freeze dried

[etat]

Saw elsewhere, and mentioned here a bit about putting wood in the freezer to dry.  Sparked my curisioty and just thought I'd ask about it.

[Brian_Bailey]

ck,  
The process is called sublimation.
The water goes from a solid (ice) to a gas without going thru the liquid state.
I always thought that it was a rather slow process and not as fast as suggested on the other forum.
But, who knows.  My wife temporarily put a halt to any experiments with the freezer in the house.

[Norm]

I wondered how much moisture we would lose in some white oak we sawed last fall and stickered in an enclosed shed through the winter. Was hoping it would freeze dry some out but checking with a moisture meter showed almost no moisture loss through the winter.

[beenthere]

Norm
Do you think it is below 30% moisture content?  If not, will a moisture meter give an accurate reading for anything above 30%?  

Back some 30 yrs ago, I'd heard of research to dry (or attempting to) walnut gun stock blanks by freezing. Seems it was just pre-freezing before conventional drying, trying to find a way to shorten the kiln schedule to less than 7 months.  Runs in my mind it was done in Carbondale, IL  (can't find any information on it yet).

[Norm]

If I remember right I checked it in late December and then a couple of times throughout winter time. It did lose a few points of moisture level but not much. Nice thing about cutting white oak in the fall is it loses moisture very slowly in our enclosed shed.

Your right in that it won't measure accurately above 30% beenthere, I usually don't pay any attention to the meter until under that. My experience in drying wood is best described as advanced rookie... no make that unadvanced rookie. :D

[Ianab]

My understanding of freeze drying is that it involves a vacumn, well at least thats how freeze dried food is made. Just leaving something in a freezer, well that old steak thats been sitting in the back for 12 months is a bit dry around the edges, but it's no where near kiln dried yet. I believe water sublimates from ice to vapour much more readily at low air pressure?

Anyone keen to build a refrigerated vacumn kiln?? :P

[old3dogg]

I just dont understand how you would move frozen vapors away from the wood ???

[Den Socling]

Mike,

The vapor is not frozen. Even when the water molecules are frozen, the ones that are close to the atmosphere can occasionally 'bounce' in the direction that releases them from the bonds that hold them together. That molecule is then in vapor state.

In a vacuum, it takes less of a 'bounce' so more break free.

Den

[old3dogg]

Okay.I guess I understand that part Den.
Tell me how you would move frozen water from the center of the wood.
I just cant believe this would work to dry wood.
Mike.

[Den Socling]

Neither do I.

[old3dogg]

Put it to rest then!?
Lets talk vacuum drying with heat and all of that good stuff 8)

[sawmillsi]

hi guys,

instead of just putting it down, why don't you try the method (i posted it on the other forum).

we have and it works great.

why does it work - i don't know ( and i'm studying a bach. applied science - forestry), but just cause i don't know how it works dosn't stop it from working.

[old3dogg]

Okay.If it works than lets give it a try.
How do we go about it then?
Hey!I use to dry wood in an RF kiln so I will try anything once.
This stuff interest me to know end!

[Buzz-sawyer]

water is strange stuf...has ayone ever conducted the physics 101 experiment:
Place water in vacum

as pressure approaches absolute vacum the water BOILs and FREEZES at the same time a sight to see!

So in in a vacum freezer .....
the change of states is affected accordingly  8) 8) 8)

[old3dogg]

Buzz.
An absolute vacuum?
Do you mean like 0 Torr?
I have seen sap boil out of hard maple at 35 torr with no heat.
What kind of chamber does it take to pull an absolute vacuum?

[Brian_Bailey]

sawmillsi,  

I'm all ears to hear about the method your using.

I'm looking for a quicker way of drying carving / turning blocks.

Thanks  

[Buzz-sawyer]

Old3dogg
 the term is APPROACHES ...the trick is the magic and very near temperatures of freezing and boiling...but the atmospheric pressure is reduced changing the boiling temp (vaporization)212f and the freezing solid state temp. 32f
....the two temps got closer as we approached vacume magic was when the two intersected.....dont recall specific atmospheres of pressure to be sure it was not absolute! (as in outer space)

[Ianab]

OK.. there is obviously something in this. :P
Theory... the freezing disrupts the cell structure, as the ice forms it expands slightly, maybe opening up the cell wall and allowing the water to escape faster? I don't know if this is correct, but it might explain what is going on... ???
As this seems to work at normal air pressure I might just have to get a couple of sample blocks in the freezer and see what happens.

[Den Socling]

To get water to boil and freeze at the same time, the pressure around the water needs to be around 4.6 Torr.

[Brian_Bailey]

Below is a link that explains the process of freeze drying.

Looks like a complicated process that would need tight controls to achieve consistent results  .

http://home.howstuffworks.com/freeze-drying2.htm

[old3dogg]

Thanks Brian.
It all sounds interesting but it would be a slow process in drying wood.There is a lot of water in wood.
Mike.

[Brian_Bailey]

Last fall I sawed out some 12 & 16/4 x RW butternut for carving.
I stickered them O/S under cover.
I live in WNY so since Dec. the outdoors temps. have been alternating between a freezer and a refridgerator.

This AM I pulled a cant out of the stack and cut it in half.
This is what I found after a good 6 mos of air drying.
The shell tested 14%mc and the core was 34% or at least that is what my meter said.
Just looking at the core shows it to wet for carving..



I'm not trying to be confrontational, I just don't see how putting a chunk of wood in the freezer for a couple of days and then in the fridge for a week will give you dry wood.
If it can be done, I would certainly like to know the method  :D :D.

[Den Socling]

Brian,

It can be done with a very strong vacuum. The atmosphere over your head is putting the pressure equivalent to a 760 mm column of mercury on your body. What was it that I said this morning? 4.6 Torr? That means that you remove all of the pressure except for 4.6 mm. If we did that to you, you would be a messy cloud drifting off into the sunset.  

Den

[Buzz-sawyer]

Old3dogg
We used a small4-6inch glass dome, on a rubbere matt seal.....using a small, loud, annoying vac. pump...

[Brian_Bailey]

Den,  

I've long been accused of being the source of smelly clouds drifting along. I've always denied any involvement, but me thinks that it's a fermentation /pressure release process :D :D.

But seriously,  I have a good market for carving wood and lately there has been a lot of interest for turning wood that is dry enough that it won't crack and distort before you can get it off the lathe.
If I can substantially cut the long wait between wet wood and dry, maybe my business could turn a profit  :D :D.

When someone posts a method for quick drying, I'm very interested in hearing about their method.

But on the other hand, I'm not interested in spending dollars to save nickles  .

[old3dogg]

So do we pull a vacuum,freeze the wood,let it thaw,freeze the wood,let it thaw,etc,etc....?
Im only asking because I dont know.

[etat]

Things I thought I knew.  Water turns to steam if you get it hot enough.  Water turns to ice if you get it cold enough.  When it's steam it goes into the air.  When it freezes it gets cold, expands, and turns hard.  Get enough water in the atmosphere it turns to rain. Mix hot, and cold, and atmospheric conditions just right it might turn into a storm. Green wood has water locked inside.  Getting it out is tricky without busting the wood. I didn't know this was a complicated question when I asked it, but I still can't get it in my head how freezing wood will help get the water out, unless it includes a vacumn as stated.  I did not know that water could be frozen and boiled at the same time. I do find that very interesting.

[Brian_Bailey]

Here is how I see it.  

The water freezes and becomes a solid (ice).

When the conditions are right, the ice (frozen water) sublimes to a gas (water vapor) and then evaporates into the atmosphere.

Hence, where the water once was, it is no longer and now dry  ;D .

When the water leaves the wood it leaves as a gas, not as a solid or liquid.
So, there shouldn't be any damage to the wood.

The question is, is it practicle to dry wood using this method?

[Den Socling]

I think there is something else. When there isn't much water left and it's trapped in places where there are few ways out, you normally raise the temperature to increase the kinetic energy of the molecules to improve their chance of bouncing out. At 32'F, their butts are probably dragging.  :D

[Brian_Bailey]

Den,  

The water is now a gas.
Wouldn't it diffuse out of the wood quite readily without any additional heat as long as the sublimation conditions were maintained?

[Den Socling]

Brian,

This is getting as bad as some of the discussions between Wim and me at the PCS forum!  :D He's the physicist who's always spoutin' gas laws (unlike most of us who only spout gas  :D) but I would guess that temperature adds energy to gas molecules as well as liquid and solid. In fact, that's why warm air rises.

Did you know that warm, moist air rises in a vacuum chamber. This is one that the physicist have explained differently. The water vapor is suppose to spread evenly.

but,  :P I'm no 'perfessor'.

Den

[Brian_Bailey]

Thanks for the info Den  .

I'm no physicist either  :D.

I'm just trying to get my hands on a simple / effective drying system for those thick chucks of wood I have.

Your vac. system seems to be the way I'll eventually head. But for now, its design is a little to complex for my abilities.
I eagerly await the results of others  ;).

I have spent quite a bit of time the past couple of days reading about this freeze dry stuff.

One thing that became apparent was the lack of info on drying wood with this method.

As a lay person this indicates to me that this isn't a viable drying method to persue.

So for now I'll just have to let father time just click along  .

I was wondering if it would be possible to magnetize the water in the wood some how. Then just place a large magnet at the end of the boards and pull all the water out ?  :D :D :D

 

[Den Socling]

Actually, the magnetic property of water is what enables RF kilns and microwave ovens. But instead of sucking it out with a magnetic field, you swing the molecules back and forth fast enough to heat them with friction.

[jimF]

In all drying, no matter at what temperature, what is happening is that the vapore in the air is applying a pressure on the water in the wood.  If the "vaspor pressure" of the water in the wood is greater than the vapor pressure in the air the water in the wood overcomes the air vapor pressure and escapes into the air.  By adding heat you give the water in the wood enough energy to overcome the air vapor pressure.  In vacuum/freeze drying instead of giveing the water in the wood more energy you are reducing the vapor pressure in the air so the water in the wood does not have such a large  vapor pressure to overcome and is able to escape in the air.

[Brian_Bailey]

The theory behind all these different drying methods is certainly easy to grasp  ???.

Now all we have to do is make the mechanical apparatus to accomplish this easy to grasp for a guy like me with limitied funds .

I'll wager good money that there would be a strong market for a small 100 / 200 bf vac kiln designed just to dry thick chucks of wood for carving / turning.