Microwaving wood to dry - Woodworking Talk - Woodworkers Forum
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post #1 of 12 Old 09-23-2020, 10:04 AM Thread Starter
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I’m nuking a 4” diameter by 5-1/2” long cylinder of holly to dry it. It’s from a live tree and it was very wet when fresh. When I was turning it to a cylinder, water was spraying off!

Anyway, I’ve been microwaving it for 2 minutes on 50% power twice a day for 8 days. The blank has gone from 1,018 grams to 750 grams. According to my calculations based on the density of dry holly, it should weigh about 690 grams when I’m done.

I’ve noticed as it gets dryer, the weigh loss gets progressively smaller. I’m down to about 3 grams of weigh loss per nuking session at this point.

I’ve got just a few small cracks on the ends. I think these will get removed by the final turning, but I certainly don’t want them to get bigger.

At this point, should I wax or paint the ends and let it lose the rest naturally? Or keep nuking? (Or paint and keep nuking?)

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post #2 of 12 Old 09-23-2020, 10:07 AM
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Let us know how it tastes when you are done cooking it.
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post #3 of 12 Old 09-23-2020, 10:13 AM
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same process (kinda) for freeze-drying animals for taxidermy.
the first week, there is a substantial weight loss. then measured
only ounces and grams for the next few weeks until there is no
noticeable weight loss at all.
I guess with wood, you would get to the point of very little to no weight loss
and the moisture content is within your expectations.
looking forward to seeing how this process works for you.
it is an often asked question here .
(a photo journal would be nice to see).

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post #4 of 12 Old 09-23-2020, 12:13 PM Thread Starter
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There’s not a lot to see, but here’s a picture of the end of the piece. You can see the small cracks. You’ll also notice where I’ve recorded the weight after each weigh-in. Maybe I just want to stop at this point because I’ve run out of space to record the weight! 🙂

I re-position it when microwaving to ensure even doneness; that’s why you see the days of the week on the end. The other end and sides have the other five days.
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post #5 of 12 Old 09-23-2020, 12:50 PM
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Dumb, unrelated question:
When I was a kid, I was told not to run the microwave empty. Whatever is in the microwave must have moisture. It is the water molecules in your food that the microwave oven heats up.

We were taught that if you were heating something very dry, put a cup of water in the microwave oven to absorb the excess energy and prevent it from being reflected back into the microwave source. Otherwise, you may damage the klystron tube or whatever they are using these days.

Is that still true? When the wood reaches a certain dryness, are you risking damage to your microwave oven? Adding a cup of water doesn't make sense - it might spread warm moist air over your wood. Perhaps a bunch of ice cubes in a covered glass container might work.

Another thought:
We have a dehydrator for making jerky and drying fruits and vegetables. It blows warm dry air over the food, which is placed on slotted racks. If you remove some of the racks, your wood may fit in our dehydrator. Would that work for the final stages of drying, or would it cause too many cracks?
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post #6 of 12 Old 09-23-2020, 01:08 PM Thread Starter
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TA - Your point is well taken. I think running a microwave empty is still ill-advised. That’s among the reasons I’m doing short durations and 1/2 power and wondering if I should keep going when the moisture reduction has gotten so low.

I’ve also read that you you keep nuking after the wood has dried, it will char in the center.

Maybe we’ve answered the question!


That said, I see no reason not to have a cup of water in the microwave with the wood. The microwaves should be driving the moisture out of the center of the wood whereas steam from the water in the cup would only get on the surface where it could quickly dry. Heck, I might even be able to make the case that it’s equalizing the expansion!

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post #7 of 12 Old 09-23-2020, 05:41 PM
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I'm not an expert but just trying to be logical.

A kiln drying operation tends to put the wood into a very low relative humidity environment and evaporate the moisture out of the wood. In a microwave oven I would think that you are boiling the moisture out of the wood.

Adverse effects? I don't know.

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post #8 of 12 Old 09-23-2020, 09:25 PM Thread Starter
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Quote:
Originally Posted by NoThankyou View Post
I'm not an expert but just trying to be logical.

A kiln drying operation tends to put the wood into a very low relative humidity environment and evaporate the moisture out of the wood. In a microwave oven I would think that you are boiling the moisture out of the wood.

Adverse effects? I don't know.

The theory is that oven or air drying dries the wood from the outside and since the inside is still wet, the outside shrinks too fast and cracks. In the microwave, the center is being heated and the moisture is driven from the inside out, supposedly maintaining equilibrium as it exits. That's the theory. Who knows what really works. So far, my piece has lost most of its moisture with only minimal cracking. Who knows what really happens...
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post #9 of 12 Old 09-23-2020, 10:30 PM
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Logical, logical, logical.

But "Who knows what really happens..."

LOL!

Rich
Just a dumb old paper boy from Brooklyn, NY
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post #10 of 12 Old 09-25-2020, 03:49 PM
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post #11 of 12 Old 09-25-2020, 07:49 PM
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The "microwave ovens cook from the inside out" concept is sorta' true, and sorta' not true. With food, the microwave energy in a microwave oven does not penetrate that deep. It penetrates down to ~1-1/2 inches. That's why some foods come out of the microwave oven hot on the outside but with a cold center. That's also why instructions say to let your food rest in the microwave oven for a minute before serving - to allow time for convection to distribute the heat more evenly throughout your food.

If wood is similar to food, then the center of the log is not getting much of the direct microwave energy. It is not being heated "from the center out." It is more like, "from the outer layer in", where that outer layer is about 1-1/2 inches thick.

A regular oven heats from the outside in, no doubt about that.
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post #12 of 12 Old 09-26-2020, 10:06 AM Thread Starter
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Quote:
Originally Posted by Tool Agnostic View Post
The "microwave ovens cook from the inside out" concept is sorta' true, and sorta' not true. With food, the microwave energy in a microwave oven does not penetrate that deep. It penetrates down to ~1-1/2 inches. That's why some foods come out of the microwave oven hot on the outside but with a cold center. That's also why instructions say to let your food rest in the microwave oven for a minute before serving - to allow time for convection to distribute the heat more evenly throughout your food.

If wood is similar to food, then the center of the log is not getting much of the direct microwave energy. It is not being heated "from the center out." It is more like, "from the outer layer in", where that outer layer is about 1-1/2 inches thick.

A regular oven heats from the outside in, no doubt about that.
Makes sense. My cylinder is 4” diameter, so I suppose the waves are making it almost to the middle. I re-positioned the piece each time in an effort to change the focal point.

I’ve reached the point where I’m only losing a couple of grams per nuke, so I’m going to let it sit for a while and see what happens.

If I do microwave some more, I’ll be sure to let it rest before serving! 🙂
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Last edited by Quickstep; 09-26-2020 at 10:09 AM.
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