Microwave Drying Wood

Microwave drying wood is a good technique to utilise where you need something dried in a hurry. I have read many articles on this matter and have heard of several ways of doing it.

The process outlined in the following text is the one I use, and I am very pleased with the results. The reason is simple. " I HAVEN'T HAD A FAILURE YET "

Sooooooooooooo lets get cooking

What's covered here:

What you need.

Of course you will need a microwave oven.
It is not really important whether it is a turntable model or not, and the power level isn't that important either. We will be using very low power levels, (defrost) so that sort of limits itself.. The only other factor is the physical size, and that's sort of self explanatory really. If the piece is too big it wont go in the door.

You will also need some scales to weigh the article with. This bit is difficult because you need a set of scale that allow you to measure accurately to small amounts (5 gram steps or so). I have a set of digital scales that are in 5 grams steps, and they perform well. If you are drying any wood by other methods, you will probably have scales anyway.

The only other things you need is time, patience and something to microwave of course.

The Technique

First, you need to prepare the piece.Rough turn the piece until the wall thickness is about 10% of the total diameter. This allows you to re-mount it and finish it off when dry, with the benefit of enough meat in the walls to allow for any oval distortion that may occur. If the oval distortion is not important, turn it to the finish you want. IT IS YOUR CALL. I AM ONLY EXPLAINING THE MICROWAVE TECHNIQUE HERE.

Speed here is really important, as some woods are prone to crack if left to air dry, so keep moving, and if you need to leave the piece in the lathe for any time, either cover it with a wet cloth, or place it in a sealed plastic bag.

• Weigh the piece and record the weight.
• Place the piece centrally in the oven, set the power level to whatever you use for defrost (3 on mine, but some even have it called defrost. clever that )
• The following times give some indication depending on the size of the piece.
• Up to 200mm (8 inch) .....1 minute
• 200mm (8 inch) to 300mm (12 inch) .....2 minutes
• over 300mm (12 inch) 3 minutes
• After setting the oven as described (defrost setting and time), start it.
• At the end of the cycle, remove it from the oven and allow it to cool.
• Leave until cool, or at least 1/2 hour.

• Replace the bowl in the oven and repeat the cycle.
• At the end of the 4th cycle, weigh the piece and record the weight

These 4 cycles are one drying program.
If you are using a non turntable oven, turn the piece upside down and rotated 90 degrees between cycles. This allows for even spread of warming.

At the end of the program, allow it to stand for an additional 1/2 hour, and then repeat the program.

Keep repeating programs until the weight remains the same between two programs. At this time it is as dry as you will get and the piece can be finished.

A few points to watch out for

• Don't try and rush things. The temptation is to crank up the heat or the time. You will be very disappointed with the results if you do. The idea is to do this slow and deliberately. Just like too much heat when sanding will cause heat cracks to appear, so will too much heat when cooking.
  Take it slow.
• Check your settings before starting the microwave. It is easy to set the thing of on the default power setting (usually high) without realising. The results are again, too much heat with the appropriate results
• Check it out with the master of the kitchen first. If you follow the instructions I have outlined here, you are barely warming the wood, so the possibility of damage to the microwave is no more than cooking your oats for breakfast. Crank up the power or lengthen the time, and you may well cause all sorts of problems. Gum pockets in the wood will boil and explode,and they are very hard to get off. As well, you can cause tainting of the oven if the heat is too great.

Why it works, or perhaps that's How it works

This is my story and I am sticking to it.

A Microwave ovens work by vibrating the molecules within a material, and this movement creates heat. The microwaves penetrate deep within objects (organic articles)

As you can imagine, the molecules in a liquid (water and sap) are easier to vibrate than those in a solid (wood). This more so with liquids that contain "sugars", such as the natural ones found within wet wood. (are you starting to see where we are going here)

As you gently "defrost" the article, you warm those liquids, which expand, forcing their way along the cellular structure of the wood. Upon exiting the cells at the outside surface, they evaporate off.

You now have a 2 fold process underway, heating/expansion moving the liquid out from the centre of the article, but also the sponge effect, of the dry outer surface drawing the moisture out from the centre.

This is why it is important to remove it from the oven and let it cool completely in the air.

You can also see that by really giving it a good cook, you will boil the liquid within the wood, and like any cooking, you will soften the fibre and damage the wood. This can manifest itself in many ways, including increase tear out on the end grain, articles that are difficult to sand to an acceptable finish, as articles that wont take oils or waxes well.

By following the method outlined, you are merely helping the natural process with a little modern technology, and instead of taking about 1 year per inch of board thickness, you can achieve the same results in a couple of days.