So, Why Bother?....

[Gunny]

Okay, I've read and/or heard many, many explanations for drying our lumber so well (as I'm sure we all do).  And now that I'm strapping the six-guns back on and have a few of my old toys plugged back into their respective outlets, someone tell me again why we need to shoot for 7% MC out of our kilns, please.

In these parts (mid-Michigan), EMC is said to be ~14%.  I just plucked some of the Cherry that's been in the rafters of the workshop since sawn and they read a nifty 11.2% +/- a few tenths here and there.  And the White Oak that got turned into my workbench reads 9% on the dot.  The Eastern White Pine we made into the 4/4 that graces the top of the project table is in the 10-11% range, throughout.  None of it has ever seen a kiln.  Neither has any of it shown the first sign of degrade.

I know that one of the primary arguments for kiln drying is the speed by which we can get the stock down to whatever percent we hope to dry it to.  But why 7%?  The old cabinetmakers I've run with for beaucoup years tolerate 12% and below just fine so why don't we all agree to dry to 10%, save ourselves substantial amounts of $$ and get the loads out of the kilns a day or two earlier than we normally do?  After all, that lumber is going to equalize itself no matter what we took it to.  I have one customer who dumps buckets of water (at the end of each day's shift) on his concrete shop floor all winter long just to keep his inventory "moist." 

I remember once reading that the only way to "free" the "bound" water in wood fibers was with a kiln and that we'd be lucky to get below 20% by air-drying--but that just isn't so.  But that was back in the dinosaur days when none of our meters or controllers had computer chips driving them. 

Any valid reason--other than "that's some standard set a million years ago"--we don't just dry to 10%.  Or 12%?  Or, for my old buyer down in White Sands, NM, to 3%?  It might eliminate the need for the disclaimer explaining EMC, etc. all the time.

Just another bone to gnaw on for awhile...

Cheers

[Brad_S.]

My understanding is we dry to the worst case scenario - an enclosed house with central heat in the middle of winter. Most people don't humidify their homes in the winter. Cold air can't hold much moisture, so when it seeps into our homes and is heated to 70 degrees, the RH in the home is often drier than any desert. The wood starts to give up it's moisture to reach this new, very low RH, and shrinks. This is when door panels reveal unfinished edges and floors open gaps between boards. Furniture built with high MC (10-15%)wood shrinks worse than that built with 6-8% wood. If allowance for this shrinkage is not built into the furnitures design, it will crack and/or rip itself apart.

Speaking for myself, most of the hobby woodworkers I sell to don't have a clue about all this. If I were to sell them 12% lumber and the piece they built went to he** the first winter, they might blame themselves and it would deflate their enthusiasm and I don't want that, or they would blame my lumber (rightly so) and not buy from me again, and I don't want that either. I only sell AD to woodworkers I know are knowledgeable enough about wood properties to deal with wood movement, the rest are steered to the KD stacks.

Then again, my understanding may be out in left field.

[woodbowl]

After all, that lumber is going to equalize itself no matter what we took it to.

  Especially down here in the deep south it's hot and humid all summer. So why wouldn't 7% MC equilize to the atmosphere? According to an explaination that I heard, and I have no way to verify it so please correct me if needed..........A customer of mine told me that when the MC goes below (?? 9% or 10% or11%, don't remember) some type of cell lock occurs in the wood thus stabilizing the wood for when it did equilize............... Someone please blow this scenario out or the water if it's not so. It's been bugging me for a long time.

[Ianab]

As a complete distraction....

When kiln dried American wood is bought into NZ the suppliers aften have to sit it up on stickers for a while to 'undry' it 
Most NZ homes are not airconditioned and the climate is very humid. We call 14%mc Kiln dried and EMC inside is usually 12-14%. This is very much a location thing, if the woods going to be used in a 7% environment, then it's best to dry it for that. If you live in a rain forest.. then 15% is more like it.

Cheers

Ian

[crtreedude]

Did someone say - Rain forest?

There are issues as you move from one place to another with wood for sure. My question is, if I were to dry wood to 7%, how long would it take to rise up say 14%?

If furniture is made on our side and shipped to Guanacaste - it can often shrink. Guancaste is a dry topical environment - pretty much stops raining for about 5 months a year.

I think the real solution is for every home in the USA to be properly humidified - a 7% environment is not good for you, causes nose bleeds and makes you catch a lot more colds. Problem solve, sell furniture AND a humidifier. 

[Ianab]

I've dried wood in the microwave down to ~0%, then sat it on my desk and weighed it each day. In a week it was up to about 10% and in 2 weeks it was ~12 % . This was softwood (cypress) so hardwood might take longer.

Woodbowl
That might shed some light on your question too. The totally dry wood will regain moisture. I believe there is something called hysteresis involved, where one piece of wood is drying and the other is gaining moisture they will still end up about 1% apart. But this isn't enough to make much difference in the real world.

Ian

[Gunny]

Interesting dialogue so far.  But no one has really addressed the issue of the "standard" being 7%--maybe it's unworthy of comment?

How many of us have listened to homeowners and hobby-craftsmen blame the kiln-operators or others for their tables splitting and/or chests-of-drawers warping as the furnitures sit within homes with 3-4-7% relative humidities throughout the six months of the heating seasons? 

I read often that the "best" way to ensure proper climatization would be to park the designated lumber in the room in which it will ultimately be set and allow its EMC to adjust to that of that particular location in the house/shop/etc.  But I'm thinking not many homeowners would enjoy that idea for very long. 

Once again, why don't we just pop it out of our kilns at 10% and let the end-product get adjusted to whichever EMC is valid for the locale (BTW: I've read that dead-stacked, kiln-dried lumber will gain/lose 1%/month MC until it reaches the regional EMC).

The most expensive and sought-after works  in this world seem to be those which were crafted centuries before kilns became the fashionable route and they certainly seem to have aged well over the eons.  Isn't kiln-drying (and I've owned and operated DH units since the early '90s) really more of an effort at immediate gratification?  As we all know, if we plan ahead and are patient, we can air-dry our lumber to just about any spec we desire.  And I agree that even the most carefully dried stick will do the darnedest things when exposed to some of our comfort zones.

Not looking to save time nor money but the savings in energy would be delightful...

[Dan_Shade]

I've read things that tend to hint at old growth wood not moving quite as much as our wood today, but who knows...

[crtreedude]

Now here is a interesting question for you.

1. Since wood aborbs, let's say a percentage a month and..

2. In the North, you don't heat ALL the time - let's say half the year.

3. So, if the humidity moves the wood from 7% to 12% over the spring and summer

What gain is there in reducing it to 7%? Perhaps the issue is the that the furniture needs to be made when the wood is the driest, but is it possible the expansion will damage the furniture too?

[woodbowl]

Interesting dialogue so far. But no one has really addressed the issue of the "standard" being 7%--maybe it's unworthy of comment

Isn't kiln-drying (and I've owned and operated DH units since the early '90s) really more of an effort at immediate gratification? As we all know, if we plan ahead and are patient, we can air-dry our lumber to just about any spec we desire.

 I don't mean to continue piling on more questions before Gunny can get an answer, but it all seems so relative...........crtreedude, when you made this statement:

 if I were to dry wood to 7%, how long would it take to rise up say 14%?

 My question is: if you AIR DRY down to 7% and allow to rise to 14% vs KILN DRY down to 7% and allow to rise to 14%. ..................Are these in fact the same two pieces of wood? I have been told that it was not because kiln dry developes a type of cell lock and stabilizes somehow. Is this true or not?

[Brad_S.]

Should we go 'over there' and pose this to THE WOOD DOC?

[crtreedude]

woodbowl,

I am not an expert by any stretch, but I have heard kiln drying pine is important to "set" the pitch - however, I suspect it isn't necessary with many other types.

It would be good for killing certain critters too.

[woodbowl]

Should we go 'over there' and pose this to THE WOOD DOC?

  The wood doc is certaintly welcome at this point.  Hey.........NYLE.......ANYBODY !          Setting the pitch,.... I assume that is to dry the tar as much as possible, or is it a tech term to discribe internal values?

[Gary_C]

The best reason is that if you want to sell the hardwood, the kiln drying standard is usually 7%. You have essentially no chance of changing that standard so just do it.

If you are making furniture and cabinets for indoor use, you must have UNIFORMLY dried wood or there will be big problems with the finished work. I once built some cabinets out of some 10 year air dried walnut and the solid glued up doors shrunk and left a gap between the doors of almost an inch. Also on the end of the base cabinet a crack developed from the countertop down almost halfway to the floor.

Pine and other softwoods should not be dried below about 10 % becaust it tends to get brittle and splinter at lower moisture contents.

One of the reasons that old furniture pieces made before kiln drying do not have more moisture problems, is that craftsmen were much more patient in those times and probably brought the wood in to set besides the wood stove for months before they would use it. Now days we have a better (faster) method called kiln drying to a uniform 7 % moisture level.

JUST DO IT.

Also they will pay you to DO IT.    

[Texas Ranger]

Drying to the 7% reduces or removes the cellular wood moisture, that water locked in the cells.  The gain or loss after that is moisture between the cell structures.  Much more stable and easy to deal with.  Some bowl turners boil their green bowls to remove the cell moisture for the same reason.

[beenthere]

The "Wood Handbook" Chapter 12 addresses drying and has a short blurb on Page 12 about what kiln drying 'means'.  

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch12.pdf

I suspect the requests for having lumber kiln dried to say 8% may be based on having better assurance that the drying stresses were carefully handled, and no cracks were created 'on the way' to that 8%. It puts more "stress" on the kiln operator to do a better job, so to speak.

The other possibility, as mentioned (hystersis), is that wood dried to 8% will not swell back to the same size as it was at 12%.  Meaning there will be more 'movement' in the wood from 12% down to 8%, than there will be from 8% up to 12%.  Less movement would be a good thing if building furniture, doors, and flooring. 'Tis my understanding anyway.

Just my thoughts on "why bother".  Gunny has some good questions to ponder.

[ohsoloco]

This is an interesting topic, since I've built furniture out of both air dried hard and softwood, and kiln dried hardwood.  I think it is the duty of the craftsman building the furniture to understand that wood will continue to move throughout its entire life (as in a piece of furniture), and to build in a way that will let the wood do so. 

One construction detail that I always try to do is to make breadboard ends on all of the tops of furniture...like a dresser, hutch, nightstand, chest, etc.  I think it's really neat to see how the wood moves throughout the year.  When I build these tops the panel is flush with the breadboards, but then winter rolls around you can see the panel has shrunk, and in the summer it often swells just proud of where it was when I constructed it.  Those old antiques were built to move 

Personally, the only reason I see to kiln dry lumber would be to kill critters, or set pitch.  I've taken air dried lumber from outside in the winter, and started building with it right away (mostly due to impatience  ).  Let it move and it will usually be just fine

[Don P]

Put me on the hysterical bus too. I understood that it equalizes a point or 2 below EMC on the "way back up" and moves less. Movement on the way up is swelling. If your joints were snug, they get tight. The other way around, if your joints were snug and the wood shrinks more, they get loose. That said I have rarely recieved wood under 10% and never under 8% that I know of.

Woodbowl, I think that is an old wives tale. Read up on collapse, I've wondered if that is where the thinking comes from. I've even had folks draw a sketch of a collapsed cell to explain what a kiln does to stabilize wood. 

[jimF]

The reason we dry to 6-8% in most of the USA when the outdoor EMC is 12-15% is that the EMC of  indoor air during the winter with modern heating can be as low as 3% (like in Michigan) and during the summer with air-conditioning the EMC also get very low.  If the wood was dried to only 12% you would see shrinkage resulting from a 9%MC change.  Where as, with drying to say 7% you would see shrinkage resulting from only a 4% MC change.  This is true for hardwood and pine as well as old-growth and secondary-growth.  If you dry it too much the wood in joints will gain moisture and try to swell and be restrained.  This restraining will” compress’ the wood and then when it redries to the lower MC the wood will be smaller and will result in loose joints.

You also need to consider that the surface changes more rapidly than the center.  The surface can range from 3% to 15%.  So while the average MC may be 9% the surface MC where checks are located can be 3%MC.  The present checks will then open up and the customer will be dissatisfied.

In the earlier days when kiln drying was not available the EMC indoors did not get very low except right next to the fire place.  Which would dry out the joints and the chair would fall apart.  We must also remember only a few of the old furniture still is around.  Most have been discarde because they did not last.  Part of the reason some did last was because the structural design allowed for movement.

There really is not a process during drying as “cell lock”.  Within the microfibrels the cellulose does reallocate bonding sites as the wood changes moisture content and is part of the reason for hysteresis and the development of drying stresses.  But cells do not “lock” or hold their moisture if it is dried any particular way.

What moisture content the wood is dried to, does not signify how much drying stress is present.  Drying stresses are relieved after drying.  While one may suppose if the wood is dried to the standard MC it is also stress relieved properly.  But plenty of wood is the correct MC and still have high drying stresses present.

The question of whether a piece of wood dried in a kiln is any different than a piece air-dried but brought to the same MC again raised to another but same MC depends on how low of EMC they are exposed to and how high of temperature they are exposed to.  There is no experimental evidence that wood exposed to say 180F is any weaker than air-dried wood.  There is evidence that wood exposed to very low EMC will degrade the strength and stiffness.  This is most likely the reason that there has been some antidotal evidence that dehumidified wood has better machining characteristic than wood conventionally dried at 180F.  At these high of temperatures the EMC within the kiln can vary 3-4%.

Ianab, 
How big were the sample you put in the microwave oven?  If they were say 1”X 1” in cross-section and 2” in the grain direction I would expect the MC to change as quickly as you mentioned.  However, the rate at which wood gains moisture is proportional to the size squared.  For instance a piece that is 12” long and 1” X 1” is 6 time larger and  so it will change MC 36 time slower.

[Don_Lewis]

Generally you want wood to expand in use. If you build a door with panels at 12% and the house gets own to 6% in the winter, you have those nasty unfinished edges. If you make a chair that is nice and tight at 12% and the house gets down to 6% the thing gets loose. If you are dealing softwoods, you want to be sure the pitch is crystalized or it oozes out or gums up the sanding belts. If you want to kill bugs and mold and stain..... etc. Lots of reasons why kiln dried lumber is preferred.

[oakiemac]

JimF or others,

If a piece of wood is kiln dried to 7% then stored and it goes up to say 12%, is that wood any better then the same wood air dried to 12%?
I have heard that once kiln dried the trapped moisture is removed from the cells themselves. If it then regains moisture it is free water and is easyier to remove and causes less movement of the board. Any truth to the above?

[Texas Ranger]

From Lumber, by Brown and Bethel, John Wiley & Sons.

"A thorough understanding of the process of lumber drying requires knowledge of the ways in which mosture occurs in wood and the mechanisms by which it leaves the wood.  Some of the water in green wood is contained in the cell cavities.  This water is usually referred to as free water.  The cell simply serves as a container for this water, in much the same way as a cup, glass or bucket may serves as a container."

"In addition to the free water in the cell cavity, green wood also contains water in the cell wall.  This water surrounds the particles and strands of cell wall substance and fills the very minute openings in the cell wall is referred to as bound water.  The bound water is held by the wood in a much more intimate relationship than is the free water.  Some of the bound water is linked to the molecules of cellulose by secondary valence bonds.  Other bound water is held in the fine openings in the cell walll betwen the cellulose molecules throught the forces of capillarity.  The bound water is very important in any consideration of wood utilization.  Because of the magnitude of the physical foreces which hold the bound water, it is much more difficult to separate wood from its bound water than it is to separate it from its free water.  For this reason, the free water always leaves the wood before the bound water does.  In fact, the bound water cannot be removed from a cell until the free water in the cell cavity is gone."

"The condition which exists in a cell when the cell cavity contains no free water and the cell wall is satureated with bound waters is known as fiber saturation point.  Changes in the moisture content of wood above the fiber saturation point have no effect on its strength.  Ther is, however, a direct relation between moisture content and strength below the fiber saturation point.  Above the fiber saturation point, changes in moisture conten do not affect the dimensions of the piece of wood.  Below fiber saturation, a change in the moisture content will result in a change in dimensions.  Most of the physical and mechanical properites of wood are influenced  by the bound water, whereas the free water has no important effect on wood properties, with the single exception of collapse."

In other words, the strength of wood is more or less dependent on moisture content.

[jimF]

oakiemac,
No, it is no easier to remove a given amount of moisture from dried wood than from green wood.  When the wood regains moisture it gains it first as bound water as described by Brown and Bethel inTexas Ranger's note.  As mentioned earlier and if you are talking about EMC not MC,  The wood brought down to 7% and then exposed to 12% EMC will have a lower MC than the one Exposed to 12%- because of hysteresis.  i know I am mixing terms and throwwing in complications, but so everyone is clear on how wood behaves.
  One addition to that quote - the acoustical properties are also effected by moisture content.

While in general practice Don is correct in that generally pitch is better "set" in dried wood than green wood.  However, pitch can still run if the board is dried.  Depending on how well it was set, it can vary from piece to piece, given that the moisture content is the same in both pieces.
  Also, using a hammer as an example, if the handle is very dry, say 5% and then swells, say 12%, it will become very loose when it redries.  Whereas, if it originally was somewhat high MC, say 12% and dried slightly, say 10% it will be tighter than the fisrt situation.

[beenthere]

....  Also, using a hammer as an example, if the handle is very dry, say 5% and then swells, say 12%, it will become very loose when it redries.  Whereas, if it originally was somewhat high MC, say 12% and dried slightly, say 10% it will be tighter than the fisrt situation.

And it was 'looser' in the 'first situation' because the fibers of the wood 'crushed' within the confines of the steel hammer head, likely having exceeded the crushing strength of the wood.   

But, there are lots of variables to take into consideration, even in the analogies offerred.