Air drying vs Kiln drying

[Chuck_W]

I have heard many arguments concerning air dried lumber vs kiln dried lumber. What method is better for producing stable furniture quality lumber?

[Ga_Boy]

Chuck,

I am confident that many will reply to your question.

At the end of the day the difference between AD and KD is kinda like asking do you perfer Fords or Chevys.

From my point of view you made a fundemintal distinction in asking your question.  Asked the question in terms of Furniture.  For the reason you should use KD material as it has been steralized; meaning the bugs within the wood have been killed with heat.

There are two primary distinctions between AD and KD.

KD material has been sterilaized and the moisture content in KD maerial can be predetermined.

My preference is for KD material but then again I am standing up a KD lumber business so I am partial. ;)


Mark

[Ianab]

Properly air dried or properly kiln dried timber should be basically identical.
It is possible to foul up either type and end up with case hardening, checking or stain etc.
Commercially kiln drying is preferred because it is controlled, predictable, fast and should have low degrade. The finished product is at a known moisture content and bug free.

However if you have correctly air dried a similar board for maybe 12 months, then bought it inside to acclimatise to it's end environment for another couple of months, it would be difficult to tell them apart.

So to the woodworker who is making something, it doesn't make much difference, so long as it is correctly dried.

To the mill, waiting for 12 months and then having a product that still needs a little further drying, is an economic disaster.

Ian

[Ga_Boy]

Ian,

Very good points.

The basis for my statements is the material will be used for a commerical product.  If the product you are building is for personnel use then your liability is non-existant.

If you are doing this for a commercial operation then your potential liability is huge for using non-sterial material.  

Your customers would not be in a very good mood,if one day they come home to find a host of critters flying or crawling around their home.  Not to mention how expensive it will be to make the permisis bug free again.

I should have clearly stated that my statements were assuming the products were for a commercial operation. :-/




Mark

[Paschale]

I've been curious about the same thing, Chuck.  The interesting thing for me is when I ask professional woodworkers at some of these woodworking shows what their preferred choice would be.  I asked Kelly Mehler (The Table Saw Book) and Marc Adams (Marc Adams Woodworking School) this question at a recent woodworking show, and both told me they preferred air dried lumber.  Kelly Mehler was really animated about the topic.  He feels that kiln drying "dulls" or "deadens" the final look and finish of a piece of wood, though I wonder if that's more subjective than anything else.  I think for him, there's a certain appeal to doing it the way all those old craftsmen used to do it.  

That being said, he did stress the importance/necessity of allowing the wood to acclimate for a few weeks to the shop or living area before actually beginning work on it.

The more I've thought about it, the primary advantage of kiln drying is the speed of the finished product.  But for me, it seems I never have enough time to be in the shop anyway, and I have more wood than I can use in a year's time, so I'll keep with air drying, and just bring in enough for a given project.  The bug thing is definitely a consideration, but I know plenty of guys who use air dried lumber quite successfully, so I'm going to build furniture with the air dried stuff I've got with confidence, just so long as it's acclimated sufficiently to my house.  For me, that was what prompted my initial questions about air vs. kiln dried:  will it be OK and stable enough to build with.  I think the answer is yes, if it's allowed to acclimate and if you build into your design allowances for movement.

[Chuck_W]

One of the reasons i posted my question about kiln vs air drying is years ago an experienced woodworker told me that KD lumber was more stable than AD lumber. The reason he gave was the sap in KD wood was replaced with water during the process. Since then I have wondered if it was really true.

[Norm]

We use air dried wood for our projects but have a kiln for commercial sales as well. Some folks insist on KD wood and others AD, as a wood seller we try to please both. My personal feelings are that wood that looses moisture slowly is better to work with but it takes so much longer to get it dried that in a commercial setting it's not practical.

[Silverback]

I think that they only reason that woodworkers would not want KD is because the've had bad experiences with certain wood species being KD with a steam kiln.  I'm thinking walnut and the big commerical kilns steaming the sap wood to match the heart wood.  It really deadens the heart wood and takes all the hues of color out of the wood.  Personally, I think that is completely ruins it.  Dehumidfied or Air dryed--6 one way half a dozen the other.

[kilndry]

A couple of clarifications and comments

Kiln dried wood is not necessarily going to be "bug free; if it has been held at the right elevated temperature for a long enough time, it can be assumed to come out of the kiln "bug free". What happens after that depends on how it is handled. If the right bugs show up looking for a meal or a place to live, they'll go after it. A kiln can kill bugs, but won't prevent reinfestation.

And what is the definition of "kiln dried"? You and I might have the same kiln and achieve very different results, or we might have different kilns and dry at different temperatures. And we could both produce "kiln dried" lumber that is totally unacceptable to the end user.

Many applications (most interior furniture and flooring, for example) require that wood be dried to 6-8% m.c. before use. This is to ensure the stability of the wood in it's final form. When you air dry wood, it can get as dry as it's surroundings given enough time. If it's outdoors, it won't get much below 12% in most of the US. If it's air dried indoors in a heated shop, it will eventually get down to the EMC of the shop. But it will take longer and may be more susceptible to degrade and stain and/or mold problems that a kiln can help prevent with good airflow and venting.

Another problem with air dried wood is the movement of the resins in some woods (particularly woods like white pine). If the pitch isn't "set" in a kiln, the pitch can/will "bleed" through a finish or run down the wall if the wood is exposed to high temperatures after installation. I have some pine paneling here in my office that has that problem. Another place it can show up is on a windowsill, where the wood can get quite hot when the sun is shining on it.


I think if you gave most woodworkers a piece of air dried wood and a piece of kd wood, both at the same MC, they would have a tough time telling them apart. But differences in the stresses and the resins might show up after they turn it into a piece of furniture or trim.

[UNCLEBUCK]

I have read many many books on wooden boat building and every book clearly prefers air dried wood over kiln dried , they say that kiln dried lumber is weaker and also the boat is capable of rotting out faster over its life span because the kiln sucks the life out of the wood . Lumberyard lumber for boatbuilding is considered junk and very brittle . But if your not building a boat to go across the ocean I say I wish I had a kiln dryer ! It just makes sense to have one .  :)Good luck with your projects

[Brad_S.]

I don't know, Uncle Buck, that sounds like another so called urban ledgend, don't you think?
That statement they're making about sucking the life out of the wood sounds as absurd as Homer Formsby saying his polish "replaces the wood's natural oils". Forgetting the fact that 99.9% of woods don't have any 'oils', there is a coating of finish over the wood so the oil never reaches the wood anyhow.

[UNCLEBUCK]

Brad_S.The authors of the books I have read are Chappele ,J.White ,Buehler and on and on . Read a book on wooden boat building and you will understand how the term sucking the life out of wood comes about from kiln drying so I dont understand your rush to judgement of urban legend but then again I think  there is a few of you that lay waiting for a argument with high tech terminology .I would rather try and help the question along instead of nose picking but do us all a favor and read a book on wooden boat building .The books I have read are from naval architects and ship builders from Maine .They are not pumped up school teachers ,they are the real deal and this is the information I am passing along on this mans thread to help him . Have fun learning about boat building and next time you go to the lumberyard you will be suprised how different you look at lumber .

[Brad_S.]

Whoa, I cry Uncle! :-[

I wasn't questioning your motives or knowledge. If I offended you, I apologize. I was merely inquiring whether that was a statement you believe or if you were just repeating it verbatim to further the thread. You obviously believe it, and that's fine. I am not trying to start an arguement with you about high tech terminology as you accuse me of, but "sucks the life out of the wood" is not high tech terminology, it is a negative ambiguous statement that doesn't explain anything.

Let me explain that I am approaching this as a wood retailer in the northern US climate. I think things in the southern climate are different.

I sell lumber both air dried and kiln dried. I sell AD to people I feel understand wood and how it behaves. If a customer doesn't understand the concept of moisture content, I really push them towards KD lumber.

If the customer buys the wood in June, and lets it acclimate in their shop for a couple months like the books say to do, nothing has been accomplished. Lumber for indoor cabinetry must have artificial heat applied to it to achieve the proper moisture content of 6-8% unless you anticipate and allow for shrinkage in your design. If they build a table with it in September, come January, the table will dry out from 15% or so to 8%, and there is a good chance the table will warp or crack. The now disheartend wood worker will either question my lumber or their techniques, niether of which bode well for future sales. They have wasted time and money, making for one unhappy camper.

Even if  lumber has been stacked in grandpa's barn for nigh on 20 years, it is still not ready for cabinetry use. The reason for this is because of modern central heating. Without a humidifier, the average home in winter here in the north is drier than a desert. First time heat is turned on in the house, grandpa's wood will start to dry out. The same principle holds true with flooring. Letting it acclimate for a couple weeks in summer is not only useless, it can actually be detrimental.

What I'm getting at in a way too round about way is why would drying the lumber in a kiln change the physical property of the wood differently than what will naturally happen in the house? If by saying it becomes more brittle you mean drier and less pliable, then yes, that's true. That's why chairmakers prefer AD wood for steam bending. But that is not a defect caused by kilning the wood,  just a property of dry wood. And it makes sense that kiln dried lumber in a boat would suck water in. It's doing what it always does - equalize to its environment. Your response makes it sound like kiln drying ruins lumber, and to a boat builder I guess it does, but to a cabinet maker, it's highly recomended.

I guess I should have used more emoticons in my original post to lighten it up, but these santas take up too much room between the lines! ;)

[MemphisLogger]

I regularly use both AD and KD material building furniture and cabinetry. AD will get down to 10% in my shop in the summer and 8-9% in the winter. KD material typically comes back from the kiln at 7-8% and if not plastic wrapped will eventually reach the same MC as the AD wood.

I prefer to use AD material if Ill be using alot of handtools on the project. It's only my observation, but AD material seems to respond better than KD. I've also noticed that KD material seems to chip out more with a router or planer. This may have something to do with the shell being dried below target MC during the high temp conventional drying that the kiln uses.

I am extremely careful to check each piece of AD wood I use for signs of pests with PPB being my greatest concern.

Walnut will have better color if AD. Certainly better than Walnut that has been steamed to color the sapwood.

AD material does bend better regardless of final MC so there may be some credence to theories that kiln drying makes the wood somewhat more brittle--again, it could have something to do with the shell having been drier than final target during th KD process.  
  

[jimF]

If a boatbuilder compares AD with the 2X4 or shleving material in the lumberyard. he's right, there is a difference. Structural lumber is dried at higher temperatures(>200F) than appearance grade lumber and faster.  If he compares appearance grade lumber (no greater than 180F) to AD, I doubt he could tell the difference.  Most structural lumber is loaded with surface checks while appearance grade is (should) not.  The surface of appearance grade is planed off so any "overly dried shell" would be removed.

[Den Socling]

OK. Now 2 cents from a vac dry guy. The baseball bat guys tell me two things. Vac dried billets are stronger that conventional KD billets and overdried billets are weakened. If vac dried billets are stronger, I suspect it would be due to the wood fibres never seeing the heat of conventional kilns - just like AD. Overdried is undoubtedly weaker and more brittle. Run overdried wood through a molder and look at the tear-out. If overdried was OK, the bat guys would be doing it to get weight down. You can overdry with a kiln but it's unlikely in AD unless you live in Tucson.

[Ianab]

Re the boatbuilding issue.

Using 6-8% wood for building a boat wouldn't be a good idea because it's equilibruim MC is going to be much higher. Like it's probably going to be stored outside, or in an unheated shed, or on the water. So it's going to settle to 10-15% depending on where you are. This probably gives KD wood a bad name with boat builders, it's usually too dry for them, hence it may be brittle and then swells up in use as it gains moisture. If it was kiln dried to 14% then used to build a boat, maybe not much difference?

So wood thats airdried to the environment the boat is to be stored in is probably the best option, even if there is some urban legends and suspect science in the reasoning.

Ian

[Engineer]

Here's my two cents from a user's perspective.

I think air dried wood is easier to work and has much better color (due to UV exposure).  Kiln dried wood seems to me to be more brittle and more "bland" - no character.  I think you can get the best of both worlds by air drying to a nominal 12-15% MC,and then kiln drying the rest of the way to 6-8%, retaining color and workability while stabilizing the wood and killing off pests and rot.

Unfortunately I find nobody to be marketing wood this way, so I'm air drying my own stacks and searching for some space in a kiln.

[Paschale]

My solution is to air dry the wood, and then bring the wood into my house until it reaches that 6-8%.  Of course, it doesn't ensure that pests are killed, but the boards are free of evidence of bugs--so I'm thinking I'd be alright.  For me, though, I just wanna work on one project at a time, so it's not a big deal to bring some wood in the house to acclimate.  If I were doing this all the time, wouldn't be very convenient.  But the way I'm looking at this, is that the projects are destined to be in my house, so I might as well finish dryin' em in the house.

[chet]

Paschale, I have been using your method for years. I didn't think da bugs really cared weather they hitchhiked in on boards or firewood.
I have since quit bringing in firewood,   ;)  but continue to stack a wack of lumber for winter projects.  

[GHRoberts]

I am told for bending in order of best to worst:

green wood, air dried, and then kiln dried.

Since boat wood and some furniture wood is steamed and then bent, I suspect that heat does not affect wood as much as people claim.

[Paschale]

Paschale, I have been using your method for years. I didn't think da bugs really cared weather they hitchhiked in on boards or firewood.

Well, Chet...it was you who gave me the idea in the first place, at the Piggy Roast this summer.   ;D  So my method is the Chet method, and this winter's the first I've tried--it seems to be working out alright.   ;D  

[chet]

I guess I been using da Chet method all along too.  :D  :D  :D

[Patty]

The Chet method has been working for us too. Catchy name, too!  ;)