Figure 6.12. Three 25 mm (1″) long lengths cut sequentially from an English oak plank were prepared and tested. Each piece started the test weighing 73± grammes, and were approximately 177 mm wide by 21.5 mm thick. Piece A was kept near a radiator for three days prior to further experiments. The weight recorded after this additional drying was 69 grammes. Subsequent microwave oven drying to 0 percent MC and a final weight of 65 grammes shows the piece started the test at 6.5 percent MC. After oven drying it measured 171 mm wide x 21 mm thick. B, the middle section of the three cut pieces, was placed outside but sheltered from rain for three days after which it, too, was oven dried to 0 percent MC. Prior to drying it weighed 76 grammes with a dry weight of 66 grammes. This equates to just over 15 percent MC, indicating its MC rose probably 7 percentage points over the three days. Dimensions of the piece just before oven drying were 177 mm wide by 21.5 mm thick. C was weighed and measured after three days of soaking in water. Its weight after soaking was 92 grammes and measured 184 mm wide by 22 mm thick. Subsequent oven drying to 0 percent MC and a final weight of 67 grammes show MC of this piece was 37.5 percent after soaking. Dividing the narrowest width (A) by the widest width (C), i.e., 171 / 184 = 0.93 or ~7 percent shrinkage from 30 percent MC, or greater. Similarly dividing the narrowest thickness by the greatest thickness, i.e., 21 / 22 = 0.955 or 4.5 percent shrinkage. The photograph was taken after A was oven dried but before pieces B and C were dried. The dimensions are very approximate as only a steel rule took the measurements. Although measurements are only approximate it’s interesting to note shrinkage factors for the tangential shrinkage (~7 percent) and radial shrinkage (4.5 percent) for this piece of oak are quite close to European oak numbers provided in various wood movement tables, i.e., 8.9 percent tangentially and 5.3 percent radially.
This is an excerpt from “Cut & Dried” by Richard Jones.
Oven drying in a microwave oven takes between 20 and 45 minutes. The average time is 30 minutes. It saves a great deal of time compared to 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 will need to be able to weigh the wood samples. I find electronic postal scales purchased at a reasonable cost from an office supplier work well enough for my needs. If you require more accuracy, more expensive scales are required. My scales provide readings in 1 gramme divisions from zero up to a maximum of 2,200 grammes, and the machine can be set to give readings in either grammes or ounces.
To dry the wood I use a turntable-type microwave oven with 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 downward 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 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, so 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 the same as described in section 6.6, i.e., a full thickness and width piece taken at least 400 mm in from the board’s end, approximately 25 to 32 mm (1″ to 1-1/4″) long. Weigh your sample and make a note of this. If the sample is already partially dried, e.g., about 25 percent MC to 15 percent 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 percent MC, I recommend you cook it at the lowest setting of the oven for no more than 45 or 60 seconds to start with.
When wood is definitely very wet, 30 percent MC or above, the first cooking should last no more than between one and a half 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 is preferable to starting again because of a burnt sample.
After the first cycle, weigh the sample or samples again to form an impression of how quickly the wood loses weight, i.e. loses water. Let the sample rest for a minute or so and re-cook it for between 45 and 60 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 five or six low-weight readings are recorded.
When this point is reached, use the formula provided earlier, i.e., MC percent = ((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 are important: Do not use the microwave oven’s high 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.
I generally find kiln-dried wood samples react differently to cooking than green or air-dried samples. It’s best 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 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.
In closing, these final, following warnings probably seem obvious, but they’re worth mentioning. Removing cooked wood from the oven requires care. It’s usually quite 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, if placed in a rubbish bin, could start a fire. Careful disposal is essential. The safest thing you can do is put the cooked wood in water when you’ve finished drying it to ensure it doesn’t burst into flames later – it can happen.
— Meghan Bates
