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Vacuum kiln dried wood problem?

Started by logbyr, March 27, 2020, 06:32:44 PM

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logbyr

Had  five 10-12" wide 9/4 white clear curly maple slabs custom dried last week in a vac kiln.  All  cut from same log.   Picked the slabs up today and was surprised at amount of cup in the wood.   Thought cup was almost nonexistent with a vac kiln.    When I got home I checked the moisture and they read from 3.4 to 4.7%.    I called guy and he said they showed them at 6%.  They dried my wood with 11/4 squares of same species.    My wood  was a tree growing in woods 5 days before going in the kiln.  They load kilns soon after sawing their wood.
Is the low moisture % why I got the cup? Is this wood going to cause problems after it acclimates to 6-8%.   
 I invested in vac drying to get bright white flat wood to market quick.    The co has been running vac kilns for years and have tons of experience.  Trying to figure out what went wrong.   Any help here?

low_48

12" wide isn't very wide. What diameter was the log? If it wasn't much over 12", that is the problem. A vac kiln removes the moisture at very low temperatures, but wood shrinkage is wood shrinkage.

logbyr

It was an 18" log.    The boards were from the outer part of the log where curl  was most intense and the boards were free of the darker heartwood.

Southside

That's the problem. Natural desire of the wood to want to cup in those circumstances. 
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GeneWengert-WoodDoc

Cup is caused because the bark side of lumber shrinks more than the heart side.  Vacuum drying does not affect natural shrinkage.  The drier the wood, the more cup.

You must be using a pinless moisture meter.  Readings are affected by wood density.  If the drying people use a pin meter, they cannot measure under 6.5% MC.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

logbyr

Gene u r correct on each of our moisture meters.   Both Wagner, mine pinless theirs has pins.


GeneWengert-WoodDoc

So long as we are talking about cup, did you know that the faster you dry, the flatter the wood will be?  Do you know why?  As the outside or shell dries, it tries to cup, but the wet core stops it fairly well.  Now, the shell is dry and twice as strong, so when the core begins to shrink and want to cup with less cupping tendency than the shell has, the dry, flat core prevents cup.

Also, if you have partly dried wood, where the shell is dry and flat, if you rewetting the shell, it will lose its strength and will be easy to cup.  So, rewetting partly dried wood increases cup.  This is why we never want to start the kiln with conditions that add moisture back to the shell, and never let the kiln get more humid when running.  This means that a breakdown in a kiln, steam, vacuum, DH, etc., has a good chance of increasing cup...depends on conditions in the kiln.  DO NOT ADD WATER OR MOISTURE BACK TO WOOD THAT HAS STARTED TO DRY THE SHELL FAIRLY WELL.  At the end of the cycle, we do add moisture back to the shell to remove casehardening and this does not affect cup.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

boonesyard

We just finished up a load of large white oak slabs 12/4 - 30"x12'. Most of the slabs were dry, but there were spots in more than half of the slabs that were still as high as 15%+, they were locked up and just would not move any more. I got some advice from a local guy with a kiln that told me to spray the whole load down with water, that it would open it up and allow it to finish. I pulled the dry slabs, sprayed down the load, and it was dry in a week. I will not forget that lesson.
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