USING THE MICROWAVE TO DRY
GREEN ROUGH TURNED WOOD
I used the microwave to dry the 11" diameter by 5 1/2 high box elder bowl to the right to speed up the drying process a little.
I cut a tree damaged by the "Ice Storm of '98" one day. I rough turned and microwaved the bowl the next day.
After giving the bowl a few days to reach EMC (equilibrium moisture content), I finished turning the bowl a few days later. (The microwave can over dry or under dry the wood; I find it prudent to wait a few days to allow the turning to adjust to the relative humidity.)
To use the microwave to dry rough turned stock (It can also dry finished turnings using the same method), I use the following procedure:
(1) After roughing to 3/4", I put the bowl into the microwave on defrost for 2 minutes. (a brown paper grocery bag will protect the microwave or some wrap with paper towels)
(2) I leave it in the microwave (with the microwave off) for at least 10 to 30 minutes. This allows the outside to cool down in a high humidity area. This will cut down on checking from the outside losing moisture too fast before the inside has lost enough moisture. Then take it out and let it completely cool.
(3) After 4 or 5 cycles on defrost, I often proceed to high for as many cycles as it takes. (I recommend that you stay on defrost; some woods will check at the high setting. At the first sign of checking, back off on the power level.) Please, never leave the room while the microwave is drying wood. A mistake in time or power can cause the wood to start smoking or worse. Even after the microwave is off, the wood should be watched for at least 1/2 hour. It has been reported that smoldering can start on the inside of an over dried piece and could result in delayed ignition. You can tell when it is dry just by feeling and even the smell is different. Also it no longer beads up moisture. If you use brown paper grocery bags or paper towels, when the bags an/or paper towels remain dry, you know that you are done. You can also weigh the turning after each cycle in the microwave. A "postal scale" is good for this method as it can detect slight loss of weight. When the turning no longer loses weight, it is dry. This is the best method of all.
(4) Then I usually let it sit for 3 or 4 days to reach EMC (equilibrium moisture content) before finish turning and finishing.
Every species of wood is a little different and reacts a little differently. If in doubt, use lower settings and shorter periods of time. This is a guide line only and trial and error is ultimately the best teacher if you wish to push the edge. However, it is probably best to use the 2 minute (or shorter time), defrost setting, half hour between cycles as a safe starting point. You'll get in less trouble if you also don't exceed those settings!
Note: The microwave is also good for getting rid of insects. I turn quite a bit of wormy red maple. A minute or two on defrost will kill powder post beetles or any other insects. (It's actually the rapid vibrations not the heat that kills the critters.)
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