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KD vs AD

Started by spencerhenry, March 04, 2005, 09:41:44 PM

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spencerhenry

this may sound like a stupid question but i have no experience with kiln drying. what is the difference between a piece of lumber that is KD with a mc of 10%, or a piece AD with a 10% mc?
99% of everything i mill is standing dead. the wettest logs i mill are just under 30%, with some logs as low as 12%. i air dry all my lumber. some like blue spruce can warp badly, but most engleman spruce warps very little. some aspen (typically wetter), warps bad also.
i live in western colorado which most of the time is like a desert. relative humidity in the summer is usually around 15%, and daytime temps are in the 80's with almost always some breeze. i think some of my 1x goes from 20% mc to under 10 in just a few days. how is air drying in an environment like this different than a kiln?

sawmillsi

Hi mate,

The biggest difference is the time it takes to get to 10%mc.

With logs, it is almost impossible to get them to 'dry' out 100%. In fact, even in desert Australia, the centre of logs will still have some moisture.

I would assume that you have taken a MC reading from the outside of the log? If so, remember one thing, unless you have probes that are insulated (except for the tip) you are getting an average moisture content reading from the length of the probe. And if the probe is only 1" long, thats the part of the tree that drys first. My bet is that if you cut the log open the mositure content reading will be much higher.

One of the biggest problems with drying fast (dosn't matter kiln or air) is if the timber gets 'case hardened' - this is where the inner part of the timber has a higher moisture content and this leads too deformities.

I think if you can air dry in a week or less (even 1 month), don't bother with a kiln.

In saying that, people have funny thoughts on kiln dried timber over air dried timber - that is they think its better, not always the case.

Good luck

Simon

spencerhenry

when i stated mc in logs, that was the boards after they came off of the mill. so it represents the center of the log. in late summer here (hot and dry) the mc doesnt change much from inside of log to near the bark. with 1x that i have cut, in july or august, a board a week old, or a month old, is only distinguishable by the color difference. i do believe that lumber under 2" dries out in a matter of days, unless it is raining.
thanks for the info

LeeB

10% is 10% no matter how it got there. I prefer airdried myself. You may need a little lower moisture content than that if the relative humidities in your area are that low. Be sure and dsign your projects to allow for possible shrinkage after the stuff comes inside. LeeB
'98 LT40HDD/Lombardini, Case 580L, Cat D4C, JD 3032 tractor, JD 5410 tractor, Husky 346, 372 and 562XP's. Stihl MS180 and MS361, 1998 and 2006 3/4 Ton 5.9 Cummins 4x4's, 1989 Dodge D100 w/ 318, and a 1966 Chevy C60 w/ dump bed.

old3dogg

I have heard some people say that KD lumber has more stress than AD lumber.
Ive never felt this was true but than again I use to be a kiln operator. ;D

Don_Lewis

Air dried to 10% is the same moisture content as kiln dried to 10% except the kiln dried may have any bugs or fungus killed by temperature and may have pitch set. Whether you need pitch set or not depends upon the ultimate use of the lumber. The stress in air dried wood in your climate can be as bad or worse as in kiln dried lumber. It is easier in a kiln to correct stress if it occurs.

malhyp

Hi there Don, I had someone say the same thing to me and i wasn't sure on what to say. What they were saying was that they would only air dry timber as it's end result was better than kiln drying. He seemed to thing that drying timber in a kiln was not the best way as you are quickly drying the timber which then results in more splitting and cracking later, where as air drying allowed you to control the process and shift timber around if it starts to react to the weather.

Cheers
Malcolm
www.timberseek.com.au
Cheers
Malcolm
www.timberseek.com.au

Don_Lewis

Your friend referred to "more splitting and cracking." There should be NO splitting and cracking and in many cases the only way to assure that is in a kiln. You will generally get much better yield using a kiln than air drying, Degrade during air drying can be quite high unless the lumber is well protected from sun and rain and the air flow is right. If you are getting splitting and cracking in any lumber, you should figure out what is going on and fix it. Lumber is too valuable to simply accept that you will loss 10% of the value by drying problems.

You sometimes hear people say that they find air dried wood "works" better than kiln dried. That just means that got some poorly kiln dried wood. If you track that down each time, that will be the conclusion.

Ianab

Malcolm, you may also be talking about some of the Aussie Eucalypt species. They are difficult to kiln dry - they need VERY gentle treatment in a kiln. That means tying up a conventional kiln for longer longer than is economically practical. Many of those species are usually air dried, then finished in a kiln. OK it takes longer, but the economics are better. The throughput of the kiln is MUCH higher and if degrade is minimal it can make sense.

I agree with Don that generally a kiln (properly operating ) will give better results than air drying, but some species handle air drying better than others. The Mac cypress that I usually work with is almost never kiln dried, again it needs a much gentler cycle than the local softwood kilns are set up to use. It does air dry easily with practically no degrade, it's naturally resistant to bugs / fungus and doesn't bleed pitch, so in that case air drying is 'better'.

Other species will air dry too fast and check, or dry to slow and stain. And because you dont have much control over the drying you are sometimes at the mercy of the weather - not a good thing.  ::)

Cheers

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

blaze83

there was anarticle in Fine Wood Working Magazine a while back about color differences in kiln dried and air dried walnut. I can't find the article in my back issues. :-\ does anyone remeber the article or have first hand experience in color variations in walnut or other woods? I find this interesting. I'll keep looking for the article too :P


steve 
I'm always amazed that no matter how bad i screw up Jesus still loves me

Larry

I dry a lots of walnut both in a DH and solar kiln.  Also air dry walnut.  Don't see much color change either way.  I'm guessing 99% of commercially dried walnut is steamed before going to the kiln...and dried at a much shorter and hotter schedule.  Steaming colors the sapwood brown and also gets rid of all the purple and red highlights in the heartwood.  Having compared walnut dried both ways I think steamed walnut is bland and down right ugly.

Stock makers prefer air dried walnut...they say it cuts differently than KD walnut...softer and sharper.  I agree.

Any other species of wood...at least in the midwest, I don't see any difference between KD and AD.

Here is a link for ya steve...index to all issues of FWW

http://www.taunton.com/cgi-bin/artresult-fw.cgi
Larry, making useful and beautiful things out of the most environmental friendly material on the planet.

We need to insure our customers understand the importance of our craft.

Sawyerfortyish

Here in N.J. we can't get our lumber below 12% air drying and some days (rainy or humid)it will go up to 15%. Like Don lewis says you can't kill bugs or set pitch without the high temps of a kiln. We just started breaking down some of the huge piles of lumber thats been air drying for as long as 4 years and kiln drying it. I am amazed at how much water runs out the drain of the kiln from lumber that is 12-15%. It runs at a pretty good drip for about a week before it stops then we turn up the temp to kill insects. Just about 90% of what we dry is oak or hardwood. The air dryed lumber has a lot more degrade than kiln dryed and takes at least a year to get the moisture content down close to 12-15%. Kiln dryed lumber has almost no degrade the boards look bright not discolored like air dryed lumber and in kiln drying I can sell dry lumber in about a month.  Air dryed lumber can still be 20%in the middle of the pile even after more than a year. We started making hardwood flooring you can't do it at 12-15% your lumber has to be down to that 8-9% range or you'll get complaints.

TexasTimbers

Quote from: Larry on November 23, 2006, 02:10:21 AM
. . . . . Steaming colors the sapwood brown and also gets rid of all the purple and red highlights in the heartwood.  Having compared walnut dried both ways I think steamed walnut is bland and down right ugly.
Would you consider condensation drying the same as steam drying in this instance regarding discoloring walnut?
The oil is all in Texas, but the dipsticks are in D.C.

Larry

Kevjay, might have mislead you a little about steaming walnut.  To properly steam walnut you need fresh green boards.  Put them in a stack without stickers.  Than into the steamer.  The steamer has some system of steam vents that spray super heated steam directly on the lumber for maybe two to four days.  When you get the desired color the stacks are removed from the steamer, lumber put on sticks, and than to the kiln for drying.  Two completely separate operations.

I can't see the type of kiln used for drying as effecting the color of walnut much at all.

Hope I made it clear...sometimes not real good bout splaining things.
Larry, making useful and beautiful things out of the most environmental friendly material on the planet.

We need to insure our customers understand the importance of our craft.

Den Socling

In steaming for color, the goal is to get the wood hot without doing any drying. Thus, you use steam to heat it.

Good air drying yields the same wood as good kiln drying. But some species, like red beech in New Zealand, will be destroyed by air drying as easily as putting it in a conventional DH or steam kiln. It just takes longer.  :D

DH kilns typically yield brighter wood than steam kilns because of the lower temperature. Steam kilns dry more quickly but the heat dulls the wood.

Tropical man

As others have said, 10% is 10% no matter how it gets there.  But kiln drying will give you more control, the ability to detension, kill buggs and fungus, and a uniform dry throughout the load.  There are drying schedules for every species and thickness.

Air drying in your very dry environment would require drying sheds and good stacking with uniform stickers.  I would try to restrict airflow also, maybe by using 1/2 inch stickers, just a thought. 

Ga_Boy

Hi Torpicalman,

First welcome to the FF, we are glad to have you here.

Regarding your suggestion of using 1/2" sitckers; in Kiln Operators School; I was taught to only use 3/4".  Any thing less may result in mold and fungus growth.

If the objective is to reduce air flow to retard predrying, try wrapping the stack with Dry Shade or burlap.

It may seem strange to do this, but the smaller air space between the layers of material will become saturated with moisture and you will have less air movement causing the growth of fungus and molds.  A 3/4" air space and the stack wraped will reduce the air flow and retard the drying but not to the point of creating conditions where fungus and mold will grow.





Mark
10 Acers in the Blue Ridge Mountains

jrdwyer

I was taught in wood class that the reason that softwoods (2x4) are dried to 19% MC was because fungus cannot grow in wood at this level or below. So in other words, it is the highest MC where fungal degrade won't occur. Do you think this is true for both KD and AD stock in both hardwood and softwoods?

I know the SPF 2x4s equalize in my walls to about 11-13% MC based on my meter.

Tom

The mills are concerned with moving product.  19 percent allows them to kill insects, prevent mold and rot, and get the wood onto the truck quickly.  There may be some setting of pitch and stabilizations gained.

The priority is to get it shipped and that's the number that says it can be shipped.

At 19 percent, there isn't much difference in Air Dried and Kiln Dried accept for the killing of insects.

WDH

For the big pine mills, volume is very important.  They typically dry dimemsion lumber to 19% in 24 hours.  Hardwoods take days, not hours.  That is why the big pine mills can cut so much volume - they can dry it fast.  However, as discussed in another post concerning the quality of lumber available at the Big Box Stores, I suspect some of the bow and twist so commom to that lumber results from drying it so awfully fast.  Take the young plantation timber with a high % of juvenile wood, dry it as fast as possible, and presto, you get sorry quality.  I only air-dry wood.  I don't have a kiln.  However, the degrade from air-drying can be high because the variation the wood sees from the environment is all over the map.  It can be too dry, then too wet for perfect drying, and the wood cycles through all this variation, yielding more splitting, case-hardening, cup, etc. than you would get under controlled drying conditions like in a well managed kiln.  However, air-drying is the only way some of us can go, so to do it right, read up on drying hardwood lumber and minimize the degrade.
Woodmizer LT40HDD35, John Deere 2155, Kubota M5-111, Kubota L2501, Nyle L53 Dehumidification Kiln, and a passion for all things with leafs, twigs, and bark.  hamsleyhardwood.com

flip

I just put some hard maple in the kiln for a few days, it was at 15% after AD for about 8 months.  The end use will be a computer desk for my mother in her new house, my father is feeling ambitious.  It all depends, if you aren't concerned about killing bugs or fungus (if it's even present) the wood just needs to be as low enough it won't move much where it will be used.  In other words, get it as low as possible then let it eqalize for a few days where the end product will sit.    For example, I took a board and made a few schedule boards for work here, the boards were 16" wide and I cut and planed them.  The board that had pith in it stayed flat the board one cut above bowed about  1/8".  I did not kiln and MC was 15%.
Timberking B-20, Hydraulics make me board quick

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