First Experience of Microwave Drying

[PaulH]

A neighbour of mine cut down a large poplar tree and gave me a large portion of the trunk , which was approximately 8" in diameter. Having cut it into smaller logs, (also about 8" in length) and splitting each through the pith, these were then sealed with a 50:50 mix of water and PVA glue.

Being rather impatient , I wanted to get turning some of it so I decided to experiment a little and part turned a pot-pourri dish to about 0.5" thickness. The wood was still very green (only cut down about 1-2 weeks) and hence the moisture was so high that droplets of water could be seen coming out of the wood as it was turned! :shock: At this stage the part-turned dish weighed 225g. To try to avoid splitting, I initially put it under a pile of the wet shavings for a couple of days but on weighing it again it had only lost a couple of grams. My impatience set in again, so I decided to try microwave drying...

I put the dish in our (1000W) microwave for 30 seconds at 10% power. After this the dish was just warm enough to feel the heat in your hands but definitely not "hot". I weighted it immediately and was disappointed that it had lost very little weight. However, I left it to cool and after about an hour noticed that the weight had dropped by about 15 grams, implying that the effect of the heat was not felt immediately but over a period of time.

I repeated this process over a period of 3 days, leaving at least a couple of hours at least between each session in the microwave. On some occasions, I did put it in for 45 seconds (still at 10% power) and this seemed to work without any adverse effects. After the 3 days, the dish stabilised at around 110g and had gone slightly elliptical - the major axis was about 1-2mm longer than the minor axis. However, there was (just about) enough thickness in the walls to remount the dish and true it up. All in all, a very successful experiment!

So what would I do different next time?

1) Take pictures of each stage and record the process more accurately to allow for more accurate repetition in future . I tended to put it in the microwave just when I happened to be passing rather than at any set time intervals. Hence I don't really know the total number of microwave sessions, but would guess at around 15-20.

2) Leave the walls slightly thicker to leave more wood for final turning stage - the final dish has quite thin walls....

3) Be braver with the microwave and go for 45 seconds, particularly towards the end of the process when there’s less water in the wood for the microwaves to heat.

I've since passed the resulting pot-pourri dish with one of these lids on to my neighbour so can’t even post a picture but I can report they were very pleased with it!

I hope this has encouraged some of you to “have a go” in the microwave – just remember, it’s better to “under cook” than have a smell of burning wood!

Regards,
Paul.

 

[AndyT]

Paul - thanks for sharing that - I'd second what you say and add just this: when I last tried the same thing, I put the rough-turned bowl on a pad of kitchen towels, on the microwave turntable dish. That meant that I could see moisture that had been forced out, making the paper slightly damp, then use dry paper the next time.

Andy

 

[Hans]

Hi Paul,

A couple of years ago I started using a microwave for drying wood.
In the beginning I logged some data hoping it would help to improve my understanding of the method. I wrote a short article on the subject for a Dutch woodturning magazine. I don't expect this article to be very helpful in general and even less so for non Dutch speakers. With the article there is an Excel sheet with data of 19 cases. It contains used power and heating time, with tables of time, weight, loss of weight per 24 hours. Hardly any text, so probably no language problems for anyone that can use MS Excel.
It is on the website of my local turning club. Excel sheet
In general I have had excellent results. As a rough guide I use one minute with power in watts equal to the weight in grams for pieces up to 500 grams.
I have learned not to use the microwave on wood that is spalted or has a soft (rotten) core.
The only time I put olive in the microwave it started smelling and smoking right away.

Hope this helps,

Hans

p.s. At your point 3) I would tend to use less power when the mass decreases, so shorter times when most water has disappeared. Would like to hear of your results.

 

[JackL]

I've successfully used microwave for drying for a number of years now and generally give the wood 20 seconds on 50% power and repeating about 4 times per day - letting to wood go completely cold after each treatment and I also use kitchen towel both during microwaving and to sit the wood on afterwards.
Don't do what my brother-in-law did though - he put it in the microwave on full power for 10 minutes The local fire brigade got there quite quickly though

 

[PaulH]

Thanks for the responses - I like the idea of the kitchen towel, although in my experience little moisture was lost in the microwave itself (based on weight loss). Rather, it was when I took it out and (as I should have said above) put it on a cooling rack to allow free air circulation all round.

Perhaps this was due to me being over cautious in terms of the actual time in the microwave, but as I said above, better to "under cook"!

Paul.

 

[Losos]

Paul / Hans

Thanks for posting this info.

I have been toying with the idea of putting some fruitwood in the microwave since I am committed to doing a few pieces by Xnas and I want it to be completely dry