Kiln Dried vs Air Dried

[Hans Braul]

I recall reading somewhere that kiln drying changes the cell structure of wood so it becomes more stable, with less tendancy to absorb moisture. What is your experience? I have a stack of 20 year old air-dried cherry that I intend to use for a table top. Would I be better off buying kiln dried for the top and using the air-dried stuff for the other parts?

Thanks
Hans

[Chris Barton]

Kiln dried wood is usually around 6-8% moisture content while air dried wood rarely get below 12-15% moisture content (unless you live in the desert). Invest in a moisture meeter and check your wood for its moisture levels. I don't know the answer to your cell structure question but, logically one would expect the center of a kiln dried piece of wood to remain more stable over time.

[Travis Johnson]

Drying lumber is a science unto its own, and this guy did not fair very well in that subject in school. I cannot say with definity that it changes the cell structure, but I believe Kiln Drying extracts the moisture in the wood that is called BOUND WATER.

If I understand what I think I do, Bound Water is inside the very cells of the wood, and therefore hard to remove. Air drying lumber removes the easy mositure, what is called FREE WATER. This is the moisture that resides between the cells of wood.

Even in air drying, or solar kilns, they do what is called a FINAL WALK. That is where the temp is driven up over 150 degrees for over 24 hours. That kills all bugs and bacteria that is present, and also "hardens" the wood. I am guessing that it is this Final Walk that might expell the BOUND WATER and therefore change the cell structure as you mention.

Hopefully someone else will jump on here and either give me credit and say yes I am close, or that I misunderstand the science of drying lumber altogether. In my defense though, i think I am pretty close.

[Steve Wargo]

I read and been told, that the only advantages of kiln drying is the fact the the extreme heat will kill any insects living in the wood and the time required to dry the lumber is decreased substantially. As far as dropping the moisture content of the wood down to 6%... Who cares? As soon as it goes into your shop it starts to increase back to the humidity level of your shop, and then to the home that your furniture is going to be placed in. I use air dried lumber whenever possible, and in some cases I feel that the colors of air dried lumber are more vibrant, especially in walnut.

[Larry Rose]

Hans, the trick to using either air or kiln dried lumber is to allow for the movement that is going to occur in both. I have used air dried walnut,cypress,oak and poplar in furniture that is in my house with no problems. I also have built some pieces from kiln dried that I didn't pay enough attn. to future movement, with predictable results.

[Cliff Rohrabacher]

Yah Kiln drying is harder on the lumber just like you said. It's not so great for steam bending. You can bend it just take it easy.
As for motion after the fact - - - Oh so many factors to consider: Wood type, temprature of the kiln, time in the kiln, ambient humidity after kilning. Even the growth of the lumber is part of the picture.

Then there is vacuum kilning. You can kiln wood with a couple of electric blankets and a vacuum chamber.

However at 20 years old wood (air or kilned), is wonderful stuff.
The older the lumber the greater the degree to which it'll have stabilized. Humidity issues aside: Wood moves more when young - even kilned wood. Time tends to stabilize it. Making stringed musical instruments one sees this movement express it'self dramatically in the thin sections. I prize old lumber even to the point of scavanging antiques for the stuff. I have some mahogany and maple set aside from the 1970s.

I submit that the notion that kiln drying causes wood to move less is entirely a function of humidity and mostly ambient humidity. ust don't make that old fashioned mistake of making a top and then binding or encasing the end grain with a nice wide board. That's a recipie for disaster.

[Michael Conner]

Kiln drying does remove more moisture, but dry is dry the 'dryness' can come from the air or from a kiln. Kiln dried lumber left in the air for a long enough period will gain moisture to the point that it might as well have been AD. Likewise, you can bring AD wood into your home for a few weeks if you want to bring the moisture content down lower. It is true that the heat of the kiln can kill 'bugs', but with wood that has been AD for 20 years, I think I would mill it near final dimension, bring it indoors for a week or so and then finish milling and building. I think it will work just fine.

[Howard Acheson]

Steve has the answer. Kiln drying does not change the cell structure (more to follow) but it does kill any bugs in the wood. It also takes the initial drying down to a lower equilibrium moisture content. But, once the wood is taken out of the kiln, it begins to reacquire moisture to come into equilibruim with the local relative humidity.

Improper kiln drying can affect the cell structure. If done too quickly or without controlling the moisture, it can cause stresses in the lumber which are caused by damage to the cell structure.

[Steve Clardy]

Kiln dried lumber was introduced years ago, with many advances in that field. It was done so the lumber could be used sooner, without the waiting time of air dried lumber.
I will use air dried lumber for myself, as I have a lot of it left over from my days of having a woodmizer bandmill.

But thats where I draw the line. I will not use air dried lumber in products I build for customers. Way to many risks of lumber shrinking after the fact. For instance, raised door panels.


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[Bob Winkler]

Don't trivialize the fact that kiln drying kills bugs. I purchased a batch of 4-year air dried lumber not too long ago. So far, many of the boards that I have cut into have shown active powder post beetle infestation. :eek:

To make sure that I could store this wood safely, I've been soaking each board in pesticide. Not too much fun, especially since some of the boards are very large.

For me, kiln dried is the only wood in my future.

Bob

[Jim Becker]

Don't trivialize the fact that kiln drying kills bugs.

This may or may not be true, depending on the TYPE of kiln drying that was performed on the wood. Not all kiln drying processes use high heat.

----
Personally, I like and use air-dried lumber most of the time, both due to availability and cost. With walnut, I'll only use air-dried as I prefer the rich, natural colors that are lost since most walnut from the kiln is also steamed.

As to moisture absorbtion and "staying flat"...no matter what method is used, the lumber will eventually get to the MC of its immediate environment and if it was dried correctly, similar results should be observed relative to stability. One should always build with wood movement in mind... ;)

[John Timberlake]

I read and been told, that the only advantages of kiln drying is the fact the the extreme heat will kill any insects living in the wood and the time required to dry the lumber is decreased substantially. As far as dropping the moisture content of the wood down to 6%... Who cares? As soon as it goes into your shop it starts to increase back to the humidity level of your shop, and then to the home that your furniture is going to be placed in. I use air dried lumber whenever possible, and in some cases I feel that the colors of air dried lumber are more vibrant, especially in walnut.

I agree 100% with Steve. I have used kiln dried and air dried walnut. Will take the air dried anytime for color and have not seen any difference in stability. Just make sure the moisture content is low enough, around 10%. Either way, you should do some of the prep work on the lumber and then let it sit in the shop for a week or two to aclimatize.

[Hans Braul]

What a fantastic forum! This is my second post and in both cases the answers I got were great.

Many thanks
Hans

[Ian Abraham]

Yes a good bake in a kiln will kill any insects present in the wood, but that wont prevent it getting infested again. If you take your nice kiln dried wood and stack it in an open shed, then it will absorb moisture and attract bugs just like the air dried stuff. They will both end up in about the same condition.

On the original question, examine the wood for signs of bugs. If it's been sitting for 20 years you will see if they are present. If it's clear then bring it inside and let if complete it's drying / acclimitisation and go for it.

Cheers

Ian

[Mike Parzych]

There are some woods where the superior color of air dried surely outweighs what I consider the small risk of bugs: as mentioned, walnut - but also in my opinion cherry, hickory, red elm. In others, such as maple, white ash, or white oak, it doesn't make much difference.

[Rick Peek]

Don't want to hijack this thread, but I was wondering the signs of
active ppb infestation. I bought alot of spalted maple & cherry
burl for an old sawer in the Adirondaks mts.It has holes in many
places but doesnt look active. I have actually cut right right to
the ends of the bore holes and not found any worms, but I'm
a little nervous about using it.

[Jesse Cloud]

Yeah, air dried is full of bugs and will shrink as soon as you put it in a project. If I were you, I would get rid of all that 20 year air dried wood. In fact, why don't you just ship it to me and I'll take the risk:rolleyes:

Truthfully, air-dried is way better for bending, keeps more of its color and is less likely to be stressed internally. A lot of times when I resaw a thick piece of kiln-dried material you can almost see it cup and warp as soon as the interior wood hits the air.

If your wood has been around for 20 years, its stable, as dry as it will get in your climate. I would definitely use it for prime applications.

[Bob Winkler]

Rick,

For me, the wood had small holes on the surface, but when I resawed it, there were a lot of tracks. At first I thought it might be old damage, but after resawing a few more boards, I happened upon some of the wormy looking things now sawed in half.

Most of this wood had at least one side with heavy bark, and that is where most of the bug tracks were. I have heard that debarked wood seems less prone to infestation. I'm no expert. I do know that I didn't want to bring the wood into my basement workshop until it was treated.

Anyway, I'm sure there are lots of good reasons that air drying is great, I'm just saying that for my next load of maple, I going KD.

Bob

[Ken Fitzgerald]

I recently turned a bowl from KD honduran mahogany. It had worms holes and the worms were still at home. Apparently the KD process pretty well mummifies them, however. When I got nearly done, the LOML loved the bowl, wanted it in the house but without the occupants. I bent a small hook on the end of a paper clip and had a devil of a time get the remnants out of the hole. They were "cooked" in place and dry. Thankfully!:D

[Nate Rogers]

The one thing that I have noticed is that kiln dried lumber tends to be more brittle, In my opinion you see a lot more tearout and chipping. Just my 2c
Nate

[Fred Voorhees]

Welcome to the forum Hans!!! Yeah, it's pretty informative around here.

[Ian Abraham]

but I was wondering the signs of
active ppb infestation.

If they are active in the wood you will see signs of fresh little piles of frass or sawdust outside the holes. Not 100% method of course, but if you see fresh dust, then you can be 100% sure they ARE there.

Most bugs that live in green / wet timber cant survive in dry wood. You will see their burrows / bodies remaining, but they die as the wood dries, kiln or no kiln. PP beetles however dont live in green wood, they only live in dry wood. So they must infest it after it's partially dried. They wont be there from the start. That also means that if you come across infested dry wood, get rid of it (of treat it to kill the bugs) so they dont migrate to the rest of your stash. If the woods green, then they aren't PPB, and will die off as the wood dries, and wont be interested in the rest of your stash.

Cheers

Ian

[Allan Speers]

Steve has the answer. Kiln drying does not change the cell structure (more to follow) but it does kill any bugs in the wood.

Howard, I'm pretty sure that kiln drying DOES alter the cell structure. I'm no expert, but I have read many times that kiln-heat changes the lignen, making it stronger / tougher. Some folks on wood.net claim that kiln-dried lumber is harder to work (esp with hand tools)

Additionally, luthiers avoid kiln-dried lumber like the plague. With the spruce (used for the tops) they feel that air-dried is more flexible, thus giving better vibration. With the backs & sides, they feel that the air-dried is easier to bend, with less chance of splintering. Maybe they're wrong, but there's five hundred years of violin & guitar making behind that theory.

[Keith Outten]

Most of the comments made about kiln drying don't apply to dehumidification kilns. The process doesn't use high heat and is very gentle. The process also is controlled so the output is superior to air drying. It takes me from four to six weeks to dry a load of hardwood if the load is absolutely green right from the sawyers mill. I never air dry before placing lumber in my kiln even though it would decrease the drying time in the kiln considerably.

[Tom Sontag]

I only jump in at this late stage to toss in what I have learned, primarily from Dr. Gene Wengert, the Wood Doctor, on Woodweb.com. There are extensive archives there on this and related subjects.

Aside from the insect issue (thoroughly covered here already, except mentioning that any wood can be infested with PPB after drying), the primary reason to prefer KD over AD is cellular memory (my term). Wood first loses free water, then around 25-30% begins losing bound water. Removing water from the cell structure difforms it greatly, like drying a grape into a raisin. If your furniture will be in arid environments like a desert or exposed to indoor heating (this includes most of us), then it needs to be dried down to 6-8% MC so that when it returns to that level, the board will not surprise in its movement. AD lumber rarely gets below 10%, so you will get movement that could ruin your table top. This problem is lessened considerably when using wood that has already established its form from being down to 7% in the kiln.

Steotypes exist on this subject because kiln drying is often poorly done, which introduces significant problems you won't see in AD wood. I prefer the color of AD, but I always try to get it into the kiln (without steam treatment which is what changes color) and down to <8%. Whichever wood you use, none of this cancels the need for all the normal precautions about using wood that is acclimated to your shop, etc.

[Mike Leone]

I bought some maple, a couple of months later I found some little piles of dust on them, yep ppb's, I built a solor kiln in my driveway and let the little critters cook. During the day it would get up to 140-150 at night down to 120. Its been a year now stickered and stacked in my gararage, I've yet to see any more activity. The solor kiln consisted a dehumidifier, and a couple of fans to circulate the air. Its was basicly a frame built around a pile of wood wrapped in black poly. The wood was 2x12 soft maple from a 1890's bank barn that was dismantled and the wood was not taken care of properly, so it was also pretty wet though. Along with the maple I got 1200 of sycamore, I don't know if ppb's like sycamore or not, but from what I've seen they kind of avoid that. You can also try bora-care or timbor however I think those are more effective if the wood is wet so it can penetrate deeper into the wood.
Check out the sawing and drying forum on wood web there is a wealth of info of infestations.