Drying (and rehydrating) of timber

[langchop]

Having some experience with woodworking, it is widely understood in woodworking that wood cyclically dries out and shrinks and then rehydrates and swells, most particularly seasonally. (Think of your front door that closes nicely in winter but scrapes and wedges in summer)[People often neglect to seal under a door!]

When we leave wood to dry/ stabilise for a few months/ years... what is it that we are actually achieving? Is it really drying out?

Lang

 

[physh]

When wood is cut down, it has a very high moisture content. "seasoning" wood allows it to lose moisture untill it has stabilized with the ambient humidity.

Unless wood is sealed, the moisture content of the wood will continue to fluctuate based on the ambient humidity.

Granted that I do live in florida, my front door shows no substantial changes as it's pretty well protected with spar finish. Also the material, mahogany, I think is more stable.

 

[mete]

All you do is stabilize to ambient conditions. When laying a hardwood floor best practice is to spread out the wood in the room and let it stand for days to stabilize. Rifle stocks are sometimes sealed on the outside with a waterproof finish forgeting to do the inside and the stock warps !! ...Much of the wood handles for knives are 'stabilized' which means vacuum impregnating with acrylic.

 

[Fred.Rowe]

Green lumber will "dry" to somewhere around ten or twelve percent, moisture content, when air dried; Put up on stick. It takes a closed, controlled, system to get it to 4 to 6 percent moisture.
This is what furniture shops use. I made custom furniture for some years. When I shipped it across country to an area with different moisture conditions than where it was built it could cause some real problems.
For knife handles, I believe stabilizing is the only way to go.

Fred

 

[Stacy E. Apelt - Bladesmith]

As to the question of drying for years. When wood is cut fresh it has moisture throughout the wood fibers. More in the softer cambium and sapwood areas, and less in the denser heart portion. As the wood slowly dries, the moisture reaches a point where the outside ambient moisture and the wood are equal.It still isn't stable yet. The point of stability comes when the core of the wood and the surface of the wood are the same. On woods that are very dense (ebony, ironwood, etc.) this can take many years. Until this point is reached, it still can crack because it is unstable .
Movement of the wood after being finished is a result of the wood gaining and loosing moisture content due to absorption. This can be reduced by stabilizing with a resin or oil. If the entire exposed surface is not sealed, the wood will still warp.
One common misconception is to relate relative humidity with moisture content. I often hear," Wood could never get to 10% moisture content here in the South where the average humidity is 80%." These are two different percentages. Moisture content is a percentage of weight. One pound of water in a ten pound log is 10% moisture content. Relative humidity is the ratio of the amount of water present in air to the maximum amount possible at any given temperature. 10% humidity means it can hold 90% more moisture, not that the air is 10% water ( and 100% humidity does not mean that the air is pure water. It would be really hard to breath during a rain storm if that were so!).
Stacy

 

[scottickes]

( and 100% humidity does not mean that the air is pure water. It would be really hard to breath during a rain storm if that were so!).
Stacy

If 100% humidity meant that the air is pure water, then you're under water. No air. Just water. That would be swimming. You can only breath underwater unaided if you have gills.

These posts have showed that some people think that they're comparing apples to apples, when they're really comparing apples to oranges.

Good explanations by all!

Ickie

 

[langchop]

Interesting posts. I agree that % moisture for wood and humidity is apples and oranges, but I imagine that it makes sense that at 100% humidity, a sample of timber whether it is at 2% moisture or 30% moisture, will not get any dryer. Similarly, at 80% humidity it will be very slow in drying, if at all.

So maybe its a matter of comparing apples with pears, rather than apples with oranges?

Thanks for your feedback

Lang