iDRY Vacuum Kilns


What mc is safe to start sterilizing

Started by forrestM, May 02, 2024, 07:39:18 PM

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I know there are probably a dozen of these posts, so I apologize. What mc is it safe to start high temp sterilization (I'm talking 140-160 f). 

Does it matter the species? And does it matter the thickness? Or is mc the only deciding factor?

I always assumed it was anytime you got to 15 percent you were safe from any harm to the wood   


doc henderson

I would say when you are within a few degrees of your target MC.  dry wood is an insulator.  water is dense a requires calories to heat up, so wet wood will take longer.  You will drop a few % MC with the heat treatment if for 24 hours.  energy is consumed with phase change of the water from liquid to vapor, but without further rise in temp. 

thicker slabs should be treated longer, till the core is over 133o
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I think the only danger with increasing the heat to sterilise levels too soon is if you exceed the safe drying speed for that species. So species / thickness / kiln design all matter. Construction pine is dried at high temp in about 2 days, and comes out of the kiln at ~19%, with any bugs well and truly cooked.  It's hard to dry pine too fast, but you can't do that with White Oak etc. 

With more sensitive woods, you would dry them down to near your target moisture, following your kiln schedule, and then crank up the heat for that last 24 hours as that's both lower risk to the wood, and more energy efficient. While you could increase the temp earlier, you would also have to let the humidity increase, to keep drying at a safe rate. So you are pumping in more heat than really needed. 
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Once you are below 25% at the core with any species or thickness of wood you are safe from causing further defects but stress caused earlier in the drying will tend to show up when you are between 15-20% as that is when the wood is unable to contain it.
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