Microwave drying

Re: Microwave drying

Dick Coers

Can't tell you how long, too many variables. Wall thickness, density, diameter, moisture level......

I wouldn't recommend experimenting with a new technique on some cherry burl. If you over do it, it will burn. And it burns from the inside out. You may see a little smoke, then a lot of smoke, but it looks fine for a while. Next thing you know, you have to drop it in a bucket of water to put out the fire.

Start slow, weigh the bowl every time you bring it out. When it stops loosing weight, stop.

Re: Microwave drying

John C Lucas

Just start slow and be patient. I start on high power for 30 seconds just to see how it goes. It should be warm but not real hot. Then you can try a minute. I personally dont go for long periods. I heat it and then do something else while it cools. I weigh it and write that down. I prefer many short cycles over the afternoon or all day. Some do longer periods on lower power. A lot of experimenting needed because there are too many variables. If you heat it too fast for too long you can ha e a fire so never leave it alone. What happened to me was there was a sap pocket inside the wall of the bowl and it got hotter than the rest and burned through the sidewall and actually caught fire. I was standing there so no damage other than the bowl. Took forever to vet the smell out which is why you never use the one in the house.

Re: Microwave drying

John K Jordan

I've only used it when in a hurry.

I've done just as described but only with smaller blanks, say 1.5" squares. It took a bunch of short cycles until the weight stabilized. I adjusted the time so the blank didn't feel too hot - I think about 15-20 seconds each time. Much would depend on the wattage of the oven. If the oven has controllable power settings a lower setting might be better to keep from burning. I took the piece out of the oven and let it cool completely each time before weighing.

Be advised that microwave chambers have hot spots and can overhead in one small area unless something is moving to help even out the energy. (I was involved laterally with a research project once on variable frequency microwave ovens so I learned a bit about the size, shape and position of the hot spots. In that design the thing moving was the frequency.) For the wood drying I used an oven with a rotating platter.

JKJ

Yes, watch out for pitch pockets in cherry........

JamieDonaldson

...as I once learned! It broke into flames and smoke alarm went off, so wife ordered me to find an oven of my own for the shop! Really convenient to also warm my coffee, but definitely dry the wood in controlled steps to avoid heat cracks as well as smoke..

Re: Microwave drying

David DeCristoforo

Marshall Gorrow has a good tutorial on his web site:

http://mgorrow.tripod.com/microwave.html

I tried this with some green madrone and I was amazed at the amount of water that came out of a few small pieces of wood!

Re: Microwave drying

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Well, yea,,,, The heat will drive the moisture out

Keith Newton

But it will still come out of the end grain much faster than the side grain, so you can cause some checking. After playing with it, I dropped back to the paper sack for a few weeks. It just didn't seem like it was worth the risk for something that really doesn't take all that long.

Sometimes when I do a turning which may have a thicker area, like carved feet, on the bottom of side grain bowls or vessels, I may set them up where I put gentle heat on the thick area for short periods.

Of course, when you put it in the microwave, you'll be heating everything. You may want that to kill some insects, and their eggs. However, you can mask the quick drying areas with plastic to slow that part down.

It is tricky, and probably the safest way is to only zap in short burst, then feel for the heat.

Re: Well, yea,,,, The heat will drive the moisture

Geoff Whaling

I have tried microwave drying methods on some far north Queensland timbers to see how they would respond, mostly small cross section spindle sized blanks. As others have said I'm not convinced that the rapid moisture loss with microwave ovens offers sufficient benefit to warrant the risk of fire and damaging the cellular structures in the wood. Some respond well, QLD Silver Ash is one but it is also renowned for its steam bending qualities.

Many of our Australian woods when being kiln dried undergo a regime of heat to remove the bulk of the moisture then reconditioning with heat & controlled moisture, often in several cycles. If this is not done correctly there can be spectacular collapse with some species, surface checking, and tempering or case hardening of the wood in others.

I only have seat of the pants testing to rely upon but I much prefer to turn air dried woods, over kiln or microwave dried wood for my spindle turning.

Re: Microwave drying

Bob Macgregor

A few weeks ago I cracked a 12" yellow birch bowl by drying it too fast in the microwave.

I wonder if the defrost cycle would work better by automatically applying pulses of power, with a stabilizing pause in between the pulses?

Re: Well, yea,,,, The heat will drive the moisture

Ric Taylor

Masking part of the wood with plastic (depending on the type of plastic) in a microwave could result in some real excitement that you really don't want as part of your day.

Re: Microwave drying

Ric Taylor

Bob, in years gone by there have been articles published that recommend using the defrost cycle. I have used it with great success using defrost for 3 minutes on and at least 30 minutes off, repeating that cycle as necessary for 7 to 10 cycles.

Careful monitoring is necessary, as others have said the wood could be burning on the inside which might not be apparent. (Microwaves cook from the inside out). What this means is you have to be nearby and monitoring for most of a day. Is that worth it? Maybe on a short-deadline project, but probably not as a routine method.

What happens to drying wood

Geoff Whaling

When we are accelerating the rate of drying for timber we are trying to reach what is termed the final equilibrium moisture content (EMC) for that timber in a given environment in a rapid, cost effective manner and hopefully without drying degrade.

Wood dries from the outside inwards. If we dry it to fast we get structural (collapse, splitting) and cosmetic (surface checking etc) degrade, too slow it suffers biological degrade (fungi, blue stain etc) so we must control the rate of moisture loss to prevent degrade.

Wood looses its free water, the water not bound up in the cellular structures of the wood, first and it is relatively easy to remove. The bound water is more difficult to remove because of the cellular structures of the wood. The outer surface of the wood dries first so it has a much lower MC than the core i.e. there is a moisture content gradient. Wood dries faster on end grain than in face grain because of the capillary structures in the wood. We have to manage the transfer of water from the core to the surface in an optimal manner.

In commercial kiln drying operations an optimal “drying schedule” is determined through research or from historical experience. The drying schedules typically involve,

• A degrade control period until the wood has lost about one third of its moisture (free water).

• Accelerated drying period after all of the free water has evaporated.

• Equalising period to stabilize moisture distribution through the timber i.e. same MC in end grain and mid board / blank. The wetter material is allowed to go on drying, but you are attempting to stop the dry material drying any further.

• Then finally a conditioning period to reduce stresses in the wood particularly to avoid or relieve case hardening, warpage, cupping etc.

This cycle may be repeated depending upon the species, drying method and intended application of the timber. Pallet timbers for local use barely have the free water removed whereas high value fine cabinet timbers undergo a complex drying schedule.

Microwave drying is not common in commercial operations, but Australian research shows it is viable and cost effective for small section short lengths of high value timber. Microwave drying is quite different to conventional drying techniques as it heats from the core outwards. This makes it more difficult to monitor the moisture gradient in the timber.

This is why it is important to permit blanks to undergo a resting cycle between microwave sessions. The rest period is both an equalising and conditioning period.

Only experimentation and experience will determine what cycle times are effective / optimal for a particular species, its MC before drying, the microwave power and the local ambient conditions.

Here in Townsville in the dry tropics I can air dry a 30 x 150 mm (1.5 x 6”) board in under a year, Wood turning blanks 300 x 75mm (12 x 3”) take approx 2 years but we generally go 3 years. Drying degrade – splitting – from to rapid moisture loss is a problem in our winter.

Re: Microwave drying *LINK*

Brad Vietje

The process is well described -- theory and all, in Michael O'Donnell's excellent book, Turning Green Wood.

I never use high (750 watt and 1000 watt MW), but generally go slowly at 50 - 60% power. Trial and error with constant vigilance at first is essential (see Lyle, etc...) When wood starts smoking in a microwave, it can really stink, and you don't want it in your kitchen.

Like many others, I generally use this only when I need something pretty fast, as time and a paper bag have worked well for me. BUT, if done right, there should be no increased risk of checking. Uniformly thin/thick is essential. A goblet with a thin stem will burst into flames when you're not looking (DAMHIKT!)

I use brown paper toweling folded twice (4 layers) under the piece, and it turns visibly darker when dampened by the escaping moisture. I run 4-5 cycles of hot/cool until the damp stain is almost gone -- then let it rest and air dry. The PT's will all be wet at first, then 3 layers, then 2, then only 1 -- only where it is in contact with the foot or base.

The other application that I've used is to get a crinkly texture on green burl wood when turned, wet sanded, and finished in green wood. A few weeks/months and a paper bag work fine for this, too, but sometimes its just pretty exciting to see what sort of texture you will get. Then I remove raised grain with a fine Scotch Bright pad -- usually on the hook-n-loop sanding pads in a hand drill, like power sanding. If you go with the direction of the crinkles, you can get a nice finish w/out taking off the texture.

Safe spinning,

Brad Vietje

Newbury, VT


Turning Green Wood -- Michael O'Donnell