Moisture Content

[Jim Murphy]

Fact 1. Today is the third woodfest.

Fact 2. I am air-drying my purchases.

Fact 3. There is an active thread on solar kilns.

Fact 4. I know a lot about surveying, but very little about air drying sawn timber.

Fact 5. There is no substitute for cubic inches or experience.

Throughout the web, there are assertions that unless wood is kiln dried, it is not going to work well. Purportedly, AD wood can only reach a MC of 12%, whereas in the kiln it drops to 6%.

My limited research indicates that wood is constantly adjusting to the relative humidity. Here's a clip from Dr. Wengert at WoodWeb:


Q.
Can you post the equation or table that gives the equilibrium moisture content (EMC) for differing relative humidities (RH)? i.e., 10 percent RH = x EMC, 20 percent RH= y EMC, etc. A.
0% RH = 0% EMC
19 to 25% RH = 5% EMC
25 to 32% RH = 6% EMC
32 to 39% RH = 7% EMC
39 to 46% RH = 8% EMC
46 to 52% RH = 9% EMC
57% RH = 10% EMC
65% RH = 12% EMC
74% RH = 14% EMC
80% RH = 16% EMC
91% RH = 21% EMC

Travis and I were discussing this Friday night, and I brought up the issue of the colonial craftsmen who produced some extremely beautiful furniture. Maybe every piece of walnut, cherry and oak went into a kiln before it was worked, but somehow I doubt it.

Even if that were the case, any wood exposed to a higher RH will see an increase in EMC. You don't have to be a weather junkie to understand that anytime it rains, the RH surpasses 90%. Sometimes it rains for days at a time, and the soaked ground keeps the RH up there for some time after.

Point 1: if you're in a hurry to work with your newly purchased wood, there is no question that drying in a kiln produces a workable product more quickly. That is NOT an issue in this posting.

Point 1A. since the rate of water loss is more carefully controlled, wood that has been KD should exhibit fewer drying faults. That is NOT an issue in this posting.

Point 2: no matter how dry you get the wood, to the extreme of oven-drying, it seems that once exposed long enough to higher RH, the EMC will adjust upward.

Point 3: hardwood floor installers bring the product into the customer's house and allow the material to "acclimate" to its surroundings. Anybody with a hardwood floor knows that the joint gaps widen in the wintertime when the RH is low, and in extreme cases, swell and buckle under high moisture conditions, proving that KD material is subject to the vagaries of the ambient RH conditions.

Point 4: most of us do not have a climate controlled shop. Therefore, when the KD wood is brought into our shop, the EMC changes to adjust to the RH in the shop. Even if you run the AC all summer, there are mornings when the temp is 70° and it's raining. Your AC will not be running, since I doubt you set the thermostat below 70°. The RH will rise.

---------------------

Rebuttable conclusion: Allowing sawn lumber to air dry to a MC of 12%, and then bringing the material into the shop for a week or so prior to milling to allow acclimation to the shop environment, will produce a wood product just as acceptable as KD lumber that is not worked until months after removal from the kiln.

Fact 6. Book learning ain't everything.

 

[Howard Acheson]

One of the primary reasons that old time furniture lasted even though it was not kiln dried is because there was no central heating. Central heating greatly lowers the winter relative humidity causing damage to solid wood.

Another point to keep in mind is that most old furniture self destructed fairly quickly. What you see now are the survivors. I saw it stated once in an antiques newsletter that the greatest value enhancer of antiques is central heating as it destroys so many older pieces making the survivors that much more unique and valuable.

Most colonial and early american furniture was made by intinerant, traveling carpenters or local shops. It was not well made which also caused much of it to become damaged. Look at a lot of "antiques" and you will see splits is wide solid wood panels.

The point is that air dried wood can sucessfully be used for furniture. However, good construction techniques must be followed allowing for expansion and contraction as relative humidity changes.

 

[walnutjerry]

The point is that air dried wood can sucessfully be used for furniture. However, good construction techniques must be followed allowing for expansion and contraction as relative humidity changes.

A windsor chair is a prime example of the statement above--------the design of the windsor is such that it does not restrict the natural movement of the wood. Keep in mind there are no mechanical fastners in a traditionally built windsor, only glue and wood wedges.

Jerry

 

[Kyle]

Furniture that was made in the 18th and 19th century was not kiln dried, but it was fully seasoned (for years!) and subsequently brought into the shop or an area to fully finish drying which was usually a heated space. In essence it was kiln dried just not in a modern method.

My house is full of furniture from the 1800's made by relatives that did not do anything but the method above and most pieces are relatively stable but some are not and exhibit warp or split from the drop from 12-15% MC that was common in NC at that time to 6% which is common in HVAC heated and cooled homes today.

I think there is a misperception that wood is not usable from air dried but I would also conclude that the appropriate steps are not taken to fully dry the wood to usable form before using thus perpetuating the misperception. The primary reasons wood is kiln dried is:

1- Kill insects and their dormant larvae.
you wont discover these until you have a finished piece of furniture and the powder posts pop up year after year

2- collapse the cells fully and strengthens the wood.

3- speed the drying process.

Kiln drying wont:

1- fix defects that are inherent in the wood ( twist, bow or warp) due to stress.

Dr. Wengert has forgotten more about sawing and drying than I have learned and I can honestly say he knows his stuff.

 

[Alan in Little Washington]

Don't forget the affects of sealing and finishing.

 

[Kyle]

good point Alan. Water evaporation and most modern finishes don't mix..

 

[Kyle]

Mike.. that myth has been perpetuated forever..I can air dry most 4/4 and 5/4 stock without rushing it in 60-90 days the temperature is consistently above 70+ degrees. 8/4 stock can take more than double that to dry adequately and thicker stock can stay wet for years.. I have had 16/4 stock retain high moisture 4 years after being sawn..

From my youth Many years ago I remember a rule of thumb for drying lumber. It was one year per inch of thickness plus one year.

So, that makes it two years for 4/4 stock. Three for 8/4 stock and so on. Does no one use that time table any more or has everyone forgot? I know my mentors prized 40-50 year old walnut and maple. They said it took that long for hard wood to dry, cure and equalize. Is that a thing of the past to be ignored in these days of rush, rush, rush?

 

[Dusty Sawyer]

One facet of this discussion that begs further detail is that of what is the definition of a 'kiln'. There are considerable differences between commercialy operated, solar, and every variation of small kilns out there. Not to mention the knowlege and attentiveness of the operator. Another aspect of every system out there is measurement of the water removed from the wood and the fact that there is more than one measurement scale or method of determining water removal; weight change, water collected by a dehumidifier, surface and spiked measurement devices.

The point is that the assertion that kiln dried lumber is automatically better than air dried is faulty. I have come across more than my share of poorly dried kiln lumber plagued by every evil incarnation of case hardening, uneven drying, honeycomb and cracks. The quality of kiln dried lumber is greatly dependent upon equipment and operator.

What has worked very well for me is to air dry the lumber for the old rule of 1 year per inch and then it in an area controlled by a dehumidifier and heat. Uneven drying is virtually eliminated due to the many cycles of heating and cooling provided by the daily temperature changes.

For most of us who do not have access to a well operated commerical kiln or equipment and experience to operate thier own, air drying works great.

David

 

[jeff...]

This is a great discussion. I can honestly say I have not had any problems with bugs even with some pine that I knew had PPB in it when I had it cut. When planing if I run across a bug hole I flood it with some cheap two part 5 min clear epoxy. I think if there were any live larvae (which I doubt after dehumidifying) they would be glued to death. But I mainly use the cheap two part epoxy to fill holes, secure knots and fix small defects.

What has worked very well for me has been sticking and air drying to around 12 ~14%, this really doesn't take to long a 2 ~ 3 months for 4/4 about twice as long for 8/4, I don;t know about 12/4 and up, I've never tried to dry anything that thick. I try and position my stacks North to South since "mostly" the wind blows from east to west. This seems to give good air flow through the bunk and have not had any issue with mold. A word of cation snakes like to get in between the layers so be careful of them and keep your favorite snake gun close by.

once air dried to around 12 ~14 % I have two options

#1 use as is for outdoor projects
#2 dry further for indoor projects

If used for indoor projects I move and re-stick in my shop along with a dehumidifier and a tarp. Run the dehumidifier till the bunk reached 6 ~8%. Make sure you check the MC in middle of a few boards as the out side edges tend to read drier than the middle till the bunk is equalized. Do this by ripping a board down the center or cross cutting in half. Measure the MC as close as you can to the absolute middle of the board.

Oak takes the longest to dry, because you have to be nice to it - you can't suck to much water out of it in a days time or it will split, so I usually give oak break from the dehumidifier during the night, which means it takes at least twice as long to dry. Other woods like soft maple, poplar and BW you can get pretty aggressive with and don't have to be so nice. The thicker the lumber the longer it's going to take to air dry and the less aggressive you can be with the dehumidifiers which also means longer dehumidifier times.

Softwood like pine and cedar dry pretty quick as does poplar and I did not measure the temperature of my tarped dehumidifier pine, but I know it got good and warm under the tarp, I may have reached the bug kill temperature I don't know. Maybe this is why I have not seen a PPB problem with the pine or any of lumber I've been drying this way? The dehumidifier method has worked well for me, but I don't think I would try and dry more than a few 100 BF at a time this way.

For larger scale, I'm going to build a 2500 BF solar kiln out on Ted's log yard. We'll see how well it works, if it works well, Teds is already talking about a second one. They also have a wood broiler to heat their house and Winston said today he knows he could get the temp up to 150 in the kiln during the winter months and all it would take is some finned copper pipe in the solar collector part of the kiln so that may be an option too.

Thanks

 

[Kyle]

One facet of this discussion that begs further detail is that of what is the definition of a 'kiln'. There are considerable differences between commercialy operated, solar, and every variation of small kilns out there. Not to mention the knowlege and attentiveness of the operator. Another aspect of every system out there is measurement of the water removed from the wood and the fact that there is more than one measurement scale or method of determining water removal; weight change, water collected by a dehumidifier, surface and spiked measurement devices.

The point is that the assertion that kiln dried lumber is automatically better than air dried is faulty. I have come across more than my share of poorly dried kiln lumber plagued by every evil incarnation of case hardening, uneven drying, honeycomb and cracks. The quality of kiln dried lumber is greatly dependent upon equipment and operator.

What has worked very well for me is to air dry the lumber for the old rule of 1 year per inch and then it in an area controlled by a dehumidifier and heat. Uneven drying is virtually eliminated due to the many cycles of heating and cooling provided by the daily temperature changes.

For most of us who do not have access to a well operated commerical kiln or equipment and experience to operate thier own, air drying works great.

David

I would agree that this is true in most cases which is why I tend to do the following for quality reasons:

1- Air dry green stock until MC reaches 25% in the core or less before kiln drying.

2- Air dry for maximum color.

3- Kiln dry to 5-6% with the correct kiln schedule and subsequently equilibrate the load to reduce case hardening, honeycomb.

MOST defects occur ABOVE the 25% moisture content level typically because of too rapid of a drying process.

the best remedy for air dried wood is proper stickering, good ventilation and keeping the stack out of direct sunlight. The sun is not your friend when air drying. unless covered my experience has been you will lose the top two layers of wood due to sun damage.

 

[Kyle]

This is a great discussion. I can honestly say I have not had any problems with bugs even with some pine that I knew had PPB in it when I had it cut. When planing if I run across a bug hole I flood it with some cheap two part 5 min clear epoxy. I think if there were any live larvae (which I doubt after dehumidifying) they would be glued to death. But I mainly use the cheap two part epoxy to fill holes, secure knots and fix small defects.

Thanks

True PPB only attack hardwoods..there are lots of other critters that love pine though..

powderpost beetles, POWDER POST BEETLE CONTROL, wood destroying insects,

from the website..

"As a rule, they enter lumber while it is being stored and cured, then later, emerge from the finished product. Old items of furniture and wood antiques are especially vulnerable to attack by the beetles.

Damage is usually
to the starch-rich sapwood of large-pored hardwoods such
as ash, hickory, oak, walnut and cherry. The hardwood
floors of new homes are commonly attacked"

all of these creatures love moisture for their starch lovin' ways.and only heat or poison will kill the larvae..

 

[Tarhead]

Has anyone used Boric Acid powder or solution on their lumber? If so how did you apply it? It is nontoxic to us and our pets but the beetles and termites have trouble with it.

 

[Kyle]

boric acid will work, but wont penetrate well and tends to be an after the fact kind of solution. I don't think it will kill larvae.

 

[jeff...]

Kyle you made a couple of great points, thanks for sharing your wisdom in this area.

1 - Cover the tops of your air dry bunks, sunlight is not a friend of green wood, nor is rain for that matter.

2 - Bugs, I completely understand what your saying and can honestly say I have not had a problem with any bugs from hardwood lumber I've dehumidified under tarp. I have had some bug problems with pine that was not dehumidified and strictly air dried for out door use. It's mainly been limited to those pesky carpenter bees in the spring and summer months. But they tunnel into kiln dried pine just the same.

I guess the question that begs to be answered is: What temp does one need to bring a bunk of lumber up to and for how long in order to sterilize it (kill any bugs and larvae)? I've read as little as 120 for 12 hours to as much as 150 for three days. I think the 150 for three days was to set the pitch in pine though. And I'm also thinking this temperature and duration would need to be reached towards the end of the drying cycle.

I'm personally very against the use of chemicals for obvious reasons.

Thanks

 

[Kyle]

Dr Wengert is pretty adamant about 135F for 24 hours minimum for 4/4 material and longer for thick stuff.

I don't have bugs all the time in my air dried stuff and usually its only in the sapwood. A lot of times you wont see them until summer May-June.

BTW my wisdom comes from screwing up wood along the way..

 

[Alan in Little Washington]

I think the bottom line is we will all do or need to follow established procedures as much as possible to meet our personal needs. While stacking and drying willy nilly may occasionally result in some good boards, you are more than likely going to get lumber with any or all of the typical problems- cupping, warping, checking, case hardening. Though following the excellent advise of Dr. Wengert and the folks at VA Tech or the info put out in many pamphlets by USDA on handling and drying lumber (all available through links here and Woodnet) does not necessarily guarantee 100% success it sure improves the odds. So we will have to weigh time and dollar costs (income and expenditures), equipment, space, waste, etc. etc. against our needs and decide what is best.


And by the way, anyone find a good cheap source for twin wall Polycarbonate yet?? :-D:-D

 

[jeff...]

Kyle, been doing a little more studing on Kiln operation, since I'm going to have to learn how to run one here pretty soon. I found the answer to my question
What temp does one need to bring a bunk of lumber up to and for how long in order to sterilize it (kill any bugs and larvae)? In the following Drying Hardwood Lumber Ag document page 13. Eugene M. Wengert is listed as one of the three authors.

"If insects pose a problem for drying at low temperatures, the lumber may need to be treated with chemicals before drying or it can be heated briefly (approximately 24 h) to 130oF (54oC) or higher, which will kill insects, insect eggs, and fungi."

(Drying Hardwood Lumber)http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf

There is also a Dry Kiln Operator’s Manual by the Ag dept located here
Dry kiln operator's manual

This may be old hat to you, but very interesting to me, I hope Alan, The Wood Butcher and anyone else who is interested in running a kiln finds these documents useful.

Thanks

 

[Wildwood]

Everything in life has it's advantage and disadvantages, and so it is with drying wood. You get more control, save time, kill bugs, and set the pitch in pine drying lumber in a kiln. Just don't forget bout those nasty chemical they apply to wood before entering the kiln.With air drying you use less energy. Just takes more time! European craftsmen always scoffed at Americans for using wood that wasn't air dried for at least 7 years.Neither kiln or air drying lumber will be without some loss. Neither kiln or air drying will stop the affect of relative humidity on wood. Wood is hygroscopic and gains and loses moisture content through out it's life. That's why we finish wood, which offers some protection from relative humidity.How many of you have used a kiln dried 2x4 that was still soaking wet? They probably didn't allow that wood to lose it's free water content before shoving into a kiln. So even with kiln drying there is usually some air drying period involved.Just because wood has been in a kiln doesn't stop it from reabsorbing moisture. Same also true with air drying. Because of moisture gain and loss wood moves, and should be planned for in the design phase before building something. I say only way to solve this debate, is plant more trees. Furniture industry just makes stuff out of particle board! I'm for more trees!

 

[jeff...]

Wildwood you bring up a good point. I think your right it boils down to personal preference.

I asked a question in another thread what factors play an important roll in the purchase of lumber. One person replied "I only buy KD S2S stock that is top quality (FS)." I can appreciate that response and if that's a determining factor for that person then that what's important to that person. Personally I think a lot of folks are hesitant to by green lumber or even air dried lumber. They would rather pay a lot more more for kiln dried. That way they can bring it back to their shop and start working it right away. There's a place for Green / Air Dried / Kiln Dried - it's all up to the person buying the lumber and their personal preferences and time.

Having had worked with both with Kiln and Air dried - I personally prefer air dry, it just seems to have more brighter color and works so much easier. But then again I'm far from being a craftsman like most of our members.

Thanks

 

[scsmith42]

A lot of good info on this thread - I won't reiterate it all.

Proper KD should result in less loss, and a faster turn of product. Although air drying does not cost energy, there is a cost of the asset value which is tied up for year(s) in the drying process. For this reason, most businesses that sell a lot of wood will opt for KD - faster return and less waste. Setting the pitch in pine is one benefit that cannot be duplicated with AD.

RE sterilizing, 130F min is a good target, but keep in mind that the center of the board needs to be at 130 for 24 hours in order to kill all of the bugs. So, 12/4 stock at 130 for 24 hours is not necessary fully sterilized - in my experience it can take 48 or more hours depending upon the initial wood temp, species and thickness.

Scott

Whether you AD or KD - be sure to start off with stickers that have a low MC - otherwise you will get mold or stain between the green sticker and the green board. You will also get better results if the stickers are all very close in thickness, and your lumber is similar in thickness. AD inside a building, out of the sun, should result in a higher yield versus outdoor drying.