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Deterioration
of Wood Panels and How to Prevent It
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Imagine that you've been
shrunken to molecular size. |
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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? |
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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. |
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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. |
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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. |
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Along
with this outflow go the vaporized lignin
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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. |
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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. |
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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.
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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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