Microwaving wood to dry it

[PhillH]

Anyone offer any advice regarding this as a technique ?

I'm thinking things like 8 x 2 inch cedar bowl blanks.



Guess who has a load of cedar coming hopefully, and I can't be bothered to wait

 

[Neil]

My advice, (I've been drying wood commercially for twenty years) - dont bother, its a waste of time for the water that you drive off. If its freshly felled the rh is around 35%. The level will fall rapidly to around 23-25%, the level at which the unbound water has dried off, and then the slow drop will occur over a number of months and years to around 15-17%, thereafter it can become drier but really only through a final kiln dry.

Wood becomes dry by evaporation from the surface and then moisture level stabilistion occurs slowly, water then evaporates slowly from the surface and then stabilisation occurs, water slowly leaching through a material that is anywhere from 450Kg/m3 to upwards of 750Kg/m3.

Microwaves make it hot, they steam a bit, feel wet on the surface and then the surface dries, but its still sopping wet in the middle. Some people swear by it as a method, but frankly I think they are wasting their time.

If you have a dimensionally consistent blank, say 8" x 2", weigh it first. I take it that this is freshly felled? The average dried weight of Cedar of Lebanon is 520 kg/m3 This means that the blank you have should weigh, dried weight, 8 inches (0.2032m) squared x 2 inches (0.0508m). In m3, that is 0.0020975 cubic metres, times 520 Kg/m3, equals a dried weight of the blank of 1.1KG, If you weigh it as it is, it will probably be around 1.5 kg. Target weight to turn it should be around 1.2kg or less.

0.3 of a kg of water is near enough half a pint of water, and if that can be done successfully in a microwave I'm the pope.

Would be interested in the statistics if you chose to give it a go, weigh it first, sticky it in the micky, time it and weigh it and record it and see what happens.

As you may have gathered, I am not a believer, microwaves aren't that clever for cooking and as for wood drying they fail even more spectacularly.

(Figures here are for a 8 x 8 x 2 square blank, if its round let me know and ill redo the calcs)

 

[paulm]

All I know is if you microwave it... do it when she's out

 

[PhillH]

Neil,

Thank you for a very erudite answer, much appreciated.

To answer a couple of your questions it was felled about 2 months back and has been stacked outside since in 3 - 4 foot logs, so in my humble opinion it's wet.

I was thinking about making some 8 inch (ish) x 2 inch blanks and microing them to dry. I may not bother after your reply, just need to get a patience injection it seems.

I'll have a think.

Thanks again for taking the time to reply as you did.

@ Paulm Point taken

 

[stevenw1963]

Can't you turn it wet Phil ?, will dry quicker that way.
Rough turn it to a near approximation of the finished size, let it dry then re-turn it to finished size.

 

[Doug]

Can't you turn it wet Phil ?, will dry quicker that way.
Rough turn it to a near approximation of the finished size, let it dry then re-turn it to finished size.

That's the way I'd go about it, I've tried the microwave method with some success on smaller blanks, but it don't think it's worth the effort.

Then again if you've got plenty of cedar might be worthwhile trying it on one blank just for the experience oh & the dogging you get when the lady wife comes home & you've stunk the house out

 

[Grump]

might be worthwhile trying it on one blank just for the experience oh & the dogging you get when the lady wife comes home & you've stunk the house out

WHOA STEADY ON ARE YOU SURE????
DON'T LOOK ETHEL!!!!
DON'T GOOGLE IT INIT?
Dogging (sexual slang) - Wikipedia, the free encyclopedia

 

[Neil Lawton]

Pope Neil does have a certain ring to it

I have a microwave in the workshop specifically for drying wood.
After Neil's post, I have come to the conclusion it must be a portal to the kingdom of the water stealing elves, because I have successfully dried blanks in it for quit a while now
A lot of people do get it wrong, and you end up with what Neil says, a blank, dry on the outside, but sopping in the middle.
You can also overdo it and cook the wood, this leads to cell wall collapse, and bits can fly of while turning with a Gatlin gun effect.
The one thing you do need for microwave drying, is the one thing you are trying to avoid, Patience!
I most recently did a load of pen blanks, these are the easiest to do, some of which I gave to Neil.
Olive, Burr Sycamore, Burr Oak and Spalted Elm mix.

850w microwave.
Fill the plate with blanks, but not touching each other.
Microwave on full power for 30 seconds, take them out, mop out the microwave, put the blanks on a tea towel or kitchen towel and leave them till the next day.
Keep doing this for 3 or 4 days.
You can monitor progress by recording the weight loss (when Cold), or with pen blanks I use a probe type moisture meter, the probes easily reach the centre of a 1" blank, I usually put a couple of sacrificial boring bits of the same wood in, to monitor.
After 4 days the Spalted Elm had a moisture content of 14%, so did the burr Sycamore, so they were put in the useable box.
After 6 days, the Olive was reading 18%, and the Oak was reading 22%.
After 8 days the Oak came in at 18%, so the Elves were still at work!

Bowl blanks are more difficult, but there is an easy way.
In Phil's case I would make a note of the weight, then do 1 minute on full power for the first day, to drive out excess moisture.
30 seconds every day therein, weighing each time before microwaving.
When you get to the point that the weight does not change, the blank is dry, in fact too dry.
Put it to one side for a couple of weeks, and it will come up to equilibrium.

 

[Doug]

WHOA STEADY ON ARE YOU SURE????
DON'T LOOK ETHEL!!!!
DON'T GOOGLE IT INIT?
Dogging (sexual slang) - Wikipedia, the free encyclopedia

Local vernacular round her for getting told off :nonono::nonono:

 

[Grump]

And the time effort and cost of energy you may as well buy a bowl that has already been made init?

 

[PhillH]

Thanks to the other Neil for another very erudite reply.

@Doug nearly spat tea over the iPhone when I read dogging

 

[Neil]

I'd go along with the viability of the pen blank scenario, and suggest that if you predrill them you'll get even better results, wouldnt suggest that you drill them with the final dim drill as they are going to move.

Bowl blanks, nope still dont go for it Neil! But I have some fresh cut sycamore so will try some as you suggest when I get back from a three day craft fair at Sandringham.

 

[Woody]

I have tried it once with an 8" x 2" silky oak bowl blank short burst on low let it cool 5 min and repeat till it is finished it did work and I did turn a bowl from it but was it worth the bother no not for me I could have made 1/2 a dozen bowls whilst drying that one bit of wood so I have never done it since

 

[chas_41_uk]

I've tried it with a couple of small bowls and it has certainly speeded up the drying time. I've got a larger deeper one drying at the moment which I haven't put in the microwave and that has really elongated, should be interesting when I try to finish it.
I haven't tried microwaving wooden pan blanks, but I might give it a go, I got a beautiful block of spalted beech a couple of weeks ago which has developed a very bad (deep) split. So a mate has taken it to work to turn into pen blanks for me. I'll try it on those.

 

[bellringer]

I've tried it with a couple of small bowls and it has certainly speeded up the drying time. I've got a larger deeper one drying at the moment which I haven't put in the microwave and that has really elongated, should be interesting when I try to finish it.
I haven't tried microwaving wooden pan blanks, but I might give it a go, I got a beautiful block of spalted beech a couple of weeks ago which has developed a very bad (deep) split. So a mate has taken it to work to turn into pen blanks for me. I'll try it on those.

Could you take some picture of a wooden pan blank

 

[Neil Lawton]

Hope the fair goes well Neil

Please do try it, I would be interested in your results, good or bad.
I've not tried the pre drilling scenario for pen blanks, but shall do so. The blanks do move quite a lot, but no more than I would expect through conventional drying
If I get chance, I will post some start to finish experiments.
I can only comment on the bowl blanks, by my own experience.
The biggest thing I have done is 8"x 3" Oak, green to finished in 4 months, with no more movement than I would normally expect from a "Dry blank" in the workshop, being brought into a summer lit conservatory.

 

[sbwertz]

I have "kiln dried" blanks in my backyard grill sitting in the Phoenix sun! I have also used a fan driven dehydrator, set at 145 degrees. takes about 4 days for a standard blank. Never tried the microwave.

 

[Neil Lawton]

And the time effort and cost of energy you may as well buy a bowl that has already been made init?

Time, 30 seconds a day and you don't have to watch it!
Effort, pressing a button!
Energy, pence!
Buying a bowl, Pointless!
Making a bowl from scratch, Priceless init

 

[Grump]

Time, 30 seconds a day and you don't have to watch it!
Effort, pressing a button!
Energy, pence!
Buying a bowl, Pointless!
Making a bowl from scratch, Priceless init

Touch'e love it.

 

[Neil]

I have an inherent mistrust of wood moisture meters, or rather hobby or craft ones, I had one and the moisture readings were incredibly unreliable when compared to the industrial meters we had at our factory. We used to buy in supposedly kiln dried. To complete the drying process we kiln dried the wood for six weeks. Once it was dried if it was to be fir rated it was put in a vacuum for twenty minutes and the vessel was flooded with a magnesium chloride solution and the pressurised for twenty minutes. The pressure chamber was drained and a vacuum reset abolished for a few seconds to remove excess moisture. The slats, at 25mm thick we're then placed back in the kiln for another six weeks to get the hrs down to 14% max.

I find it impossible to believe that the centre of a decent size blank could be dry other than after a significant period or either air or kiln drying, but as always I remain open to being convinced!