Wood Movement and Lumber Acclimation – What is it?

Prospective clients sometimes ask why it may take so long to build a piece of furniture. Aside from other factors such as the actual crafting and finishing is the aspect of allowing lumber to acclimate to the environment before construction begins.

To understand why this is necessary, it helps to understand the nature of wood. Most people have experienced a door that would stick in the summer and yet be perfectly fine in the summer?

That is because as humidity levels rise, wood absorbs the moisture from the air causing it to expand. When the humidity levels drop again, the wood will allow the moisture to escape again and will contract.

When lumber is first cut from a tree and cut into boards, it is very high in moisture content. The moisture then begins immediately to evaporate which if not controlled, can cause the newly cut boards to warp, twist and/or cup. There are 2 methods for controlling this process. Air drying and Kiln drying.

In both cases, lumber is stacked with spacers (called stickers) between each layer to allow maximum air flow around each board allowing an even rate of moisture evaporation from all sides of the boards. It is then either placed into a big oven (called a kiln) or a covered enclosure for air drying.

lumber stickered for drying

When we purchase lumber, it has generally been dried in a kiln first bringing it down to a moisture content of 6-8%. It then sits on a shelf until purchased – usually indoors away from the elements and that allows it to begin slowly absorbing some moisture back into the wood. When it is brought to our shop, it then sits in a stickered stack for 2-3 weeks to allow it to acclimate to the local humidity conditions before milling.

Lumber that is milled straight from the mill without acclimation can react by warping as it absorbs or emits moisture and become unsuitable for the purpose that it was milled for. It also may not react at all, but why take the chance?

Wood will always change with its environment, but we can minimize the impact it has on how that movement affects the integrity of its designed use.

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