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Deterioration of Wood Panels and How to Prevent It

  Imagine that you've been shrunken to molecular size.  
You find yourself inside of a tunnel. It looks like the inside of a giant pipe perhaps an aqueduct. Now imagine as I like to do myself that you're Orson Welles, starring as Harry Lyme in "The Third Man". It's the scene where you're escaping through the Vienna sewers. Got the picture?
  Now of course, this tunnel is not really a sewer.
Let's say, instead, that it's a wood fiber. The "bricks" that are the main structural component of this tunnel are made of cellulose, the most abundant substance in wood. Cellulose, like stone and brick, is a remarkably stable and durable material. Here, the "mortar" that holds the cellulose "bricks" together is made of lignin. Lignin is a resin-like glue that binds wood fibers together, and it's what makes wood "woody". But unlike the stable cellulose, lignin is slowly volatile, and it is easily broken down by UV radiation and by oxygen.
  Standing in this tunnel, when the air is still,
you'll notice the build-up of an odor (no, not that odor!) like a piece of Masonite®, which is the lignin slowly vaporizing out of the walls of the wood fiber. If the vaporization of the lignin "mortar" is allowed to continue, the cellulose "bricks" will eventually collapse.
  Flowing past you through this "pipe" are a stream
of molecules, primarily H2O and the various components of air: oxygen, nitrogen, CO2, etc. When the air pressure and humidity levels on the outside of the wood rise above the levels of those inside, the levels seek equilibrium and these materials flow into and through the pipe. When pressure and humidity outside of the wood drop, the flow reverses, and the contents of the pipe flow out.
  Along with this outflow go the vaporized lignin
molecules. (In art conservation terminology, this loss of lignin would be called "off-gassing".) When clean air and moisture flow back into the pipe, the odor is gone. But fresh oxygen and moisture start to break down more of the lignin. Volatilization of the lignin continues, and the odor/lignin build-up returns until the pipe is flushed out again. As time goes by, this cycle repeats itself over and over again. Time and tides, they say, wait for no man. Not even Orson Welles.
  From this vantage point, it may become
more obvious how to stop the loss of lignin, which would otherwise eventually cause the deterioration of the wood fibers: Paint the outside of the fibers, and plug the ends. Once the fibers are sealed off from the air and the ends are tightly closed, the air inside the fibers becomes saturated with lignin vapors. When this "vapor point" is reached, further volatilization of lignin stops.
  Preserving the lignin in a wood panel is like putting
a stopper on a bottle of turpentine. If the bottle is open or the edge of the panel is not sealed the turpentine (or lignin) will easily escape through the open end. Put a piece of tin foil over the top of the bottle or perhaps a coating of acrylic gesso (which is permeable) onto the edge of the panel and you will slow the loss of gasses, by a little bit. But if you put a tight cap on the bottle, or a tight vapor-barrier coating on the edge of the wood, you will safely preserve the contents within.
For tips on how to seal panels
and reseal exposed edges,
click here: (page will open in a new window)
< panel sealing tips >


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