Guitar Repair and Restoration
01/05/10 - Winter, Humidity, and Your Guitar.

Winter is now upon us and this is a great time to talk a little about the importance
of humidity and how it affects your guitar. Most manufacturers try build guitars in
a controlled environment, one that is relatively stable both in temperature and
relative humidity. The climate in which guitars are typically built is suitable for
most areas of the country but owners must take extra precautions in places
where temperatures can fluctuate from one extreme to the other; such as here in
Indiana. The ideal humidity range for most guitars is 45% - 50% RH. When outside
temperatures drop the air dries out bringing the relative humidity down.  Once
the humidity drops below 40%, guitar woods begin to dry out and physical
changes will occur in the instrument. These changes, when left unchecked, often
result in the following most common symptoms:

Sharp Fret Ends - Wood fingerboards shrink across their width, metal frets don't.
The result is the protrusion of the fret ends beyond the side of the fingerboard,
making the fret ends feel sharp and the guitar uncomfortable to play.

Low Action - Most prominent on acoustic instruments. As the top becomes dry,
the wood shrinks, and the top of the guitar will gradually flatten, reducing the
string height over the frets.   

Cracks - These are more common on acoustic instruments. Top cracks occur
when the wood dries to the point that the forces across the grain are such that
the wood grain structures fail and are torn apart. This can also result in the
failure of the back and top center seam on acoustics.

Finish checking - This is most common on instruments finished with lacquer or
varnish. Rapid temperature changes cause movement of the wood. These
finishes, being brittle and thin, cannot expand like the wood and are subject to
cracking. The result is a web like pattern of small cracks across the finish.

You can prevent most of these problems by using and maintaining a case
humidifier for your guitar when it is stored. Guitars that are left out for display
should be kept in rooms that are humidity controlled if possible. When
transporting an instrument from outside to inside during the winter, allow it to set
in its unopened case and gradually adjust to the warmer indoor temperature
before opening the case. Bringing a cold nitrocellulose or varnished finished
instrument indoors and immediately opening the case may expose you to the
unfortunate experience of watching and hearing the finish crack right before
your eyes.
Top crack due to low
Sinking top cause by excess drying/low
A bridge crack that occurred in
combination with top sinkage.
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