# Kiln drying



## Jaydd0531 (Aug 17, 2017)

I am making a kiln. Using a dehumidifier and fans. Its not getting the moisture out. I check it in the kiln and its at 6 then take it out and the next day its back to 12-13. So I think temp is the problem. I was reading some things and it said in a dehumidifier kiln can use up to 160 f. My kiln is 12x6x4. Wood is 1.5 to 2 inches thick. What is a cheap and safe way to get the temp up and what temp is good? Will a space heater get temp high enough?


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## Steve Neul (Sep 2, 2011)

A dry kiln does more than moisture content. I forget what the temperature is but it's higher than 160. The wood needs to be heated through to the center of the wood in order to kill any insect infestation the wood may have. Then if you don't have sufficient temperature for long enough the wood dries on the surface and is still wet inside. This is probably why you are having fluctuating moisture content readings. The wood is wet in the middle and after taking it out of the kiln the moisture in the middle seeps to the surface.


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## Jaydd0531 (Aug 17, 2017)

So what is the best way to get the temp up? I'm afraid a space heater won't go that high. I don't want to waste my money on it if it won't get high enough. The article I was reading said 133 f for 4 hours would kill any bugs.


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## Tennessee Tim (Dec 15, 2010)

Steve Neul said:


> A dry kiln does more than moisture content. I forget what the temperature is but it's higher than 160. The wood needs to be heated through to the center of the wood in order to kill any insect infestation the wood may have. Then if you don't have sufficient temperature for long enough the wood dries on the surface and is still wet inside. This is probably why you are having fluctuating moisture content readings. The wood is wet in the middle and after taking it out of the kiln the moisture in the middle seeps to the surface.


Sorry Steve....1/2 truth and 1/2 WRONG!!!
Yes most kilns do more than drying..... BUT the extreme temp has nothing to do with drying, IT'S ONLY for sterilizing!!!! There's a LOT of factors to this equation that I don't have time to type this a.m.



Jaydd0531 said:


> So what is the best way to get the temp up? I'm afraid a space heater won't go that high. I don't want to waste my money on it if it won't get high enough. The article I was reading said 133 f for 4 hours would kill any bugs.


ONCE the wood is correctly dried to the desired goal THEN depending on species all I believe sterile at 133 for 4 hrs AT THE CORE/CENTER....and some pines require 165 I believe to set the sap pitch from running.

There are several articles I and a few other "driers" have done here on the basics. IF you can find Dr. Gene Wengert (?spelling?) aka"the wood Dr" has written many books on drying.

It's obvious you haven't read enough prior to this project (don't feel alone, I've missed the mark before also) You have to get the moisture correct to the core and it's not fast with a homemade DH. I have one posted here I call the super kiln. Sorry lack of time this a.m. to go further....please search the posts here re DH kilns and drying......DON'T dry TOO fast!!!! This causes other issues.....as I said above...LOTS of equations to be done correctly.


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## difalkner (Nov 27, 2011)

The sawmill I use has two solar kilns. After cutting Walnut for me he leaves it outside a week to 10 days and then in the kiln for about 5 weeks to the inch of thickness to get it down to about 10% to 12%. He has a dehumidifier in the large kiln, shown below, but only fans in the smaller kiln. Seems like the last time I got Walnut the temp was about 120° but it was a cloudy day about 10 in the morning. I'm sure it gets much hotter during the day.

















David


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## Steve Neul (Sep 2, 2011)

Tennessee Tim said:


> Sorry Steve....1/2 truth and 1/2 WRONG!!!
> Yes most kilns do more than drying..... BUT the extreme temp has nothing to do with drying, IT'S ONLY for sterilizing!!!! There's a LOT of factors to this equation that I don't have time to type this a.m.
> 
> 
> ...


Well don't forget the high temperatures expedite the drying process so more wood can be moved through the kiln. This takes precedence over sterilization for the lumber mills.


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## Tennessee Tim (Dec 15, 2010)

Steve Neul said:


> Well don't forget the high temperatures expedite the drying process so more wood can be moved through the kiln. This takes precedence over sterilization for the lumber mills.


WELL don't FORGET high temperatures causes OTHER issues when drying wood ALSO. Drying too fast causes stress in the wood and as many has found when ripping the wood moves, twists, pinches....a lot of this has to do with drying too fast...it's called...

Case hardening describes lumber or timber that has been dried too rapidly. Wood initially dries from the shell (surface), shrinking the shell and putting the core under compression. When this shell is at a low moisture content it will 'set' and resist shrinkage. The core of the wood is still at a higher moisture content. This core will then begin to dry and shrink. However, any shrinkage is resisted by the already 'set' shell. This leads to reversed stresses; compression stresses on the shell and tension stresses in the core. This results in unrelieved stress called case hardening. Case-hardened wood may warp considerably and dangerously when the stress is released by sawing.
(wikipedia)

In a small DH kiln most aren't set up to counteract/attempt to reverse the reaction via steam....ALL of this has to be done under strict guidelines and timing.

One trick to your DH kiln is the water coming out keep it measured...or at least do as I, watch the pan/bucket until a week has gone by and very little moisture has been captured for the whole week.... I put a weather station RH reader in the return air area and watch the MC/RH #'s, once the RH/MC and temp (I dry at a lower than normal 90 deg) I then watch the water output. Google RH vs MC and you should find a chart that shows temp and RH and the cross over to MC...
here's one by Dr Gene... http://www.woodweb.com/knowledge_base/Humidity_Temperature_Wood_Moisture.html
and by wagner
https://www.wagnermeters.com/rh-and-mc-an-environment-of-change/


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## Tennessee Tim (Dec 15, 2010)

difalkner said:


> The sawmill I use has two solar kilns. After cutting Walnut for me he leaves it outside a week to 10 days and then in the kiln for about 5 weeks to the inch of thickness to get it down to about 10% to 12%. He has a dehumidifier in the large kiln, shown below, but only fans in the smaller kiln. Seems like the last time I got Walnut the temp was about 120° but it was a cloudy day about 10 in the morning. I'm sure it gets much hotter during the day.
> 
> View attachment 309873
> 
> ...


 A note on this speed

Solar drying and ADing has a natural stress reliever the increase and draw to wood cells that happen during the day relieve themselves at night in a relaxed state......think of it as grippin a wet sponge in your hand tight for a minute then relaxing it...the water pushes out but deforms the wood/sponge, but when you loosen the grip (the night) it inhales air until the next squeeze/draw.

It's really simple IF put in simpler terms for me anyway.


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## Jaydd0531 (Aug 17, 2017)

From what I read you can take a 1% out per day. The wood has been air dried for about 6-8 months. It is about 13-16 before I put it in the kiln. I am making coffee tables with the wood so some crackers are ok. I am surfacing the wood with a router to straight it out so warps aren't a big deal eather. The pieces are about 4-5 ft by 20-40 inches wide. Could I bring the temp up during the day then cool down at night?


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