when wood is dried at too aggressive a schedule in a conventional kiln and like Beenthere says, the outside of the wood dries faster than moisture can migrate out from the core.  This leads to all sorts of problems with stability, internal stress and checking.

Ian

Would high heat (120*) and single digit humidity cause the wood to case harden???

[SwampDonkey]

Even just air seasoning wood, like in the attic, causes the surface layers to stretch, put under tension and the core compressed because it wetter. When drying it not rapid the tensile stresses are relieved as soon as adjacent layers dry and shrink, allow more shrinkage on the surface. Too rapid drying causes a permanent set because of rapid shrinkage from moisture loss. If stresses that develop exceed the maximum strength of the wood perpendicular to the grain, surface checks will form. As drying progresses, the core shrinks further, but the set shell prevents normal core wood shrinkage. Stresses become reversed and honeycoming can occur in the final stages. Can remove casehardening in softwoods with prolonged storage at air temps. Or elevated temperatures toward the end of drying can relieve it in softwoods/hardwoods. Casehardening can occur when air drying as well at pronounced high temperature drying.

[Textbook of Wood Technology]

I'm pretty sure attic drying can cause it in some circumstances if it's quite dry air and not high humidity. Uninsulated attics up here tend to have high humidity when we get hot spells and same with barns. I have not noticed any casehardening in my hardwood, but I have noticed some surface checking on thick stock not planed and down on ground floor level. Rough lumber wicks moister away into the air quicker. So just because you see surface checks , it does not means it's case hardened.

[WDH]

If it was for an extended period of time, I would think so.  In natural air drying or solar kiln drying, the wood heats up and dries during the day at lower humidity.  At night with no heat from the sun, the humidity increases and allows the wood to equalize somewhat between the shell and core.  This is called conditioning, and it happens naturally in air drying and in solar kilns.  In commercial steam type kilns, this natural conditioning does not occur, so the kiln conditions have to be set and monitored to dry the wood in a controlled manner so that the case hardening condition does not develop.  In many kilns, at the end of the cycle, water is sprayed on the wood to condition it to where the shell and core are in equilibrium.

120 degrees and low humidity would damage some woods like oak if left that way for too long.

[beenthere]

Imerc
What species are you dealing with, being as there apparently is some suspicion that the attic caused some casehardening. What was the history of the species before going into the attic?  

Thickness of the wood?

Any pics of some cross sections of the wood in question? Surface checks? internal checks?

[IMERC]

Imerc
What species are you dealing with, being as there apparently is some suspicion that the attic caused some casehardening. What was the history of the species before going into the attic?  

Thickness of the wood?

Any pics of some cross sections of the wood in question? Surface checks? internal checks?

Ummmmmmm....
no answers, I neglected to ask....

the Q was presented and I said that the wood could have case hardened because of our low humidity and how hot it could get in an attic...

things spiraled from there.. (you chose direction)

what and why I said what I said was drawn on this tid-bit..

a very long time back or several life times ago I was the labor on branching, bringing to the saw, loading, feeding, slabbing, catching, moving to the attic and stickering X/4's of black cherry, oak, walnut and hickory...
all this wood sat for years waiting to be turned into flooring, trim, cabinets and etc for my parents home....
come that day and the resaw process started it was declared that the wood had case hardened....
It looked like good wood to me but what do I know... I'm just a dumb kid.......
the interior was a different story - stringy/fibrous like a frayed coarse rope and you could break apart the wood fibers with your fingernails... it was as though the wood fibers had separated and the wood was turning(ed) pithy (sp? good word?)... it broke into soft tooth picks comes to mind...
a log left to lay that rotted from the inside out might be a good way to describe it...
some of the planks had a little of this in segments along it's length and others were in that condition for most of the length...
this is best that I can remember it... but no matter... my old man and his declared it was all my fault... buck sawed it all into firewood...
when I mention to dear ole dad that not all the wood was in that shape I was just told to go back to work...

so now my curiosity is peaked and I would like to learn more about wood becoming case hardened for my benefit...

[Tom]

IMERC
To ease your mind, I would feel 99.9% in the right, declaring that your labor had nothing to do with Case Hardening the lumber.   If you had been sloppy in stacking it, you might have been responsible for some crooked boards, but even then, boards will warp regardless of the care used.   I'd venture to say that the sweat of your brow more than covered any responsible decision made for where and how the lumber was dried and how much maintenance it received in its storage, on your part.

It's a good lesson for all adults who pick on the smallest and the ignorant.   You can't make yourself a bigger man by cutting down the ones around you.   Feel pride in the fact that your job was done properly.  You got the wood into the attic.  If there was a fault, let the man who decided to put the wood up there accept the blame. 

[IMERC]

Tom...

I know better than that being my fault...
I knew then "it wasn't me"...
or a bazillion other items....

kinda like...
we know it ain't yur fault but we're gonna blame ya anyways...

Yesterday???
Day before???

when ever it was when the Q came up about using wood stored/cured in an attic for fine furniture I remembered that little fiasco and gave my answer based on that....

and now because of the rekindleing I'm looking for the whys, fors and how comes...

[tyb525]

Sounds like it could be dry rot?

[beenthere]

Imerc
I could say without a doubt that what you describe wasn't case hardening (as Tom said too).
Rot is likely what it was, and sure sounds like it may have been getting rainwater on it while in the attic.
(and tyb has that covered)

Sorry you took the heat. But sounds like you are holding up well regardless.

[IMERC]

Sounds like it could be dry rot?

that was some time ago...
but can a massive amount of wood all be affect the same way???

[IMERC]

Imerc
I could say without a doubt that what you describe wasn't case hardening (as Tom said too).
Rot is likely what it was, and sure sounds like it may have been getting rainwater on it while in the attic.
(and tyb has that covered)

Sorry you took the heat. But sounds like you are holding up well regardless.

what you and tyb makes sense...
but the rain water intrusion didn't happen...
1. we lived downstairs and would have noticed...
2. if it did I would have been delegated roof repair guy the day before the 1st leak showed up...
the thing about the case hardening stuck because I was reminded every so often about that..
so could it still have been rot???

as far as holding up now...
try and pull that fault/blame deal now....

what was - was...
but it isn't now...

GUYS!!!!!

thanks for the help...
the reading material I have will take a while to digest but I'm on my way...

[SwampDonkey]

I'm wondering if it was a combination of honeycombing and collapse from the case hardening. Seems like it was some time ago from your literary meanderings, and memories fade. ;)

May have been rot. But, if it's that bad the roof of the structure and rafters must be in a real bad way and probably the walls of the house as a result are water soaked and rot in them to.

[IMERC]

I'm wondering if it was a combination of honeycombing and collapse from the case hardening. Seems like it was some time ago from your literary meanderings, and memories fade. ;)

May have been rot. But, if it's that bad the roof of the structure and rafters must be in a real bad way and probably the walls of the house as a result are water soaked and rot in them to.

the building and roof were all in fine shape...
trust me...
there was no water intrusion...

[SwampDonkey]

That being said, I rule against rot in the lumber. ;)

[WDH]

If this was in CO, is the humidity there very low for several weeks at a time in the summer?

[IMERC]

If this was in CO, is the humidity there very low for several weeks at a time in the summer?

oh yes...

[IMERC]

If this was in CO, is the humidity there very low for several weeks at a time in the summer?

we had years (7?) of drought ...
bone arid dry and hot...

[WDH]

There is your answer.