Microwave drying advice

[Roger Wiegand]

We have a club project that involves making some vases using wood from a local arboretum that has helped us out a lot over the years. The vases need a glass tube insert and most of the wood available is green (cut a couple months ago with the ends sealed), so will warp as it dries. If it moves too much there's a possibility of breaking the tubes,

I've been experimenting with microwave drying of blanks and feel as though I am making very little progress. Advice on technique would be appreciated! I've never tried this before, having never had to work to a deadline with green wood before.

The blanks are about 3" x 6-8" honey locust. I've turned them round and drilled an undersized 1-1/8" hole 5-6" deep into them. I started, based on reading interweb advice, by microwaving 20 seconds on high in a 900 watt MW and then allowing the blank to cool for 30 minutes. The last couple cycles I've upped that to 30 and then 40 seconds. I'm 8 cycles in and the blanks have lost no more than about 20 g from their initial 500-700 g weight, about a 4% loss. They actually have lost more sitting on the counter overnight than in the MW cycles. My moisture meter said the log started at about 19.7% MC (I don't have a pin meter, so can't measure the now round blanks). Measuring with a caliper there's insignificant out of roundness detectable so far.

Are my expectations of greater loss per cycle out of line? The blanks are barely warm after microwaving. How much MW time is reasonable-- I really don't want to blow them up, but I'd really like to get them dry enough ASAP.

 

[hockenbery]

Are my expectations of greater loss per cycle out of line?

. Suggested reading Tony Hillerman- Seldom Dissapointed.
Got to attend a talk by Hillerman - Great evening.

My experience is with small bowls longest grain in the bowl is on the bottom or the side grain edge. For a 10” bowl 3” plus or minus.
You have 6-8” of long grain. You have a couple things in your favor.

Honey locust won’t shrink much Radial: 4.2%, Tangential: 6.6%,
Those shrinkages are from dead wet to oven dry. You will have a bit less
Using the full shrinkage your 1 1/8” hole will be a dry dimension of something like 1.07775 x 1.05075
These numbers are pretty close if the top has paralell grains lines and not too far off for other sections of logs.
A shrinkage of 0.04725 x 0.07425 this a little less than a 1/16/and a little more than a 1/16

If you have an 1/8” lip on your tubes and do an 1/8” oversized hole the lip will hang on the top of the hole and you have an 1/8” left for shrinkage. When you think it is dry you should be ok.

My bowl formula:
I used to teach classes that met once a week for 3 hours. One of the things we did was to rough turn a 10” diameter bowl to about an inch thick. The students would dry it during the 6 off days in the microwave.
3 minutes at 40% power, at least 30 minutes rest. Repeat. 8-10:cycles. It is dry.
Then the next class they had a dry bowl.

I use to be able to get most of the drying done while watching a 1 hour NFL game.

 

[Don Wattenhofer]

Don't forget the seasonal changes in moisture will cause expansion and contraction and it does not take much to break a glass tube. How about making a tube from copper water tubing.

 

[Doug Freeman]

How about making a tube from copper water tubing.

Doh! (Homer Simpson) Great idea.

When I have done vase for glass inserts, I wet turn the OD, drill a ~1/4” undersize hole, and hollow the ID (leaving a 1/2” or so at the bottom to hold the glass). The hollowing provides faster drying with less stress. Dry the wood, remount, turn the top and bottom glass holding areas to size - intention is for the glass to be removable, so it will wobble just a bit, supported by the bottom of the hole. I tell recipients to remove the glass to fill with water, so the wood doesn’t get wet.

 

[Dean Center]

It's been quite a while since I've nuked any wood blanks, but I did not have any trouble getting them dry when I did. They were 1.25-1.5" in diameter, and I nuked for 30-100 seconds wrapped in brown paper or newspaper, starting at short time and working up. I wasn't very patient when I was younger, and I'd only wait 15-20 minutes in between nuking episodes. The paper picked up a whole lot of moisture and needed to be changed periodically. They got pretty hot when they got close to dry and a couple of times, my impatience would create some scorching on the blanks and the paper might smoke a bit, but no real fires.

With the hole drilled in the center of the blank, your wood thickness would be a bit under 1", so I'd think you could get this to work. If your paper isn't picking up much moisture, try a little longer run. If they come out too hot to handle, shorten the time a little. (As noted, IME wet blanks can handle more time; dry blanks are riskier) As long as you're watching while it's getting nuked and can push the stop button if things start smoking, you really can't get into much trouble. (Unless the blanks are stinky when heated, in which case you'll be in the cat house for a month when Mrs. Wiegand finds out.)

 

[Roger Wiegand]

Design of this project was not in my hands. Sounds like I should be safe nuking them more aggressively, and that maybe it doesn't matter so much for this species.

 

[hockenbery]

Design of this project was not in my hands. Sounds like I should be safe nuking them more aggressively, and that maybe it doesn't matter so much for this species.

Do be sure you hole is larger than the tube. +1/8 should work

Sounds like fun project for the club.

 

[Pat Cogdal]

I don't know when your deadline is but I have been experimenting with desiccant drying with blanks about that size. I put a bunch in an over sized Ziplock filled desiccant and let them sit for a week or so. The blanks were spalted maple and were at about 16% going in, 6% when i pulled then out and bounced back to 7-8% and stable weight sitting in my studio. I bought an 8lb bottle of desiccant from Amazon for about $35 which can be recharged in the oven when the color changes and can be reused. I like that it's a set it and forget it method.

 

[Michael Nathal]

I microwave rough turned hollow forms that are very wet for up to 5 minutes at a time at 100% power. I start at 3 minutes and take out the piece. I am shooting for steam coming out of the entry hole. If it is not yet steaming, I put it back in for another 2 minutes. Once it is steaming, I pull it out and let it cool on a wire rack with the entry hole facing up, you can feel the very hot moist air coming up out of the hole like a chimney. I let it cool for 30-60 minutes and weigh it. Then repeat. You get a feel for how much microwaving time is needed based on the weight loss. At first, on a 1000 gram piece, I might get 100-120 grams loss on each iteration. After 2 or 3 iterations, the weight loss may be only 50 grams or less, and I reduce the microwave times accordingly.

I feel the steam is desirable as it reduces stress in the wood, reducing cracking. Somewhat like boiling.

 

[Alan Weinberg]

I microwave rough turned hollow forms that are very wet for up to 5 minutes at a time at 100% power. I start at 3 minutes and take out the piece. I am shooting for steam coming out of the entry hole. If it is not yet steaming, I put it back in for another 2 minutes. Once it is steaming, I pull it out and let it cool on a wire rack with the entry hole facing up, you can feel the very hot moist air coming up out of the hole like a chimney. I let it cool for 30-60 minutes and weigh it. Then repeat. You get a feel for how much microwaving time is needed based on the weight loss. At first, on a 1000 gram piece, I might get 100-120 grams loss on each iteration. After 2 or 3 iterations, the weight loss may be only 50 grams or less, and I reduce the microwave times accordingly.

I feel the steam is desirable as it reduces stress in the wood, reducing cracking. Somewhat like boiling.

I’ve only done 2-2.5 minutes at 100% so you are much braver than I. You’ve given me the courage to try longer times.

 

[Kurt Manley]

I’ve only done 2-2.5 minutes at 100% so you are much braver than I. You’ve given me the courage to try longer times.

I dabble with some microwave drying, not enough to expertly comment but the length of time that is appropriate definitely depends on the microwave itself. 5 minutes in my old cheapo microwave that I have in the garage is probably equivalent to 1 minute in the brand new microwave in our kitchen.

 

[Mark Jundanian]

I don't know when your deadline is but I have been experimenting with desiccant drying with blanks about that size. I put a bunch in an over sized Ziplock filled desiccant and let them sit for a week or so. The blanks were spalted maple and were at about 16% going in, 6% when i pulled then out and bounced back to 7-8% and stable weight sitting in my studio. I bought an 8lb bottle of desiccant from Amazon for about $35 which can be recharged in the oven when the color changes and can be reused. I like that it's a set it and forget it method.

Just to confirm, Pat, your drying the blank, not a rough turned piece?
It so, how large a blank have you done, and what types of wood have you dryed this way?

 

[Pat Cogdal]

Just to confirm, Pat, your drying the blank, not a rough turned piece?
It so, how large a blank have you done, and what types of wood have you dryed this way?

So far just some small pieces of spalted maple mostly. 3x3 -3x5 that I use for bottle stoppers and such. But just blanks so s far.