Conditioning Phase of Drying Cycle

[Downhill Cutter]

I'm relatively new to the Forum.  I tried a few hours ago to float this subject, and I don't see my post (speculating operator error) , so if I'm redundant, I'm redundant. 

I'm just at the end of my first charge in my Nyle L53.  I have 1000-1200 bdf of 10-12/4 red oak slabs reaching 8-12% as measured surface and @3/4" deep with my Orion Moisture meter.  In the beginning of the cycle I successfully case-hardened the load per Seth at Nyle.  Thanks, Seth, for advising me to throw 5 gal of H2O on the floor, turn off the compressor, and heat it up to 140 for a couple of days.  Problem solved. . .  with any luck we killed bugs at the same time, if I understand correctly.  

Question:  Is it advisable for me to run a conditioning cycle on my material?  I don't read much about that, but is it customary to do so?  If I'm right, I'd dump about 1 gal of water on the floor, turn off the compressor, and run it at 120 dry-bulb for a day.  I'm just a little twitchy, as it's my first load and I'm pretty green at this process.

Thanks in advance

Downhill Cutter

[WDH]

I do not purposely condition at the end of sterilization by adding water to the floor, but I do turn off all the fans and shut the vents (compressor is already off during the sterilization phase) and let the load sit and slowly cool down for a day or two to let everything come to equilibrium.   

[YellowHammer]

The 8 % to 12 % spread on moisture for such thick wood concerns me, it should be much tighter at the end of the drying cycle, which may add considerably to the drying time, especially with oak.  

If that's measured from board to board, then I suggest getting them all down to 8% before removing them.  

If it's measured internally on individual boards, then they have wet spots, and definately need to stay in the kiln until the moisture gradient in the board is very close.  This may take many days.  If it was white oak, it might take weeks.  

There is no need to dump a gallon if water in the kiln because there is enough that much and more in the lumber itself that will be driven out when reaching sterilization level temps.  I personally don't do a water dump unless it's necessary.  

When all the wood is within a percent, or under 8%, jack up the temps to 145 F, close the vents, turn off the compressor, and heat the kiln up and when done let it sit and slowly cool down over a day or two.  The wood will relax and behave noticeable better.  

  

[GeneWengert-WoodDoc]

Conditioning is the process of adding moisture quickly and at higher temperatures to the surface or shell of lumber to cause the surface to try and expand, which (because it cannot expand) creates stress that cancels or offsets the casehardening stress, which was caused by attempted shrinkage early in drying.  (Sometimes, conditioning is the term used for equalization...not a correct use.)

The prong test is used to measure casehardening, but it requires no moisture gradient within the piece of lumber in order to be accurate.  That is why putting it a microwave for 30 seconds or so after cutting, will help create uniform MC.  It is likely that varying moisture at the end of drying means that equalization has not been 
done, so the prong test will not work.

Note that when air drying, the high humidity every morning relieves the stress put in the previous day, so there is no casehardening stress, or need to condition, WITH WELL AIR DRIED LUMBER.  This is why some people report putting a small amount of water on the floor relieves stress...there was no casehardening stress in the first place.

[Downhill Cutter]

Thanks, Doc.  You folks are really helping me think-through this process.

FF and its members make a dynamic community.

Thanks to all of y'all for being there.

[scsmith42]

I've had good success with adding moisture via a high pressure misting system.  I dry a lot of oak and was never pleased with the "water on the floor" in my container kiln.  The misting system really works well and combined with a Delmhorst Kil-mo-trol system I can precisely match the core and shell MC%.

[mredden]

Conditioning is the process of adding moisture quickly and at higher temperatures to the surface or shell of lumber to cause the surface to try and expand, which (because it cannot expand) creates stress that cancels or offsets the casehardening stress, which was caused by attempted shrinkage early in drying.  (Sometimes, conditioning is the term used for equalization...not a correct use.)

The prong test is used to measure casehardening, but it requires no moisture gradient within the piece of lumber in order to be accurate.  That is why putting it a microwave for 30 seconds or so after cutting, will help create uniform MC.  It is likely that varying moisture at the end of drying means that equalization has not been
done, so the prong test will not work.

Note that when air drying, the high humidity every morning relieves the stress put in the previous day, so there is no casehardening stress, or need to condition, WITH WELL AIR DRIED LUMBER.  This is why some people report putting a small amount of water on the floor relieves stress...there was no casehardening stress in the first place.

Okay, I'm trying to wrap my mind around this and my thoughts are not clear or organized. It sounds like conditioning is something to avoid  through proper, "well air dried lumber." I assume that some degree of case hardening occurs in all air drying, but that it becomes detrimental at some point where the case dries too much faster than the the rest of the board. I have a thousand questions about avoiding detrimental case hardening, but I'll start with one;

Is there a way to measure/monitor case hardening? Let's assume I measure the moisture at approximate pin depth of 3/8" and again at 1/3 or 1/2 the depth of the board with nails or hammer pins. Is there a percentage difference that can warn me that detrimental case hardening is a threat?
Thank you for sharing your expertise with noobs like me.

[GeneWengert-WoodDoc]

Nope.  No easy way, as there is a MC gradient that makes the prong test invalid.  BasicallY, air dried lumber gets a small amount during the day and relieves it at night.   Casehardening is created the first five to ten days of initial drying, when the shell is trying to shrink but the core is still too wet to shrink.  The details are discussed in DRYING HARDWOOD LUMBER.

[YellowHammer]

The moisture gradient in wood and its effects on the drying rate and for that matter, the final behavior of the piece is a very complex relationship.  To make matters worse, each species as it own characteristic moisture loss "signature" and becomes very apparent when air drying or using low temperature dehumidification kilns.  So tests like the prong test, although very useful (I run them routinely) are invalid until there is virtually no moisture gradient, but getting to a no moisture gradient is very difficult in some species and thicknesses, which is what prompts the desire to run a prong test in the first place, to try to understand what is happening to specific pieces, or kiln loads of wood.  

The properties of each species moisture releases rate and depth of release rate, which is a function of the actual cell walls and how they collapse, is something that you will develop a "feel" for with experience, but is definitely not as simple as throw a bucket of water in the kiln.

For example, some species, such as cherry, will absolutely drop water out, with no restriction, virtually independent of thickness of for that matter temperature.  It will have a vey steady release rate, from core to shell and develops virtually no gradient, even on thick wood, whether it was air dried initially, put in the kiln, or whatever.  Interestingly enough, cherry doesn't even have a very high allowable moisture release rate.

Walnut, on the other hand, has a very high allowable moisture release rate, leading one to believe that removing moisture from it will be easy, like poplar.  However, due to many reasons, walnut is one of the most ill behaved wood in a low temp DH kiln, and frequently "locks up" during the drying process, sometimes having huge moisture gradients in boards.  I've had it as high as 18% MC in one board and the board next to it in the pack be at 7%.  Add the fact that walnut is usually is very internally stressed wood to begin with, then thats why it is such a pain to dry correctly, get a low moisture gradient, get it done in a reasonable time, and have straight boards.  Walnut is a pain.

Other species can be difficult also, just when you think to have them on the ropes.  Maple, for example, will trick you.  It has to be dried fast to keep from sticker staining, but sometimes the high delta in the wet bulb/dry bulb will cause all kinds of moisture gradients, that need to be addressed later in the cycle, once past the sticker stain stage.

In your case, thick oak is a real challenge, red oak having a tendency to lock up about halfway or near the end of the cycle, when air dried initially, making it very difficult to achieve any kiln condition that will allow it to dry close to its max drying rate.  It really has to be beat into submission at the end of its cycle.  Interestingly enough, it has a completely different drying characteristic from green, especially in the beginning of the cycle, when it wants to seriously overdry and blow through its max drying rate curve and surface check before you know whats happening.  White oak is red oak on steroids, and thick white oak is even worse still.

There are ways to overcome these obstacles, but different approaches for different problems.  From a practical standpoint, there are only so many knobs you can turn once the wood is in the kiln, such as heat with compressor, heat with no compressor, very high heat with no compressor, time, frustration and patience when the wrong method is chosen.  Oh yeah, not to mention the electricity bill.

There are things to do when air drying, also, and in that case, Mother Nature can help or hurt, and I try to pick the wood that I'm sawing based on all of that, if possible.  For example, I just milled up about $80,000 retail bucks of 9/4 walnut because it is now a perfect time to air dry it and get it down in a hurry without wasting time in my kilns.  Near 100F temps every day outside in Alabama, very high humidities at night, going into the end of summer when it gets dryer and dryer every week.  Perfect for thick walnut.  A natural kiln schedule.

However, these are just about the worst conditions for maple.

Anyway, there are many times that a load of wood seems to fit the description of case hardening, but in reality, in my experience, it there are many more variables involved that manifest themselves as issues with drying wood, and case hardening is just a good catch all term that encompasses many of them.  Some times drying difficulties and the inability for a piece of wood to come down in moisture gradient is as simple as it has a cold 😁.  Or rather it has a bacterial infection and won't release its moisture.

[Southside]

There are ways to overcome these obstacles, but different approaches for different problems

YH - you forgot about the chickens....:D

[WDH]

That is the last resort.

[YellowHammer]

Yep, when it's time to implement the magic and Vodoo techniques, it's pretty much a last resort.  

[WDH]

That part is an art, not a science as it is not included in Drying Hardwood Lumber. 

[scsmith42]

Robert, incredibly well written post above.  I salute you!

[boardmaker]

I agree with Scott, great post Robert.

Scott, do you have any way of tracking how much water your are adding with your system.  I'm curious?

[scsmith42]

I agree with Scott, great post Robert.

Scott, do you have any way of tracking how much water your are adding with your system.  I'm curious?

I don't have anything in place, but it would not be that hard to put a meter on the incoming water line.
Mine is controlled by a separate RH sensor.  When conditioning, I can enter whatever RH% that I'd like and the system will maintain it.

[Southside]

What kiln system are you running Scott?

[scsmith42]

What kiln system are you running Scott?

I have four solar kilns and two Nyle 200 series (one not installed).  The fogging system is in the Nyle.

[Downhill Cutter]

I feel like I've just been sat down and talked to by a bunch of Uncles . . . who KNOW stuff.  

Beautiful post, YH.  Thanks so much!

Thanks Dr Gene!

And Thanks, Scott!

Jerry Cox

[PA_Walnut]

Walnut, on the other hand, has a very high allowable moisture release rate, leading one to believe that removing moisture from it will be easy, like poplar.  However, due to many reasons, walnut is one of the most ill behaved wood in a low temp DH kiln, and frequently "locks up" during the drying process, sometimes having huge moisture gradients in boards.  I've had it as high as 18% MC in one board and the board next to it in the pack be at 7%.  Add the fact that walnut is usually is very internally stressed wood to begin with, then thats why it is such a pain to dry correctly, get a low moisture gradient, get it done in a reasonable time, and have straight boards.  Walnut is a pain.

Tru'dat! I just took some 5/4 walnut out that was measuring 6-8%.  Unstickered it and did some spot checks with the pinless meter: 10-12%. "Naw, this can't be right..." Got out the trust Delmhorst pin meter and yep...was right.  Back in the kiln with it. :-\

[GeneWengert-WoodDoc]

When you say that it was measuring 6-8% MC in the kiln, how were you measuring this?  Did you make a temperature correction if you were using a meter?

[PA_Walnut]

Gene, I was using Delmhorst 2000. The issue was that I didn't access the whole load. I pulled a few boards to spot check with Wagber pinless, and discovered it.

Cutting into big slabs to measure, presents an issue for pin unit.

More time &!heat solved the MC issue here.

[xlogger]

When I check the mc with my Delmhorst hammer I set the temp on 120 degrees in the kiln and walk right into the kiln and check a few places. Should I let the meter stay in for awhile or checking right away is ok? Most of what I dry is live edge and I drive the pins in the bark edge.
I'm thinking about getting a Kil-mo-trol to use. Question on it. So would if I get one would I pick a few spots and drive in pins leave them for the whole drying time and just let the wires run on floor?

[GeneWengert-WoodDoc]

You can take readings right away if the meter is not cold; a cold meter would result in condensation on the meter, probe, and wires.  If you get condensation, you will get a reading like 12% MC when the pins are not in the wood.  If the meter is colder than the kiln by very much, put the meter and probe in a plastic bag before entering the kiln and let it warm up for 30 Minutes or so.

With the Kiln-Mo-Trol, the pins are inserted while stacking the lumber at selected depths and wires run to the outside of the kiln to a box with a switch.  The switch lets you switch from probe to probe.  probes are not removed until drying is done.  Note that knowing the MCs between 30% to 10% MC does not do you much good as the defects develop and worsen at higher MCs, and overall the probes tend to require double checking final MC values.  Pins are usually driven to 1/4 of the thickness and not into the core.  On thicker wood, two depths are used...1/8 thickness and 1/4 thickness.

Note that putting needles into the edge of a bark-edged slab, even with longer needles, means you are usually measuring the outer sapwood, which does dry faster than the inner sapwood, which dries faster than the heartwood.

[xlogger]

So with large live edge slabs that have been air drying for months you unstack part of them to put in pins? Lots of work there.

[scsmith42]

So with large live edge slabs that have been air drying for months you unstack part of them to put in pins? Lots of work there.

With a Kil-mo-trol, yes. Usually we call pivot a board out of the side of the stack, insert pins and cable and then slip it back in. 
This method won't work with wide slabs though.

[xlogger]

So Scott I know you do more slabs than me, how do you keep track of MC when drying? If I understood Danny correct he does it about the same way I do. Correct me Danny if I'm wrong.

[WDH]

Here is my method.  Air dry the slabs first to below 20%.  Target is 15%.  Put in kiln and set wet bulb at 120 and dry bulb at 75 so that the compressor runs full out.  Watch the water in the bucket.  At the start, I will get 9 gallons of water (Nyle L53).  As a few days goes by, less water per day.  After about 7 days (depending on species, oak takes longest), there will be only a little water in the bucket after a 12 hour period.  I know that I am getting close.  When I am down to about 1/2 cup or so and things seem at a steady state, I check the moisture content with the Delmhorst J2000X with the long insulated pins and the slide hammer.  If the moisture content is 9% or less, I close up the vents, shut off the compressor, and set the dry bulb temp to 150 degrees.  Once the temp hits 150 degrees, I run the kiln like that for 24 hours, then I am done. 

If I am still above 9% when there is minimal water in the bucket, i.e. the load seems to have stalled, I close the vents, shut off the compressor, ramp the temp to 150 degrees.  When I hit 150 degrees, I open the kiln doors for about 3 or 4 minutes to dump any water vapor.  Then run for another 8 - 10 hours and repeat with the doors open for about 4 minutes.  When I get below 9% moisture content, I hold the temp at 150 degrees for one more 24 hour period, then I am done.  That glides me in to 8%, my target here in Georgia.

My method involves a little science and a lot more experience.  Since I capture every drop of water exiting the kiln, I know when the compressor has done its job and it is time to check the final moisture content.  I also know whether it is time to sterilize or if I need to turn off the compressor and use higher temperatures to coax that last little bit of moisture from the wood. 

Since I can only do one thing at a time (do everything alone without any supplemental help), usually the load sits in the kiln after I am done and have shut everything off with the vents still closed and the kiln bottled up for another 24 hours, giving the load time to relax and equilibrate and I have time to unload and process the wood.  I do not unload the wood until I start planing  it and get it racked in the climate controlled space.  The worst thing for me is to unload the kiln but not be able to process the wood until some time later.  I learned that the priority is to process and rack the wood in a timely manner after it leaves the kiln.  If the outside humidity is high (usually is ) and I let the wood sit under the shed for a while, the boards most exposed on the outside can gain moisture and bow a little, and I despise that.  All boards must be flat without any twist or cup. 

[scsmith42]

So Scott I know you do more slabs than me, how do you keep track of MC when drying? If I understood Danny correct he does it about the same way I do. Correct me Danny if I'm wrong.

My method is similar to Danny's, except that I use solar kilns as pre-driers (and for some species they will dry all the way).  In the Nyle, I'll set the temp at 150 and the RH% at 30% or so.  Once the load is done I'll crank up the RH% with the misting system for conditioning - starting while the load is still warm and I've turned off the DH unit.  I'll bring it up to 40% RH and then shut the misting system down.  
As the temp drops the RH% may increase a bit, but since the lumber is absorbing the moisture it usually stays pretty close to the 40% target.  I'll let the load sit for 24 hours sealed up before unloading.

[YellowHammer]

I use little bit of both techniques, air dry, then for wood that I know will stall, like walnut, I'll put it in the solar kiln, then finish it in one of the Nyles.

I do the sterilization to 150F, vents closed, compressor off.  Then I left it "rest" for 24 hours, put it in the warehouse, and let it sit with stickers and weight until is gets dead stacked a few days later.

[Nebraska]

Thank you all for the time and the knowledge. Learn something new everyday, so the day isn't wasted. Enjoyed the read.

[xlogger]

ok I'm pretty similar with you guys on that. Biggest question is how do you check you MC on slabs while drying? I usually don't even try checking till I see no water dripping and check with slide hammer on bark edges and try to find a place where bark has come off.  

[WDH]

I go in the side like you do.  As deep as the pins will go without the bottom touching the wood. 

[YellowHammer]

I go in from about a half a dozen places, on different slabs, in places the customer won't see.  I'd rather punch a few extra holes than find out after it's been pulled from the kiln.  

Another way to confirm the slabs are dry is to feel the air in the kiln after the load has been sitting for its rest period.  If the door opens and it's dry, that's a good sign.  If the air is slightly moist, that's not a good sign and time to start slide hammering to find out what's wrong.  

The true test of a whether a slab is dry is to lift it, and feel it.  After deadstacking a few thousand slabs, you'll be able to just "feel" if it's right or not.  If it's dry on the outside and wet on the inside, the little alarm bell in my head will know it and I'll start trying to find out what the problem is.  After all, if it's dry, it's dry, and dry doesn't weigh as much as not dry.  It won't have a slightly clammy feel, and the grain will have a collapsed look.       Sometimes a wet slab spot will even sound different when it gets deadstacked and dropped on the pile.  All are subtle clues something isn't right.  Look and listen for them. 

 
 

[Stephen1]

I go in the side like you do.  As deep as the pins will go without the bottom touching the wood.

You do not allow that little washer gaurd to touch the wood?

[GeneWengert-WoodDoc]

The small washers are used to prevent electrical contact of the probe itself with the wood. When using washers on both needles, it is ok to drive the needles all the way in.  However, the extra 1/8" will not make any difference.  Plus, if the needles break, having the extra 1/8 inch, when not driving all the way, to grab on with a vice grip is really helpful.

[moodnacreek]

While I have nothing to add to this as I do not have a kiln, I find these posts very interesting.  At my sawmill air drying has been the only way for a long time. What I have learned is that some woods will not really air dry on sticks, out doors or even in sheds. Elm and hickory are bad and w. oak all goes for trailer planks, even clear stock.   To you kiln driers, keep up the good posts.

[WDH]

I go in the side like you do.  As deep as the pins will go without the bottom touching the wood.

You do not allow that little washer gaurd to touch the wood?

No Sir. 

[PA_Walnut]

I am in the same camp with @YellowHammer and @WDH . Air dry it well, then put some heat to it until it stalls, then close'er up and put the heat to it. 

I have some 12/4 walnut slabs in right now that were about 20%. Some is down to 8-9% and other slabs/spot are 16-17%. Stalled out at 120°, so pushed it up to see what happens today. Once they are are down and closer, will run it up to 155° and shut all the vents. Sometimes I dump some water in too. I find that the added MC, evens things out and then I vent it and continue for 24 hours. 

With 50" crotch slabs in there, don't wanna ruin the load!   Maybe I need one of them new-fangled kiln things where I just push the big red button and magic occurs.  ;D :D

[WDH]

That red button thing is called a vacuum kiln ;D.

[PA_Walnut]

That red button thing is called a vacuum kiln .

I know...being sarcastic. It's been dubbed the, "Devine Drying Device", replacing all knowledge gained over the past century or more. :D

[Stephen1]

I am in the same camp with @YellowHammer and @WDH . Air dry it well, then put some heat to it until it stalls, then close'er up and put the heat to it.

I have some 12/4 walnut slabs in right now that were about 20%. Some is down to 8-9% and other slabs/spot are 16-17%. Stalled out at 120°, so pushed it up to see what happens today. Once they are are down and closer, will run it up to 155° and shut all the vents. Sometimes I dump some water in too. I find that the added MC, evens things out and then I vent it and continue for 24 hours.

With 50" crotch slabs in there, don't wanna ruin the load!   Maybe I need one of them new-fangled kiln things where I just push the big red button and magic occurs.  ;D :D

I'll let you know how the big red button works. #shelby78 dropped off 2 50" walnut slabs for me to dry. They just fit in the kiln. They are on the bottom of another load of 4/4 and 6/4 walnut and cherry. 

 

 

[PC-Urban-Sawyer]

Looks like the magic has already occurred. That's some beautiful slabs.

[Stephen1]

 

You can just see the edge of the 2 big walnut slabs .
That is my walnut/cherry and then another customers Cherry on top.