micro wave drying technique
Ok here is my dilemma
xmas is coming and i have over 50 roughed out bowls that is a conservative number. these bowls have been roughed out starting back in oct.
none are dry enough to finish turn. I need a few suggestions on how to speed up the process, i.e. micro wave drying, alcohol drying etc.
can anyone assist?
allen
Microwave temperatures and charring experiment
I have been successfully microwaving some of my bowls. I turn my bowls to final thickness and shape and sand if they aren't too wet. I let them warp as they want as I microwave them. I normally use the bowl in a plastic bag method for 30 - 60 seconds per cycle at full power. I take the hot bag with the bowl out, dry out the bag and let the bowl cool a little and dry slightly before weighing it. When I first start I can lose 5 - 10 grams per cycle. The bowl is still warm when I put it back into the bag and re-zap if for the next round. As you approach total dryness the weight losses drop off and the bowl will start to distort more. I try to stop before I get it too dry. If I go too far, after the bowls are cool they will actually pick up moisture from the air and the weight will increase. Over drying the bowl can also really stress the wood.
My hypothesis is that once the bowl is warm or hot and it is losing water that the heat helps to relieve the stress in the wood. For that reason I don't let the bowl cool down to room temperature. If I need to leave it overnight then I leave it in the bag and let the water equilibrate. In a recent case with a poplar bowl that weighted about 400 grams, it took about 15 cycles to get it down to its final 210 gram weight.
All of the comments above about charring the inside of the wood prompted me to do an experiment. I took a cutoff from one of my apple bowl blanks and zapped it for 30 second intervals at full power. This wood was about 6 x 2 x 2 and weighed 320 grams so it was much thicker than any bowl I might make. I drilled a hole for a metal thermometer that I inserted after taking the block out of the microwave. Here are the results (sorry that the software pushes all the columns together):
Time Weight (grams) Temperature (in Fahrenheit)
0 320.2 68
30 319.7 140
60 316.2 205
90 310.7 210
120 304.0 217
150 298.2 226
180 294.4 244
210 292.0 257
240 289.0 270
270 287.0 288
300 285.4 302
3600 287.2 96
I was pretty surprised that after four cycles that the temperature was above the boiling point and that it kept raising with each 30 second cycle. I stopped after 10 cycles and let the block cool. After an hour the block had gained back almost 2 grams of water so I had gone past the equilibrium point. Since my basement is about 50% relative humidity at 65 degrees, my Lee Valley moisture content chart says at equilibrium, my wood should be about 9.3% water. Back calculating, this block started out at almost 22% water.
When I cut the block in half I didn't observe any charring on the inside. There was a hint of something burning after the tenth cycle but nothing very strong. My guess is that if you kept heating it up that in a couple of more cycles there could be some charring. 300 degrees is pretty warm but charcoal production must start somewhere above this temperature.
My recommendation is to use full power for short times and measure the weight loss as you go. You will be able to tell by how the bowl feels as you approach the endpoint. The largest bowl that I have done like this was about 15 inches by 5 inches tall and it only took 45 seconds to inflate the bag. I wouldn't go longer than a minute.
There is one other thing that I feel that I must disclose. Some of the woods that have a disagreeable smell when turned will have a very strong (and worse) smell when microwaved. I more or less got used to the smell of the swamp poplar but my wife didn't. This tree made some very pretty bowls but the logs aren't allowed in the house because of the smell.
Cheers and happy holidays!
David