adding moisture back in during sterilization or pitch setting?

[forrestM]

whenever I finish my load and go to set pitch or sterilize I keep losing moisture in my wood even with the compressor off. 
I try to stop it early, so I have room to fudge, but It always ends up drying it more than I want it to. 

I don't have humidification on my nyle l200m, but does something like dumping lots of water on the floor do anything to keep the wood from drying out further? Or do I just have to start sterilization even earlier?

thanks,
forrest

[K-Guy]

Hi Forrest
What is happening is called equalization. The core moisture moves to the surface evening out the moisture content. The dryer boards will also pickup moisture from the wetter boards. I would recommend stopping your drying a percent or two earlier to get the desired result. 

[forrestM]

Does adding water do anything?

[KenMac]

As this is also of interest to me, at what mc % would you knowledgable guys recommend starting sterilization? ???

[YellowHammer]

I pull down to 8.5% kiln drying.  Then I will sterilize and equalize for 24 hours after the kiln chamber reaches 150°F.  Then I will turn everything off, including the fans to trap the air, and let the chamber slowly cool.  When I open the doors the next day, I'm at 7% to 7.5% moisture on the dot every time.   

[K-Guy]

Adding moisture is usually done to relieve case hardening stress. This can be accomplished by adding 5 or 10 gallons of water to the kiln when you do your sterilization.  I recommend 5 for 2000 bf and 10 anove 2000 bf. It will relax the shell if it is case hardened and not add any appreciable amount of moisture to the wood.  Don't put the water directly on the wood,  if it has impurities it may cause staining. Pour it on the floor, it will naturally evaporate. 

[GeneWengert-WoodDoc]

If you have a room that is at 110 F and 50% RH, and there is no way for air to leave or enter, when you heat that room to 150 F, the humidity will drop to under 3% RH.  This low RH will dry the wood in the room (the fibers on the outside of the lumber) to about1% MC.  However, the moisture leaving the lumber will increase the RH in the room, which will slow drying and prevent the wood from getting tom1% MC.  Remember, that 1000 BF of water will, on the average, lose 3 to 4 gallons of water for each 1% MC loss.  Also, if the kiln room is tight, it will not take too many gallons to increase the humidity in the room to 7% EMC.  So after perhaps 1% MC loss in a tight room, the humidity drop by going to 150 F will be offset by the moisture coming from the drying lumber and further drying will stop.

 If you are following this discussion, then you can appreciate that if you add maybe 8 gallons of water, more or less depending on the size of the kiln, to the room (the floor might absorb water, but we want the water to evaporate into the air), then when going tom150 F, the added water will evaporate rather than moisture from the lumber.  If we add even more water, the higher humidity will cause the surface of the dry lumber to swell, which will remove drying stresses or casehardening.  Of course, air dried lumber has very little if any casehardening stress. (Remember that stress prongs must not have a moisture gradient in order to show the true amount of stress.)

As you get more sophisticated, you could add water using a mist sprayer, such as sold by SII Dry Kilns, but this likely will require more heat to reach 150 F.  Or you could even use a small electric boiler to inject moisture.

Hope this makes sense.

[KenMac]

I pull down to 8.5% kiln drying.  Then I will sterilize and equalize for 24 hours after the kiln chamber reaches 150°F.  Then I will turn everything off, including the fans to trap the air, and let the chamber slowly cool.  When I open the doors the next day, I'm at 7% to 7.5% moisture on the dot every time.  

Thanks YH. Does this relate to all thicknesses of lumber? My load of pecan dropped from average 6.5% to about 5% during sterilization, but I left the chamber at 150 for 2 1/2 days. Surface mc has since come back up to about 6%. I think I apparently overshot my goal by 1% or so. Make sense to you?

[forrestM]

Thanks to everyone!

does adding water back in effect pitch setting? or do the resins still boil off in a humid kiln room?

[forrestM]

Now I'm wondering what reduced pitch setting times will result in? I keep having to turn the kiln off before I get to a full 24 hours because the wood is already dry and I can't keep it from drying. It's white pine only 7/8" thick. Im getting maybe a little over 12 hours at 160. Maybe 20 hour at 140 degrees while I'm on the way to and from 160. 

It's just paneling, and I'm going to be planing and shiplapping the material. Is sap going to run down my walls or am I at least protected by all the time it has been at least 120 degrees? Might gum up the planer though? As long as my house doesn't get 120 degrees, which I hope it won't. 

Thanks!

[YellowHammer]

KenMac
That sounds right, rebound is normal after some sterilization, and I look for it.  The amount of the initial pulldown and the rebound is dependent on the individual kiln and also the thickness and wood species, as well as the length of time of the cycle.  I don't like to go much more than 24 hours for the heat cycle.  It's always best to start little high, and walk it down on a new species, thickness, etc.  Overdrying is not a good thing and really makes the wood "grumpy" and adds more defects such as bow and twist.  I liken to to over cooking bacon.  Once the bacon is burned, its burned, and bad things happen.  It curls up and twists.  The same with wood.

I've done the sterilization cycle as high as 10%, but anything above that doesn't work as predictably.  So if I'm at 9.5%, do a 24 hour cycle and it rebounds back to 8.5% at 120°F then I just turn the compressor back on and pull it down the rest of the way.  So I'll fine tune the drying level after the sterilization.  Since we are dealing with just a few percent, timing is important, the longer the cycle, the more moisture will come out, and if the kiln leaks at all, it is acting like a direct fire kiln and is continuing to dry the wood, ever so slightly.  So the longer the cycle, the more the drying.  

After awhile, with any species, the final cycle is second nature and can be nailed on the dot.  However, it there is a miss, its best to miss high on the final MC and just polish it off with the compressor.

As far as pitch setting, the idea isn't to remove the moisture but to evaporate off the volatile hydrocarbons that are in the pitch or sap, and let the sap crystalize.  The volatiles come off at the temperature of the pitch setting temperature, so the higher the temps, the more the fumes come out of the sap and the more the pitch solidifies, and is "set."  Whether or not there is still liquid phase pitch is based on the volatilization temperature as well as the time of the process.  The vapors must have time to get from the interior of the wood to the exterior and vaporize.         

[forrestM]

Is it safe to say, with the kiln having had run longest at 120 db, I am protected from sap running at least up to 120 degrees?

[forrestM]

Well I finally really looked at the charts in the nyle instruction manual. If I don't want to lose any more mc past what I want, and at the same time remove moisture from the wetter boards, all I have to do is keep my db and and wb depression where it needs to be for emc. So if I'm equalizing / pitch setting my syp to 9% I want a db of 160 and wb of 144...depression is 16 giving me an emc of 9 percent. 

So if my wb drops too low I'll add water to the floor. If it gets too high I'll open the doors. And after 24 hours hopefully all will be where it needs to be at nine percent. I currently have maybe a 5 percent mc spread from board to board 

Have I got the theory right?

[YellowHammer]

The theory is correct but if the WB gets low, let it keep dropping a couple percent to try to hurry up and get the stubborn wood down and then when it hits the proper moisture level the spread should be much lower because drying us asymptotic to the EMC value.  

Opening doors or power venting us very effective.  

The sap will generally not run up to the max sustained temperature of the kiln.  However, it must stay at that temp long enough for the pitch to solidify.  

[forrestM]

I assume the equalization process goes much faster when the kiln is at higher temperatures? 

[YellowHammer]

Yes, also dryer wood comes up in moisture faster than wet wood wants to come down.  So I target the wettest wood to a certain moisture content and if I overshoot a little on the other, dryer stuff, I let that come up and equalize to my target. 

[forrestM]

Well I did the opposite - stopped it when the driest stuff was as dry as I could stand. So I guess we'll see how it goes. Feeling a little more confident now, though. 

I sealed off the unpowered intake vent and that seems to be keeping a lot more moisture in the kiln. I just keep checking to make sure my depression is on target. 

When you get up to high temperatures you can really tell where your leaks are!

[GeneWengert-WoodDoc]

I will try to keep this simple, but accurate, yet technical.  This is an excerpt from a "Ask the Wood Doctor" column in FDMC magazine (Furniture Design and Manufacturing and Cabinetmaker).

Pitch is not one chemical, but consists of certainly 100 variations of the basic sap or pitch molecules.  Pitch is only in the needle trees.  

The pitch molecules that are shortest, or lowest molecular weight, are somewhat runny at room temperature. When heated, these short molecules get very softer and liquid-like (runny) and eventually begin to evaporate rapidly (and smell pretty nice) at maybe 110 or  120 F.  Other pitch molecules that are longer in length and have higher molecular weight, are only slightly soft and somewhat thick and ooze slowly at room temperature.  They begin to evaporate rapidly at 140 F.  Then, some are pitch molecules that are moderately hard at room temperature but will soften and flow slowly if heated by the sun or a electric baseboard heater, or fireplace, or sandpaper, etc.  These might evaporate quickly at 160 F.  And then there are some that are really hard at room temperature and evaporate at 180 F and some at 200 F and so on.  

Note...To keep this simple, I used a few example temperatures above, but actually, there is some pitch that evaporates at room temperature, room temperature plus 1 degree, room temperature plus 2 degrees, and so on all the way up to 240 F.  So, do NOT think that there are specific groups of pitch.  Rather pitch is a continuous large group that has fast evaporation of some molecules at virtually every temperature.  So, what this means is that the hotter the lumber is heated, the more pitch that is evaporated.  It also means the pitch remaining, as you go hotter will be harder at room temperature.

So, each pitch molecule has a temperature at which it "boils."  As the pitch molecule cools below this "boiling" point, pitch becomes less and less runny, and more solid.

In short, the hotter you go, the quicker the pitch evaporates and the more of the pitch that is soft at room temperature plus a little evaporates, resulting in only hard pitch remaining.

Thickness of lumber plays a role in how fast the "boiling" pitch can leave the wood.  Thickness can also influence how long it takes for the center of the lumber to reach a certain temperature.  A common value for heating at around 180 F is 24 hours for nominal 8/4.  Note that this heating can occur within the normal kiln schedule or may be a special heating cycle at the end of the schedule.

When thinking of room temperature, some homes with an exposed beam ceiling might have heat near the ceiling that is much hotter than down near the floor where the humans are.  Wood used or exposed in an outside screened porch in GA in the summer could easily reach 100 F.  Wood being sanded with dull sandpaper can easily reach over 180 F, causing some brief flow of pitch until the wood cools.  For all these reasons, heating to set the pitch is ideally done at 180 F.  However, 160 F does a pretty good job, but may not cover all subsequent exposures.  Also, 160 F might be best at 36 hours.

[Stephen1]

I just dropped in to learn a littl bit!