RogerM":p0jl71ov said:
I did a did a MC check by putting a block of the oak in question in the microwave for 5 mins at a time, and took it out at the end of each period to weigh it and to do a moisture check.
Here's a safe way to check the moisture content of wood in a microwave.
Edit. Since originally posting I went back to my original manuscript on the subject and did some editing to make it read better. Here's the new version. Slainte.
Microwave Oven Drying
Oven drying in a microwave oven takes between 20 and 45 minutes. 30 minutes is about average. It saves a great deal of time over drying wood in a regular oven. It does however require care and attention to details. Poor methodology and mistakes in the procedure usually lead to problems and failure.
You need to weigh the wood samples and electronic postal scales purchased at reasonable cost from an office supplier I find work well enough for my needs: if you require more accuracy then better, more expensive scales are required. My scales provide readings in 1 gramme divisions from zero up to a maximum of 2200 grammes, and I can select readings in either grammes or ounces.
To dry the wood I use a turntable type microwave oven that has several power settings. The only two settings I use are the very lowest setting and the next higher setting which is Defrost – your oven is likely to have a different configuration, but whatever marked settings are available restrict yourself to the lowest one or two power levels. As the wood is heated moisture evaporates from all exposed surfaces, including the bottom face resting on the turntable; three to five paper kitchen towels laid under the wood absorb and dissipate the condensed moisture drawn downwards from the wood. If you’re testing several samples make sure they don’t touch each other because this can concentrate the energy and can lead to smoking and possibly fire.
If the wood starts to smoke at any time during the drying procedure the sample is ruined and you need to start again with a new sample. Smoking during the cooking means you have burnt away some of the wood volume and weight measurements taken thereafter are inaccurate. This is why I mostly restrict myself to the lowest power setting and short bursts of heat. The second lowest power setting, Defrost on my microwave oven, is seldom used, but I do sometimes use it for the initial drying cycle of very wet wood.
The ideal wood sample is a full thickness and width piece taken at least 400 mm in from the end of your board that is 25- 32 mm (1” to 1-1/4”) long. Wood near the end of a plank or board is unrepresentative of the board as a whole—moisture exchange is more volatile at a board’s end than nearer the centre. Weigh your sample and make a note of this. If the sample is already partially dried, eg, about 25% MC to 15% MC, cook the wood at the lowest oven setting for between one and a half and two minutes in the first cycle.
If you know the wood is already below 10% MC I recommend you cook it at the lowest setting of the oven for no more than 45 or 60 seconds to start with.
Where wood is definitely at fibre saturation point (FSP, 30% MC) or above it’s my experience that the first cooking should last no more than between 1-1/2 and three minutes with the oven at the second lowest setting. Even in this circumstance I prefer to use the lowest oven setting: it takes a few minutes longer to dry the wood but I prefer that to starting again because I’ve burnt the sample.
After the first cycle weigh the sample or samples again to form an impression of how quickly the wood loses weight, ie loses water. Let the sample rest for a minute or so and re-cook it for between 45 and sixty seconds and re-weigh.
Continue with this routine until you can’t measure any weight change, i.e., less than 0.1 of a gramme variation if you are using highly accurate scales. My scales read only to the nearest gramme so I stop cooking when I get five or six low weight readings the same.
When this point is reached use the formula MC% = (WW–ODW/ODW) X 100, where WW is Wet Weight of the sample, and ODW represents the wood sample’s Oven Dry Weight.
The following cautions should observed.
Do not use high microwave oven power settings. The internal heat built up in the wood needs to dissipate and high settings cause rapid heat build up, smoke and even fire.
The more wood tested in one go, the more time is required to complete the job. This is useful because after the initial heating of a large batch you can rotate from one sample to the next in the oven with short bursts of cooking for each piece. This gives each sample a break between heating cycles thus reducing the chance of overheating any one piece.
It’s my experience that kiln dried wood samples react differently to cooking than green or air dried samples. It’s easier not to mix samples of very different moisture contents and different wood species during the test, but it's possible if you proceed with care.
Being sure that the wood sample or samples is, or are, truly oven dry requires patience and careful weighing using accurate scales. It’s better, and safer, to use several short cycles in the oven at low settings than it is to try and rush the job using a higher setting for extended times. The latter strategy usually results in burning the wood and failure.
The following final closing warnings probably seem obvious, but they’re worth repeating. Removing cooked wood from the oven requires care. It’s usually hot and can and does burn skin – you probably don’t need to ask how I know that! Use an oven glove or heavy leather work gloves. Also be aware that at the end of testing, and unknown to you, wood might have charred on the inside: it can smoulder and burn, and could set fire to rubbish in bins, etc. Careful disposal is essential. The safest thing is to put the cooked wood in water when you’ve finished to ensure it doesn’t burst into flames later—it can happen.
