Sawdust on the Floor

I’m not sure how the subject got started, but I found myself and some friends talking about dumb things we have seen or heard of that belong in the urban legend category. I don’t know how much of what was said was true or how much was just guys having fun together, but one tale brought me up short. The story went something like this, a new wood turner ( no names here), had obtained some freshly sawn lumber that he wanted to turn into bowls. Not wanting to turn the lumber green, and not wanting to let the lumber dry naturally, he decided to speed up the process.

Here is where the story may have been edited by the teller, but the guy decides that he will dry the wood out in his wife’s new microwave oven. The story went on to tell how much family disharmony this act created… no kidding!!  There isn’t a good idea in this tale that I could find. I won’t go into the obvious problems that the idea creates, like the possible contamination of the microwave, or the possible fire hazards, or the ire that may arise from his wife. What I will talk about is the fact that you can’t fool Mother Nature.

Here’s a fact, all wood, whether it is freshly sawn or has been air drying for years will have some water in the wood. We can’t get it all out. The amount of this water is known as moisture content. Moisture content is stated as a percentage. The calculation for the amount of moisture in a board is;

initial wet weight of lumber-dry weight of the lumber X100/ Dry Weight of the lumber

To accurately measure the amount of moisture content in any piece of lumber you would either need a very accurate scale that has an accuracy of a half gram, or a moisture meter. Scales that are that accurate are very expensive and are not readily available. The Lignomat Meter shown at the right, is a much more convenient method of measuring lumber MC. A lot of woodworkers and wood turners often take along a portable MC meter when shopping for lumber as there is no way to accurately gauge the moisture content of a piece of lumber just by looking at it or feeling it. Another item you may wish to have with you when shopping for wood for a specific project is a spray bottle filled with distilled water. Spraying the wood will give you an idea of the grain patterns and highlight any defects that will show when your project has been finished.

Lumber that is used for furniture building will have a moisture content of 8% to 10%. A tree that has just been sawn will often have a MC of 75% to 200%. The moisture content has to be reduced before the lumber is usable.

Wood has to be either kiln dried or air dried. While kiln drying is faster, most home woodworkers, or wood turners simply do not have the equipment to the job correctly. When wood is kiln dried, air movement, temperature and humidity must be all closely controlled. Many of today’s industrial kilns use advanced computer controls to keep these variables. The costs to obtain and maintain a kiln are not economically feasible for most woodworkers or turners.
Air drying requires that the lumber be stacked horizontally, with stickers between each board. These stickers allow air to flow completely around the board, which is essential in the drying process. The general rule of thumb, is that it takes one year of air drying per inch of thickness in the raw lumber.

When wood dries, the outside of the wood dries first, then with time the moisture on the inside will migrate to the surface, and is evaporated. If wood were to be dried too fast, say in a microwave, the surface of the wood will actually dry and shrink while the inside of the lumber would remain wet. The resulting lumber would either warp, buckle or bow, making it totally unusable. This effect on wood is known as “case hardening”, and should be avoided at all costs.

As for the guy in the story, let’s hope it was just a story. Microwaves cook from the inside out, and while this is good for food, it is not the way that nature wants wood to be dried.

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