Air dried vs kiln dried

[jteneyck]

Obvious you didn't follow good procedure in your air drying.  A few thousand years with no kiln says you can do just fine.  Read the book referenced above and do it right. 

Where space is not a problem stacks of hardwoods are often air-dried outside to ~13% before they enter the kiln, as a money-saving measure. 

It's how it's done that counts. Kilns aren't magic, they just create RH rather than take what's free.

 
I've read the FPL publication on Air Drying Lumber.  This passage has always stuck with me.  

Advantages and Limitations
The greatest advantage of air drying lumber when compared
with drying by other processes is low capital costs. However,
as the value of the wood increases, kiln drying green wood
becomes more feasible. Species such as beech, birch, and
maple are often kiln dried green from the saw. The limitations
of air drying are associated with the uncontrollable
nature of the process. The drying rate is very slow during the
cold winter months in the northern sector of the country. At
other times, hot, dry winds may increase degrade and volume
losses as a result of severe surface checking and end splitting.
Production schedules depend on changing climatic conditions
of temperature, relative humidity, rainfall, sunshine,
and winds. Warm, humid, or sultry periods with little air
movement encourage the growth of blue stain and aggravate
chemical brown and gray stain.

John

[CStan]

I've had way more trouble with air dried than kiln dried, and I wouldn't even consider it at this point unless I had the time and space to store it in the house a minimum of two to three months before using it. I used to be a believer until my wallet told me I'd better quit believing before it's too late.

[Bill Tindall]

 
I've read the FPL publication on Air Drying Lumber.  This passage has always stuck with me.  

Advantages and Limitations
The greatest advantage of air drying lumber when compared
with drying by other processes is low capital costs. However,
as the value of the wood increases, kiln drying green wood
becomes more feasible. Species such as beech, birch, and
maple are often kiln dried green from the saw. The limitations
of air drying are associated with the uncontrollable
nature of the process. The drying rate is very slow during the
cold winter months in the northern sector of the country. At
other times, hot, dry winds may increase degrade and volume
losses as a result of severe surface checking and end splitting.
Production schedules depend on changing climatic conditions
of temperature, relative humidity, rainfall, sunshine,
and winds. Warm, humid, or sultry periods with little air
movement encourage the growth of blue stain and aggravate
chemical brown and gray stain.

John

What follows applies to the large (100,000 bdft /day production) mills I am familiar with in the southern Appalachians.  They have a lot of money at risk and tend to do things cautiously.  Small operations might do anything.  Product from a small operation may vary in quality of the drying.

Species that dry quickly like poplar will be dried soon after sawing because these species can be dried in a few days.  4/4 oak takes about a month to kiln dry from green to target 7% in a kiln.  That is a lot of expensive building to be occupying.  Hence, slow to dry lumber will be predried in some way or another down to a moisture content to where careful control of the drying rate becomes critical for product quality (about 30% moisture content).  At about 30% the lumber begins to loose water from the cells and shrink. Drying defects come from uncontrolled shrinkage from uncontrolled moisture loss.  None of these mills would risk free flow air drying high value lumber below about 30%.  

Predrying can be done outdoors under favorable conditions.  The picture shows 4/4 oak air drying outside the huge kins at Baillie mill in SC.   This same mill has acres of sheds with draped sides where the lumber is air dried under controlled conditions of air flow.  These sheds are used when free flow outside air drying is too risky.  And there are predryers where the air drying rate can be controlled.  They are like kilns with fans but not heated .  

When the drying rate is controlled by choice of when the lumber is sawed and where and how it is stacked, quality air dried lumber can be produced.  And if you have the bad luck of weather, when the lumber is sawed, or ignorance of what is important, air drying can produce case hardened oak, moldy poplar, etc.  I have dried thousands of bdft of easy to dry poplar and cherry down to about 15-20% and finished it off in a crude kiln, successfully.  I never risked do it yourself oak drying.  There are too many ways of failing oak drying.

[CStan]

That's certainly a nice operation and more than a cut above Joe Blow with a chainsaw mill, sticker stock, and a little land out behind his double-wide.

[jteneyck]

That's certainly a nice operation and more than a cut above Joe Blow with a chainsaw mill, sticker stock, and a little land out behind his double-wide.

Or your typical small time lumber milling operation.  But maybe I'm doing OK because I have successfully air dried many thousands of BF of red and white oak outside over the years.  I've also got bitten a couple of times, which is why I don't cut white oak in the summer anymore.  

John

[rwe2156]

That ain't necessarily so.  I've had white oak split and/or honeycomb something awful because it air dried too quickly.  Air drying has the potential for more defects of all kinds compared to kiln drying.  Almost no control vs. almost complete control.  

John

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?

[CStan]

Or your typical small time lumber milling operation.  But maybe I'm doing OK because I have successfully air dried many thousands of BF of red and white oak outside over the years.  I've also got bitten a couple of times, which is why I don't cut white oak in the summer anymore.  

John

My experience with air dried wood is that it's never really any better than KD, but when it's worse it's utterly horrible.  There's very little upside, but a boatload of downside. With little to gain, why bother?

[jteneyck]

My experience with air dried wood is that it's never really any better than KD, but when it's worse it's utterly horrible.  There's very little upside, but a boatload of downside.  With little to gain, why bother?

Bill explained why pretty well above.  It's about money for the large operations.  For the rest of us air drying is the only option if you want to mill some lumber but don't have access to a kiln.   No argument that KD wood is more consistent if the kiln operator knows what they are doing.  Air drying has a whole bunch of variability even for someone who really knows what they are doing.  But if it's your only option you either take it or leave it.  I'd say my success rate is better than 90% so I'm willing to take the risk.  

John

[CStan]

I'll die not having made nearly the things I've wanted to make. It would be even worse if I'd taken a detour into sawing and drying my own lumber.

My hat is off to you.

[iclark]

  What is honeycombing I've never heard that term.

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.