Air dried vs kiln dried

[Bill Tindall]

I had not either until I experienced it.

After a tornado came through the area, I wound up with oak (probably white, I thought) slices that were 12"-14" thick. The trunk was ~30" x 36". I kept one slice as a rough-edged bench outside my shop.

I prepped some of it into roughly 12" cubes for turning and sealed them. The sealing did not work well enough.

After 6 months or so, I noticed that the end grain of one of those cubes was starting to resemble a slice through a honey bee honeycomb. All across the surface on both end grain sides, voids had opened up (lots and lots of them) where the grain was pulling apart. The voids seemed almost as dense as the voids in perf metal.

It appeared that the wood inside the cube dried faster than the wood on the sides of the cube. Thinking about it now, I suspect that it was red oak and that my sealing job worked better on the sides than on the end grain.

Sorry, but I was so disgusted that I did not take any pictures.

 When wood dries it shrinks.  Once dry it becomes rigid.  When not dry it is sort of rubbery and can deform under stress. The situation begins with the piece of wood drying fast for any reason- hot day with a breeze or in a kiln out of control.  

Step 1 The outside of the wood becomes dry while the inside is still wet and swollen with water.  The outside becomes rigid because it is dry.  The inside is under wicked compression because the outside is trying to shrink and it can't because the inside is still wet and swollen.  

Step 2   Eventually the inside dries and because it dries it shrinks.  The inside tries to shrink but it is attached to a shell that became rigid early in the drying at a larger volume than if the wood had dried slowly so the inside was not much drier than the outside.  This situation reverses Step 1.  Now the inside is under wicked tenson trying to pull away from the rigid shell.  The lumber is "case hardened" at this point.  This stress can be relieved at this point by injecting steam into the kiln.  Air drying, you are screwed.   

Step 3   Because oak is very weak  at the rays the tension splits the wood at the rays opening up voids as the inside tries to accommodate the larger volume of the rigid outside ring.  

The damage usually does not extend to the outside of the lumber.  To an untrained eye the lumber can look just fine till it is cut.  To the trained eye it will look lumpy.  When cut the inside will have lens shaped voids and there is nothing useful that can be done with it.  Most commonly encountered in 8/4 oak.

[Bill Tindall]

 When wood dries it shrinks.  Once dry it becomes rigid.  When not dry it is sort of rubbery and can deform under stress. The situation begins with the piece of wood drying fast for any reason- hot day with a breeze or in a kiln out of control.  

Step 1 The outside of the wood becomes dry while the inside is still wet and swollen with water.  The outside becomes rigid because it is dry.  The inside is under wicked compression because the outside is trying to shrink and it can't because the inside is still wet and swollen.  

Step 2   Eventually the inside dries and because it dries it shrinks.  The inside tries to shrink but it is attached to a shell that became rigid early in the drying at a larger volume than if the wood had dried slowly so the inside was not much drier than the outside.  This situation reverses Step 1.  Now the inside is under wicked tenson trying to pull away from the rigid shell.  The lumber is "case hardened" at this point.  This stress can be relieved at this point by injecting steam into the kiln.  Air drying, you are screwed.   

Step 3   Because oak is very weak  at the rays the tension splits the wood at the rays opening up voids as the inside tries to accommodate the larger volume of the rigid outside ring.  

The damage usually does not extend to the outside of the lumber.  To an untrained eye the lumber can look just fine till it is cut.  To the trained eye it will look lumpy.  When cut the inside will have lens shaped voids and there is nothing useful that can be done with it.  Most commonly encountered in 8/4 oak.

[jteneyck]

I'm sure there are exceptions in any particular board.  Maybe that's b/c of where I live compared to 5% humidity.

Can you say that board wouldn't have done it kiln drying?  What is honeycombing I've never heard that term.

Sorry, but you're not going to convince me kiln drying doesn't result in a more stressful board.  There is no debate that stress is released during drying. Is it better to do it slowly or rapidly and risk the case hardening issue, too?

I think any particular board with a lot of internal stress such as twisting grain or grown on a hillside is destined for issues regardless of how its dried.

The was a day when all furniture was made air dried lumber, wasn't there?

Honeycombing looks like this: 

It happens when the exterior dries too fast.  It freezes the exterior into a rigid crust, if you will, so it can't shrink as the interior dries later on, resulting in interior cracks called honeycomb.  Those cracks can be huge.  

Stress is not released during drying.  By the very process, stress increases during the drying process, and defects occur when the stress exceed some value. Believe what you will.  Kiln drying is a controlled process.  Well controlled kilns follow schedules that prevent defects from developing.  Air drying has almost no control.  Why would you think air drying results in less stress?  

Yes, long ago all wood was air dried.  That's all they had.  They also only had horses for transportation.  

John