This is a request for more information regarding drying wood in the microwave. The following process (described in detail below) has worked for me but the results have not been totally consistent. Different species? Also if the wood has been air-dried at all there seem to be microscopic cracks which are not healed by the microwave, but are in fact accentuated. This happens when a log has been sawn and the end left to dry. This raw edge must be removed to get beyond the hairline cracks - 2 to 6 inches depending on how long the log has been dried.

Microwave Drying Scientifically

Have you dried a turned wood piece only to have it check on you later? Do you wonder how long to "zot" a piece? How do I know when to stop microwaving? Will the microwave harm the wood? If you have asked yourself these questions then read on and hopefully I will be able to answer some of your questions.

Let me talk a bit of theory to establish why the following microwave drying steps work. My theory based on what I have learned about wood over the years. A reminder though that theories are just that: theories. So if you have a theory that contrasts to mine I would welcome a discussion. However I do know that the technique descibed below works, so if that is all you want and forget the theory then scroll down to the next title which describes the process.

Factual item 1: Checking occurs in wood when the wood shrinks as it dries too rapidly. I theorize that this happens as the fibres shrink the "glue" or lignin that binds the fibres together fails and cracks develop between bundles of wood fibres. Some woods are much more prone to checking than others, apple wood being a prime example of a wood that checks very easily and quickly. Branch wood also checks more easily than the main stem of the tree.

Factual item 2: When wood is heated it bends more easily than when it is cold. My theory is that the wood fibers become "softened and flexible" and slide over each other when they are hot. This is what allows the wood to bend when it is heated and stay bent when it is cooled in the bent position.

Factual item 3: Wood may be heated to the boiling point of water - 100 C - without deterioration .

Theory of why the heated wood does not check: When the wood is hot the fibres are soft and the lignin allows the fibres to slide relative to one another thus the wood does not check while it is hot. However it will still check if it is allowed to lose moisture while cold or at room temperature even after it has been heated. When water starts to boil it changes to steam - this is a good indicator of the temperature of the wood. (When steam is being produced.)

Instructions for microwave drying wood.

First the disclaimer: This process has worked for me but I can assume no responsibility for how it works for you. And as a suggestion, do not start with a $100 burl but rather try out some "junk" wood that you do not care that much for - you may even be surprised.

Second as an alternative to the technique described below: Some microwaves have a "Sure Simmer" setting which essentailly allows you to keep the item in the oven at the boiling point for a set period of time. If you have and use this setting then the bag technique below is obsolete. Simply simmer the item for as long as required to drive off all the moisture. 3/4 inch thick wood requires about 15 minutes simmer time in my microwave. To keep the moisture level high place the wood in a plastic bag that has a few holes either pierced or just do not tie too tightly.

Here is the technique for "older" microwaves:

1. Weigh the wood and record the weight. (This is optional but a good thing to do initially as it will give you an idea of how much moisture is being driven off.)

2. Seal the wood inside a plastic bag. A ziploc bag works well. This also keeps the microwave oven clean.

3. Place wood (which is inside a bag) in the microwave. Set microwave at full power and the timer for two minutes.

4. Turn on the microwave. Watch the bag. When it starts to blow up - fill with steam not "go bang". Turn off the microwave. This is a good temperature indicator. If the wood is heated too long steam will be generated inside the fibres and strange effects occur such as hidden fissures.

5. Remove the bag and wood from the microwave. Open the bag and remove the wood from the bag. The wood should be hot. Cover the wood with a paper towel and allow to stand for 5 minutes. Dry the inside of the bag.

6. Seal the wood inside the plastic bag again (If you want to place some paper towel in the bag use dry paper towel).

7. Place wood (which is inside the bag) in the microwave. Set microwave at full power and the timer for two minutes.

8. Turn on the microwave. Watch the bag. When it starts to blow up turn off the microwave. Note how long it took to heat to "steam generation". For small pieces about 50 seconds.

9. Remove the bag and wood from the microwave. Open the bag and remove the wood from the bag. The wood should be hot. Allow the covered wood to stand for 5 minutes (Do not allow the wood to cool down too much). Dry the inside of the bag.

10. Now you have an idea of how long to heat the wood after a five minute cooldown. If you weighed the wood this is when you should reweigh it and see how much moisture you have lost. Note the new weight.

Note: When there is no longer a significant weight loss you will have dried the wood sufficiently. Over drying the wood does not harm the wood but it does waste electricity and the wood will take on moisture from the ambient air once it is allowed to rest.

11. Reheat the wood every five minutes for the time you established in step 8 until there is no significant weight loss OR until the wood "feels" dry after the five minute rest. You no longer need to place the wood in the bag except to keep the oven clean and humidity high. Some woods leave a distinct odour which may not go with your reheated Kraft Dinner. But wrapping the wood in paper will keep humidity high too.

Remember that the wood needs to be kept hot while it loses moisture and moisture removal should be kept slow by keeping the wood in a relatively moist environment. (Use the pierced bag technique described above or paper wrapping.)

Thin pieces may only have to be heated three times while thicker pieces will need several sessions in the microwave. The number of sessions depends more on the thickness of the wood than on the overall weight. Large thin pieces dry just as fast as small thin pieces. However the larger pieces will need slightly longer sessions in the oven as determined by the bag inflating with steam.

 

This picture shows the difference between two pieces that started out the same size and shape. The one on the right was microwave dried. Both from a walnut branch - a hard wood to dry without checking.

 

If you try this technique send me an email and let me know how it worked. {Click Here}to send an Email.

 

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John Williams

Microwave Drying the Science Way

Contact John / Home / Links / View work / Biography / Lathe Hints & Tips / Woodturning Glossary