Vacuum drying wood?

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PatrickR

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Has anyone here tried this to dry blanks for stabilizing?
It would seem to make sense and be a more efficient way to do it and is considered the fastest way to dry fresh cut wood.
Any thoughts or experience?


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Dehn0045

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Haven't tried it myself, but definitely a good method for small volumes of highly figured woods (arguably its the best method for drying these woods). A couple of nuances to the setup - really should use a continuous duty vacuum pump, a method for heating the wood inside the chamber is best (heating the outside of the chamber isn't very effective due to the insulating effect of the vacuuming - direct heating mats are often used), a trap for water that comes off of the wood.

You could probably do a few pen blanks in a regular wood stabilizing vacuum pot, but larger bowl blanks and such would probably require a more appropriately designed setup.

Here is a good video on the topic:
 

PatrickR

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My concern in using a stabilization system to dry wood in that way would be that some of the water would end up in the pump oil , which wouldn`t enhance pump life expectancy .

Thanks. I change the oil regularly per the units instructions.


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PatrickR

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Haven't tried it myself, but definitely a good method for small volumes of highly figured woods (arguably its the best method for drying these woods). A couple of nuances to the setup - really should use a continuous duty vacuum pump, a method for heating the wood inside the chamber is best (heating the outside of the chamber isn't very effective due to the insulating effect of the vacuuming - direct heating mats are often used), a trap for water that comes off of the wood.

You could probably do a few pen blanks in a regular wood stabilizing vacuum pot, but larger bowl blanks and such would probably require a more appropriately designed setup.

Here is a good video on the topic:

Thanks. Good info. I will watch the video.


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Curly

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PatrickR

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Thanks all!
I watched the video and it was a big help.
Now I get the concern about water. For my use it shouldn’t be much concern though as I would only be using it for already dry wood and only pen blank size. I think I’ll give it a try.
Curly - the book is talked about in the video. I’ll get it ordered.


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greenacres2

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Patrick--I picked up a used vacuum oven, which does a great job of even, controlled heat, and with long run of tubing was able to condense vapor prior to it getting to the pump. I think a desiccant chamber in between i'd be more comfortable with my vacuum pump.
earl
 

PatrickR

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Patrick--I picked up a used vacuum oven, which does a great job of even, controlled heat, and with long run of tubing was able to condense vapor prior to it getting to the pump. I think a desiccant chamber in between i'd be more comfortable with my vacuum pump.
earl

I’ll keep an eye on ebay for one.
Thanks Earl


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Dehn0045

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Food dehydrator worked reasonably well for me with pen blanks. It's a fairly aggressive method, so cracking is kind of an issue, but it's dead easy.
 

PatrickR

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I have been using a toaster oven, but think there has to be a better more efficient way.
I looked at the Vac ovens on ebay but they are more than I would spend.


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Dehn0045

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Air flow is an important factor not to be underestimated. Dehydrators run at lower temps but higher air flows, 24-36 hr is the dehydrator is enough for pen blank sizes. Placing blanks HVAC duct is another option, a little less aggressive but a few weeks should do the trick for small blanks. When it comes to highly figured woods and burls I think the vacuum method is really the only 'fast' method, otherwise you are forced to balance cracking with drying time.
 

greenacres2

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I’ll keep an eye on ebay for one.
Thanks Earl


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Keep checking--took me a few months of watching for Lab Oven (dental labs use them without vacuum, medical labs tend to have vacuum). Shipping is a killer as they are heavy. I ended up with an old analog Precision Science for $50 and it was 12 miles from my house. I draw a vacuum when i first turn it on, then seal the valves. After a few hours i release the vacuum and keep the door slightly ajar at 220 F. The oven is brick lined so the mass holds the temp very steady.

Haven't used a toaster oven since i caught one on fire in my garage. Had a PID on it, and to get consistent 220-ish had to set the PID to kick on at 154, off at 156. Interesting to watch the temp rise AFTER the element turned off--from 200 to 350 or so. The toaster oven was a brand name--purchased new, and was still pretty new when it lit up. Never again--not even for toast.
earl
 

PatrickR

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I’ll keep looking.
That’s scary. I have mine on a PID and get about 8-10 deg rise after shut off. I just read that the combustion point of wood is 500*. Yours got crazy hot.
You shouldn’t need that much heat using vacuum. At about -28 vac water will boil at 100*.
The wood will have to be heated to 200* for stabilization no mater what, so if that temp is going to crack or warp it the cooler temp options don’t matter. I would like a more efficient way to do it though. Running an oven at 220 for days at a time seems a waste.


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Dehn0045

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The wood will have to be heated to 200* for stabilization no mater what, so if that temp is going to crack or warp it the cooler temp options don’t matter.

Do you mean for curing the resin? If so, my understanding is that there is much less risk of cracking when curing versus drying. If using vacuum you can achieve bone-dry without high temperature (this is the only practical method to do so that I am aware of for the home hobbyist). I suppose one could achieve bone-dry at low temperatures using extremely dry air or nitrogen or other chemicals, but I have never heard of this being done. I have heard of soaking in acetone to more quickly achieve EMC, but I don't think this method will get you bone-dry.
 
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No big surprise right? Vacuum lowers the boiling point of water... you go far enough into vacuum and the water boils off even at room temp. :) Vacuum ovens just get the whole thing done even faster. Like Earl said, they can be expensive though so used market is the place to go if you're not doing this as a full time gig.

I've been using a lab oven set to 140F to dry my wood prior to casting. Usually in a couple days it's bone dry and ready to go. It's a low heat that reduces the risk of cracking and we get things dry quick because the lab ovens are constantly circulating air. It's a LOT safer than risking a fire with a toaster oven or smoker for drying wood...
 

PatrickR

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Do you mean for curing the resin? If so, my understanding is that there is much less risk of cracking when curing versus drying. If using vacuum you can achieve bone-dry without high temperature (this is the only practical method to do so that I am aware of for the home hobbyist). I suppose one could achieve bone-dry at low temperatures using extremely dry air or nitrogen or other chemicals, but I have never heard of this being done. I have heard of soaking in acetone to more quickly achieve EMC, but I don't think this method will get you bone-dry.

Yes, curing the resin. You could be correct but I have not been able to do it that way yet. I may try with desiccant beads first. I need to get a triple beam scale. The cheap digital I have is not accurate enough.


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1080Wayne

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Yes, curing the resin. You could be correct but I have not been able to do it that way yet. I may try with desiccant beads first. I need to get a triple beam scale. The cheap digital I have is not accurate enough.


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Bought dessicant beads about 5 years ago and never got around to trying them . Last night took a piece of chokecherry which had been drying in my basement cool room for at least 10 years , about 1.5 inch dia , no end checks , 204 gm . Measured about 8% on Timber check gauge . Weighed it , put it into a ziplock bag with an equal weighed amount of dessicant , left it on floor air register . High -27C forecast today , -30 tomorrow , -22 by Saturday , maybe -6 by Tuesday , so furnace getting a good workout , and the dessicant should stay warm . Will weigh check the wood weekly .
 

Dehn0045

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If my memory is correct, when I dried with the food dehydrator it ran at about 135 degF. I ran it for about 2 days and the moisture content of the wood stalled at about 3% after 1 day. This makes sense given this chart ( https://www.woodworkerssource.com/wood-moisture-content.html ) and the relative humidity being about 10% (probably around 70 degrees and 80% humidity in the garage, but I can't remember exactly the ambient conditions). I don't think 2-3% is good enough for stabilization but it might be for casting, my understanding is that even 1% would be too much for good stabilization. I would be very surprised if desiccant beads are able to get below 2-3% moisture without heat.
 

PatrickR

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Desiccant may or may not work but is the path of least resistance so I’ll give it a shot before trying vacuum.
For stabilizing 0% is the goal. Only the best moisture meters are reliable below 5% so weight has to be used.
I need to do more research on desiccant to see if heat would be needed or helpful.


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PatrickR

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Bought dessicant beads about 5 years ago and never got around to trying them . Last night took a piece of chokecherry which had been drying in my basement cool room for at least 10 years , about 1.5 inch dia , no end checks , 204 gm . Measured about 8% on Timber check gauge . Weighed it , put it into a ziplock bag with an equal weighed amount of dessicant , left it on floor air register . High -27C forecast today , -30 tomorrow , -22 by Saturday , maybe -6 by Tuesday , so furnace getting a good workout , and the dessicant should stay warm . Will weigh check the wood weekly .

I’d be very interested in your results.


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1080Wayne

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Yes, curing the resin. You could be correct but I have not been able to do it that way yet. I may try with desiccant beads first. I need to get a triple beam scale. The cheap digital I have is not accurate enough.
I just use a cheap 4 inch pad , 5kg x 1g scale , plenty accurate for this kind of work . Just make sure the piece being weighed is centered on the scale . I only worry about 0.1 gm amounts when using pigments .
Desiccant may or may not work but is the path of least resistance so I’ll give it a shot before trying vacuum.
For stabilizing 0% is the goal. Only the best moisture meters are reliable below 5% so weight has to be used.
I need to do more research on desiccant to see if heat would be needed or helpful.


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Agree 0% is the goal , but question the absolute necessity of it for effective stabilization , given that the vaccuum will boil out any remaining moisture , albeit prolonging the process a bit . I never double check my oven dried pieces to see if they are in fact bone dry .
 

PatrickR

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Desiccant may or may not work but is the path of least resistance so I’ll give it a shot before trying vacuum.
For stabilizing 0% is the goal. Only the best moisture meters are reliable below 5% so weight has to be used.
I need to do more research on desiccant to see if heat would be needed or helpful.


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After having some blanks in desiccant for a week this looks to be a slow process for my use. I’d guess its good for wet wood, removing gross amounts of water but to pull the last amounts of moisture out of dry wood its just too slow for me.
Next, on to the vacuum drying method.


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