Author Topic: freeze dried (Read 2511 times)

etat · « **on:** April 18, 2004, 07:17:37 pm »

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 · « **Reply #1 on:** April 18, 2004, 07:58:12 pm »

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 · « **Reply #2 on:** April 19, 2004, 05:42:06 am »

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 · « **Reply #3 on:** April 19, 2004, 07:36:15 am »

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 · « **Reply #4 on:** April 19, 2004, 12:53:19 pm »

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.

Ianab · « **Reply #5 on:** April 21, 2004, 04:58:30 am »

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??

old3dogg · « **Reply #6 on:** April 21, 2004, 05:20:51 pm »

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

Den Socling · « **Reply #7 on:** April 21, 2004, 05:54:35 pm »

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 · « **Reply #8 on:** April 21, 2004, 05:58:56 pm »

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 · « **Reply #9 on:** April 21, 2004, 06:06:17 pm »

Neither do I.

old3dogg · « **Reply #10 on:** April 21, 2004, 06:11:00 pm »

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

sawmillsi · « **Reply #11 on:** April 21, 2004, 08:10:00 pm »

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 · « **Reply #12 on:** April 21, 2004, 08:25:28 pm »

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 · « **Reply #13 on:** April 21, 2004, 08:29:54 pm »

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

old3dogg · « **Reply #14 on:** April 21, 2004, 08:38:23 pm »

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 · « **Reply #15 on:** April 21, 2004, 08:53:23 pm »

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 · « **Reply #16 on:** April 21, 2004, 09:11:51 pm »

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 · « **Reply #17 on:** April 22, 2004, 02:27:26 am »

OK.. there is obviously something in this.

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 · « **Reply #18 on:** April 22, 2004, 05:21:05 am »

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 · « **Reply #19 on:** April 22, 2004, 06:47:04 am »

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

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Author Topic: freeze dried (Read 2511 times)