Air dried vs kiln dried

[Gregor1]

Will you use air dried lumber, if the moisture content is low enough, and would would that moisture content number be?  Thanks  Greg

[MichaelMouse]

Will you use air dried lumber, if the moisture content is low enough, and would would that moisture content number be?  Thanks  Greg

Is it winter or summer?  Will it be indoors in heated/air-conditioned space?

All my stuff is air-dried, and winter is ~5% indoors.  Summer ~10%.  So use it by season and environment.

They normally KD to 8% on hardwood, but it will try to follow the RH the instant it comes out. Softwoods vary from construction grades SD at ~15%, and select grades at the same 8%. See FPL Forest Products Laboratory -- Forest Service -- USDA (fs.fed.us)  for the variations due to RH and hysteresis.

Build tight panels in summer, a bit looser in winter and realize the wood will move.  Allow for it.

[jteneyck]

Will you use air dried lumber, if the moisture content is low enough, and would would that moisture content number be?  Thanks  Greg

Depends upon what you mean by air dried.  If it's air dried outdoors it will never get below about 12% where I live.  I'm not using it to make furniture or cabinets at that moisture content because that's higher than the highest seasonal moisture content of wood in my house.  But if I bring it into my basement shop it eventually will drop down to about 7 - 9%, where my shop varies between winter and summer.  At that point it's fine to use.  

It doesn't matter whether it's air dried or kiln dried, as long as it's in harmony with your shop when you build with it all will be fine.  Hopefully, your shop will be consistent with where the piece eventually will reside, too, but that's not a requirement with proper build technique, as MM said.  Kiln operators in many parts of the country, where I live included, will dry wood to 6 - 8% average moisture content.  Of course it doesn't stay there but that's where acclimating the wood to your shop and knowing how to build comes in.  

John

[Bill Tindall]

Will you use air dried lumber, if the moisture content is low enough, and would would that moisture content number be?  Thanks  Greg

Commercial lumber is dried a bit below 7% moisture content then steamed back to relieve drying stress to the typical target of 7% .  Because the lumber has been taken below 7% it will respond to humidity changes some less than lumber air dried to 7%, but this difference is not a compelling reason to avoid well air dried lumber.  

Glue will cure slowly in 12% lumber (data from Titebond tech service) and this lumber can shrink significantly when exposed to low humidity.  I would not risk building with 12% lumber, but I am told this moisture content is typical for English "KD" lumber.  

Some will claim that air dried walnut is more beautiful.  Indeed the reds are lost in drying and for sure lost in steaming.  After the furniture sits in a sunny room for a year you will not be able to tell the difference.  These reds are fleeting in sunlight.  (Walnut has a green cast fresh off the saw, but that does not last in the presence of oxygen and sunlight either)

Build with lumber insufficiently dry for the build's home and it will become an experience you will never forget, or repeat.

[iclark]

I have seen many comments over the years about color differences for some woods between air-dried and kiln-dried. I keep seeing comments about that for cherry and black walnut. AD seems to be the preference.

There are also reports that some woods are easier to steam-bend AD vs KD.

[2Goober]

I've heard that air dried has better color. One reason I stick to kiln dried is because I've always heard it kills the bugs in the wood. I think air dried lumber would have less internal stresses?, maybe. What do you guys think?

[Bill Tindall]

Stress comes from drying too fast.  Ideally while drying the difference between the internal moisture content and outer is kept modest (modest moistrue gradient).  It would be typical to have 100,000 bd ft in a modern kiln and the customers for this lumber are demanding furniture factories buying trailer loads.  Be assured that every precaution is taken at the kiln to prevent stress in the dry product or that trailer load will be headed back for a refund.  A difficult to dry species like oak will be predried in structures that will maintain control over the rate of drying before moving to the heated kiln.  Temperature and air flow in the kiln will be adjusted to remove a target amount of moisture each day.  Finally, the lumber will be briefly steamed to relieve any stress that crept in.  

There is no control of drying rate with the stack of lumber in your back yard.  Put up a stack of red oak in a hot climate in the summer and it will case harden from drying too fast, and probably check and warp. Put up the stack in the winter and it may sticker stain.  There are lots of ways to ruin lumber that large operations know how to avoid.

Lumber can be air or kiln dried to yield a quality product.  And in either case the product can be ruined from improper drying.  If you would like to place bets put your money on an operation with 100,000 at risk . I have experienced poorly dried lumber from small kilns operated by someone that didn't or couldn't control drying and hobbiest drying in their back yard.  

It has been my experience that advocates of air dried lumber are woodworkers that do not have access to kiln dried lumber,  or do not have access to a kiln to dry lumber they produce.  Claims of superiority are made to cover up the lack of KD access.  Nothing lacking in properly air dried lumber.  But claims of superiority cannot be demonstrated.

[jteneyck]

Bill is mostly spot on.  Kiln operators have control over the drying conditions for any species and thickness they load into the kiln in order to minimize losses during drying, or problems afterwards.  I saw and dry most of my own hardwood, sell some too.  I air dry it first because of my limited kiln capacity.  I can get more loads through the drier per year by first air drying it, so that's what I do.  However, there is a much higher risk of degrade during air drying than if I put it straight into the kiln, and sometimes I've gotten bitten.  Think blue stain on sycamore, checking in white oak.  I've had to adapt to what I cut at particular times of the year to avoid problems.  

The one species where air dried, or at least unsteamed wood looks much better is with walnut.  Commerical kilns intentionally steam the wood to make the sapwood look more like the heartwood.  But doing that makes the overall color look muddy and eliminates the reds, greens, and purples that are in air dried walnut.  Those colors will be softened after a few months/years when air dried lumber is turned into a piece of furniture, as Bill said, but the clarity of the grain remains much better than in steamed walnut.  

Wood air dried outdoors and which has never been brought down below 12% MC or so is far better for steam bending than commercial kiln dried lumber.  

Commercial kiln drying kills all bugs.  Air drying does not, nor does a solar kiln like I have.  If I know there are bugs in the wood I either dispose of it, or I put it in my small electric kiln where I can take it up to 140F long enough to kill any bugs.  

You can't go wrong buying commercial kiln dried lumber from a reputable outlet, but you may be able to get more variety (species), custom cutting, and lower prices from a small lumber miller/drying outfit.  Look around your area to find out what's available to you.  

John

[Bill Tindall]

The experiment:  Typical 8/4 piece of fresh sawn walnut plank.  Split to make 1" pieces.  Steam one.  After steaming KD the steamed piece.  Air dry the mate.  Set in south exposed window in eastern Tn for months of June and July.  Photograph. 

More data.  Period furniture would have been made with air dried lumber.  Even see any reds in it after a century or two?

More data.  Any walnut not colored during finishing will bleach to a light beige after a few years by a sunny window.  I refinished a lacquered walnut conference table that the owner thought was some different wood because it was so light, about like aged ash.  

If the walnut will be stained the color before staining hardly matters.  If not stained, or oiled which creates color,  the reds are fleeting upon exposure to sunlight.  It is for this reason I always stain my walnut pieces with burnt umber mineral pigment stain.  (burnt umber is a redish brown pigment which provides a natural looking color to walnut.  )

[Roly]

Will you use air dried lumber, if the moisture content is low enough, and would would that moisture content number be?  Thanks  Greg

Besides moisture, kiln dried will set the sap if you are using pine or fir which if air dried can be a messy issue.   Roly