Woodworking Talk banner
1 - 17 of 17 Posts

·
Registered
Joined
·
3,604 Posts
Discussion Starter · #1 ·
Man, I hate to keep asking these rudimentary questions, but at this stage, I have little opportunity to ask smart questions!

I've been reading about microwaving turning blanks to dry them. I can easily understand how microwaving will drive the moisture out, but won't the relatively rapid exit of moisture cause cracks and checking?
 

·
Senior Member
Joined
·
7,222 Posts
It can. It can also cause burning.

At my local woodturning club meeting of a few months ago, someone had a bowl on the "show-and-tell" table with an obvious burn mark in the middle of the bottom of the bowl.

When the person was called up to talk about the piece, he mentioned he was using a microwave to dry the bowl blank fast. He had smelled burning, but could not see anything.

It was only as he hollowed out the blank did he find the burn mark.

Microwave dries from the inside out, so it will be hotter in the middle.

If you do decide to try this, read all that you can. Several threads on the forum. Short period of microwaves, then longer period of rest, and repeat. I only tried using a microwave once, and this was to attempt to heat up a piece as a test of bending. I burned the towel used to provide the moisture for the test. It lost the moisture a lot faster than I expected. I was lucky, no flames and the smell of smoke dissipated before my wife returned home.
 

·
Registered
Joined
·
2,609 Posts
The most important thing is not to use the kitchen microwave.

I've dried a couple of rough-turned bowls using this method, it worked fine.

I let it get hot enough that I could see steam coming out of it. To my way of thinking, since steam heat is the method used to bend wood, this shouldn't be dangerous.

When the weight change (i.e. water driven out) from each nuking session gets smaller, I reduce the duration of the blast in an attempt to avoid burning.
 

·
Registered
Joined
·
3,257 Posts
I guess I'm a much more patient man. I never heat my wood all that hot. I start out at 30 second intervals and feel how hot it is. I increase the time until it's quite hot. Hot to hot to handle but quite warm. I let it cool almost completely between cycles. For a bowl 3/4" thick this can take a lot of cycles and the better part of a day. What I do is simply, whenever I think of it or come in the house for something I'll put the bowl in for 30 seconds to a minute, pull it out and go do something else. If I'm in the house all day I can dry a bowl pretty quickly. If I'm back and forth between the house and shop it might take a day a half to dry the bowl, maybe 15 cycles or more.
I have almost caught a bowl on fire so never never never leave it alone. Stay there watching it. What happened on my bowl was it had a sap pocket inside the wall of the bowl. Apparently this got hotter than the rest of the bowl and actually burned a hole in the side of the bowl. If I had not been standing there who knows what would have happened. It took the better part of a month to get the smell out of the microwave. Good thing I'm single.
 

·
Registered
Joined
·
763 Posts
It's not "rapid" loss of moisture that causes cracks, it's uneven loss of moisture. Microwaving, in theory, can remove water evenly throughout the wood so no tension builds up. I believe it is possible, and I have had some success with it, but the microwave is just a tool which still requires good technique to get consistent results.

I have burned the center of blanks and I have cracked a few too. 2 things I have learned are: let it cool down between cycles and wrap it in a paper bag or open plastic bag so the surface doesn't dry too quickly while cooling. Hopefully you can come up with some tips to share with us after you figure it all out.
 

·
Registered
Joined
·
1,048 Posts
I have successfully use the microwave to dry wood with the weighing method. I start out by applying anchorseal the ends. I place the wood in the microwave for 45sec at 50% power. Yes the wood gets warm but by my method I do not rush the process. I have an old microwave above my shop and every time I go upstairs I nuke the wood. I have an electronic scale and if I have more than one piece I number them. I also have a bathroom up there so at least two times a day the wood gets nuked. Remember also to set the wood on two culls to get air circulation around the wood after nuking. I also set the wood on a paper towel in the microwave.

I have used this method for calls, S & P Mills, and pen blanks most of my drying has been for burls. with burls I coat all sides with anchorseal. Actually during the nuking process the anchorseal boils/melts into the wood.
 

·
Registered
Joined
·
763 Posts
Bob Willing said:
I have successfully use the microwave to dry wood with the weighing method. I start out by applying anchorseal the ends. I place the wood in the microwave for 45sec at 50% power. Yes the wood gets warm but by my method I do not rush the process. I have an old microwave above my shop and every time I go upstairs I nuke the wood. I have an electronic scale and if I have more than one piece I number them. I also have a bathroom up there so at least two times a day the wood gets nuked. Remember also to set the wood on two culls to get air circulation around the wood after nuking. I also set the wood on a paper towel in the microwave.

I have used this method for calls, S & P Mills, and pen blanks most of my drying has been for burls. with burls I coat all sides with anchorseal. Actually during the nuking process the anchorseal boils/melts into the wood.
Thanks, Bob. Now I feel like I have a substandard shop since I don't have an upstairs with a kitchen and bathroom. Do you have a media room and pool table up there too? I'm pretty proud of my shop but my bathroom is a hackberry and hose bib.
 

·
Registered
Joined
·
95 Posts
Never dry the entire blank. First of all, it takes too much time. Second, the moisture must travel so far from the insides to get out, that the moisture gradient setup causes excessive checking/cracking. Third, the heat gets trapped and can/will cause burning/charring. Would rough the bowl, then MW dry it and re-turn.
 

·
Registered
Joined
·
380 Posts
I have also used a microwave to dry blanks. It really boils (pardon the pun) down to moisture content and the microwave absorbing qualities of the material. Microwaves do not cook things from the inside out. sometimes the inside of an item is hotter but this is due to the properties of the item. ie. Sometimes the middle has more water content or various other reasons. I can easily see how wood, with all its variations in density, grain, mineral content etc, could heat unevenly and experience thermal runaway causing burning on the inside. I have seen one of my blanks pop open while water was boiling out. (just an experiment).

I say all that to +1 on all the other advice. going slow usually gives better results.
 

·
Registered
Joined
·
1,048 Posts
Thanks, Bob. Now I feel like I have a substandard shop since I don't have an upstairs with a kitchen and bathroom. Do you have a media room and pool table up there too? I'm pretty proud of my shop but my bathroom is a hackberry and hose bib.
I guess I am lucky to have my island up here in the north. We built a gambrel roof garage in 2002 after we bought this property. The garage has an apartment above 16' X 24'. In 2005 we built our home which has an attached garage so now the original garage is my work shop. My wife and I lived in the apartment for 6 months while the house was being built.

Back to drying: when I dry a bowl I put it in a corn sack (the kind that corn is sold in for feeding deer) wrapped in news paper for about 3 weeks, but when it comes to small items the nuking is the best. One more methods is to set the anchorsealed piece by furnace for a couple of weeks.
 

·
Registered
Joined
·
95 Posts
Just so you guys know, and I apologize in advance if this sounds too teachy, but as a former engineer I feel compelled to explain how microwaves work.

Microwaves use a form of electromagnetic energy that is identical to all other forms of EM waves (like visible light, IR, and infrared), except that the wavelength are longer than infrared. The waves are similar to that used in radio, but are special in that they are in a range that causes harmonic oscillations in water molecules.

AS the water molecules (in whatever) get to vibrating wildly, they heat up. That heat is transferred to the surrounding material (whatever that might be). The longer you send the waves in, the hotter things get.

Microwave ovens are relatively low energy as compared to radar for instance, and will only work for a short distance from the source (inverse square law).

MW's actually enter the material being cooked a short distance, depending upon factors such as density and water content. Higher the water content, the short the distance they enter without being absorbed and converted to heat. In some things (a cup of water for instance) that are relatively narrow, they actually do cook from the center out. Same thing for ears of corn and chicken wings, etc... Something big and thick (like a roast, or a whole bowl blank), does not allow them to travel very far into the material before they are absorbed and converted to heat. So, for big thick things, they cook kinda' from the middle of the outside in AND out. The heat generated transfers both in and out from the point of absorption. The reason that it SEEMS like everything cooks from the inside out, is that for thicker items, the heat moves into the center and can't easily get back out. The heat near the surface can get out into the air much more readily. Hence, a microwaved roast will be much hotter in the center than the outside. When cooking with conventional heat form the outside (any fire or standard oven), the opposite is true, as the heat from the surface must get all the way in to heat the center.

Okay, lesson over.
 

·
Registered
Joined
·
380 Posts
Just so you guys know, and I apologize in advance if this sounds too teachy, but as a former engineer I feel compelled to explain how microwaves work.

Microwaves use a form of electromagnetic energy that is identical to all other forms of EM waves (like visible light, IR, and infrared), except that the wavelength are longer than infrared. The waves are similar to that used in radio, but are special in that they are in a range that causes harmonic oscillations in water molecules.

AS the water molecules (in whatever) get to vibrating wildly, they heat up. That heat is transferred to the surrounding material (whatever that might be). The longer you send the waves in, the hotter things get.

Microwave ovens are relatively low energy as compared to radar for instance, and will only work for a short distance from the source (inverse square law).

MW's actually enter the material being cooked a short distance, depending upon factors such as density and water content. Higher the water content, the short the distance they enter without being absorbed and converted to heat. In some things (a cup of water for instance) that are relatively narrow, they actually do cook from the center out. Same thing for ears of corn and chicken wings, etc... Something big and thick (like a roast, or a whole bowl blank), does not allow them to travel very far into the material before they are absorbed and converted to heat. So, for big thick things, they cook kinda' from the middle of the outside in AND out. The heat generated transfers both in and out from the point of absorption. The reason that it SEEMS like everything cooks from the inside out, is that for thicker items, the heat moves into the center and can't easily get back out. The heat near the surface can get out into the air much more readily. Hence, a microwaved roast will be much hotter in the center than the outside. When cooking with conventional heat form the outside (any fire or standard oven), the opposite is true, as the heat from the surface must get all the way in to heat the center.

Okay, lesson over.
Cool. I am not an engineer but I am a science junky. So is my previous post correct? As a general rule...microwaves do not cook from the inside out. It has much more to do with how the microwaves are absorbed, which molecules are agitated and the thermal qualities of the material.(correct?)

I apologize to those who have no interest in the science side of the OP's question.
 

·
Registered
Joined
·
557 Posts
PSDkevin

I do believe you are correct. Microwave excites the molecules throughout the item and on thicker pieces the heat is trapped inside causing it to keep "cooking". Trapped thermal energy has to do something.
 

·
Registered
Joined
·
3,604 Posts
Discussion Starter · #16 ·
I've got a pretty powerful microwave, so I'm taking baby steps. I'm "nuking" my peppermill blank for thirty seconds, let it sit for about 15 minutes, then give it another 30 seconds. I'm doing this once a night, no more. I rotate it 90° on it's long axis each time and the platter in the oven is spinning during the 30 second cycle. I hope that since I'm, going slow, I won't have any internal charring, but since it's a pepper mill, the innards get drilled out anyway. As soon as I get a scale, I'll start weighing it between nukings, but for now I know I'm a long way from dry.


I guess I'm lucky; my wife is undaunted by having a chunk of wood in the microwave.
 

·
Registered
Joined
·
95 Posts
Yes and No. Center will ALWAYS be hotter in microwaved things; in thin things, it's because the energy is absorbed more in the center since it can reach there before being captured by a water molecule; thicker things stay hotter in the center because the heat simply migrates there and can't get out as fast as the heat near the surface.

Guess it depends on your definition of "cooking." If you mean true cooking then YES. If you mean heating directly from the microwaves being absorbed then, No.

No real difference in end result or effect though, except for us geeks.
 
1 - 17 of 17 Posts
Top