Microwave drying - using paper bags?

[Phily]

I've done a fair bit of microwave drying - with mixed results (the best being a very nice blank with a charcoal interior!) though I'm starting to get better after sticking with "defrost". I read an article the other day on the long term seasoning of blanks using brown paper bags (or similar). The concept being that the timber dries from the outside first and thus starts to crack well before the inside has had chance to dry. By storing in a paper bag, moisture is trapped around the surface thus helping to equalise drying times thereby minimising cracked/wasted blanks.

This got me thinking - will the bag technique aid with microwave drying? I'm trying a piece as I write but wondered if anyone has had any long term experience trying this? If so, has it helped?

[Toymaker Len]

I have tried a few different styles of microwave drying and now think that the best is to wrap the piece in a teatowel or similar piece of cloth. You keep blasting it on full power for one or two minute intervals until the weight begins to drop, then down to one minute intervals until the weight evens out then leave wrapped in the cloth to cool off slowly. Just be really careful of hot steam blasts coming out of the piece as you unwrap it to weigh. I use welding gloves.

[Phily]

OK, so that sounds like the same principal as the paper bag but a towel perhaps keeps the moisture closer to the surface. I've been zapping for 5 mins on defrost due to the "ebonising" achieved on full. I'll give the short high blasts a go though. How long do you leave between the nukes?

[Toymaker Len]

No time at all between blasts. I just unwrap, weigh, take note rewrap and blast again. You are trying to get the temperature in the piece of wood over one hundred degrees so that most of the internal water blasts out as steam. I find that the weight steadily decreases then levels out. That is when you stop. If you keep going past this point the inside of the piece will shoot up to combustible temperatures and this is where you are getting the ebonising. First hint of that is the smell of smoke when you open the microwave. If you keep going then the whole piece will spontaneously combust and totally burn up your entire credibility and you will lose microwave right for ages.

[Phily]

Interesting, that's quite different to the approach I've been using - 5 on defrost then cool right down. Weigh and if the weight is still dropping, another 5 ...........

The Manchurian I've been playing with was only cut down 1 week ago so it is was very wet. Unfortunately the batteries on my scale went flat so I had to guess how dry it was. The last zap opened up a few cracks so I must have guessed wrong and over dried. Fortunately I have a stack of it so I'll give your approach a try with a fresh piece as soon as I get some new batteries.

Cheers
Phil

[Paul39]

I have an old cheap microwave that has a mechanical timer and only one setting.

I wrap my bowl blank in 3 - 5 sheets of newspaper and heat until I see steam on the window, shut it off and then let it sit as is until I come by the mw again. Repeat several times and set the still wrapped blank aside for a month or two.

When I have gotten in a hurry the blank usually cracks, sometimes it cracks anyway. Red Oak will almost always crack. I have also charred the edges of a piece or two.

I have 10 - 20 bowls in various states of completion so there is always something to turn. I also have a pile of dry off cuts that I bought.

[Phily]

Hi Paul, it seems that wrapping is the consensus (slow or quick drying) and I agree that patience is the key. I like your idea of zapping then leaving for several months - I've tended to do the opposite, but have lost a heap from cracking - though it can work with the right timbers. Attached is a photo of an Elm bowl I finished today. This was was cut green around 12 months ago and left in blocks to dry. I turned a blank last week and zapped on defrost - about 5 x 5min. I then let it rest for a few days before turning. It has been turned very finely so it will be interesting to see how it goes over the next few months.

[Paul39]

Phily,

That is beautiful. I think it will be fine.

Elm wood was valued for its interlocking grain, and consequent resistance to splitting, with significant uses in wagon wheel hubs, chair seats and coffins.

From: https://en.wikipedia.org/wiki/Elm

[Dalboy]

Someone did an experiment and recorded his readings using a microwave oven. Take a look HERE

[Phily]

Wow, some people really get caught up in the detail!! This thread spawned an aligned discussion under the Green Woodworking Forum. I've decided to run a bit of an experiment myself on very green Manchurian Pear with one piece bagged. I'm interested to see if there is any difference in the cracking (I'll also weigh and see if there is any affect on the drying time). I have to remember to buy the (little flat round annoying) batteries for my scales but hopefully will have some results to post later in the week.

Thanks for the feedback & info Paul. I love the spalting in this particular piece!

Cheers
Phil

[Phily]

Following a discussion in the green timber forum I ran an experiment to see which approach achieved the best outcomes. The details and results are provided at:
https://www.woodworkforums.com/f71/microwave-drying-test-im-wrapped-196042
in the green timber forum.
Cheers
Phil