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Why Finish?

By Greg Heuer posted 04-24-2015 09:44 PM




Wood is hygroscopic. It tends to absorb moisture from the air. Wood used for furniture and architectural woodwork is dried to a moisture content of between 6-10%, which is to say that less than 10% of the weight of the wood is water. That small percentage is bound up in the cell walls.

One of the reasons we finish wood products is to retard the cells from taking on moisture. In the case of a table top, a finish helps prevent permanent damage to the table if the spill is wiped up quickly. Most finishes are not strong enough to fully prevent the taking on and giving off of moisture as the wood tries to come to equilibrium with the relative humidity around it. As a result, long term exposure to free water will almost always damage even the finest wood or wood product.

Dry Rot is one of the most unusual terms. It is caused by fungus digesting part of the cellulosic structure of a wooden building member, often a window sash or sill. The resulting wood seems dry or brittle, but the cause is moisture. The fungus can only grow in wet wood.


Wood Movement      


Because wood is hygroscopic, items made of wood will move when exposed to extremes in ambient relative humidity. As the cells take on water they swell, pushing joints tighter, expanding the width of boards, and changing the nature of the structure of the wood. When that same wood dries out, the crushed joints open, the boards become narrower, and the cellular structure of the piece may be so damaged that it is smaller than when it was first machined.

The boundaries of the extremes are under 20% relative humidity, and over 80% relative humidity, with a swing of over 30 points in between. Finishing will retard wood movement because it retards the exchange of water between the wood and the environment.

There is one thing every woodwork manufacturer can Guarantee – Wood Will Move when exposed to those extremes. That’s OK if and when we engineer for the inevitable wood movement.

Engineering for movement can (and should) be made part of the design aesthetic. The addition of set back surfaces, reveals, sliding joints, and decorative mouldings can make all the difference.


When all is said and done, we finish wood because great finishing makes wood more beautiful. Warm reds and browns are traditionally celebrated as the classic wood finishes from centuries past. Rich, deeply polished surfaces are still highly prized.

#woodworkingtips #finishing