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Synopsis: Wood shrinks and swells with humidity
changes, and this can damage your guitar. Find out
how to prevent this.
Probably the single
most exasperating problem in caring for a high
quality guitar is the tendency of the wood to shrink
or expand with changes in humidity. Many expensive
repairs could be avoided by understanding the
effects of humidity change and by applying simple
remedies available for dealing with them.
Nowadays almost all
inexpensive guitars are made of plywood. Although
plywood is mediocre as a tonewood, it is reliable
and durable and withstands considerable atmospheric
abuse. High quality guitars, on the other hand, are
made of solid wood throughout, which is the primary
reason they sound so much better than less expensive
instruments. Solid woods, however, are more
vulnerable to humidity change because they shrink
and expand much more than plywood.
When humidity is
very high, a guitar may sound, for want of a better
word, "soggy." It lacks volume and projection and
has a dull lifeless tone. It is also possible for
a guitar to suffer structural damage when humidity
is very high. A common problem is "bloating" in the
back, especially if the back is made of rosewood or
some other very hard wood. This is caused by wood
expansion and, particularly, changes in the
back-to-brace glue interface. In rare instances,
this condition can lead to a glue joint failure.
Another problem that can be caused by extreme
humidity, as in tropical areas or a very damp
basement, is deterioration of the glue and
potentially even the wood itself.
The effects of very
low humidity, which is a more common problem in
temperate regions of the world, are often even more
serious. Extreme moisture loss in the tonewoods
sometimes makes a guitar sound brittle or "plinky,"
even though it may seem to have a peculiar, nervous
liveliness. At a critical point in moisture loss,
the accumulated stress brought about by uneven wood
shrinkage relieves itself by producing one or more
cracks and sometimes even glue joint failures.
The key concept in
understanding humidity is what meteorologists call
"relative humidity." This refers to the ability of
air to retain and take on water, or to dry out
moisture-containing objects it surrounds. The
higher the temperature of the air, the more water it
will hold before saturation and precipitation
occur. Relative humidity is the amount of moisture
present in the air expressed as a percentage of the
total moisture the air is capable of holding at that
particular temperature. It is humidity relative to
temperature. If the relative humidity is 40%, then
the air, if maintained at a constant temperature,
will theoretically hold 59.999... % more moisture
before precipitation occurs. As the relative
humidity rises, the air takes on additional moisture
more and more slowly. When relative humidity drops,
the air accepts moisture more rapidly.
When relative
humidity is very low, things (like guitars!) dry out
fast.
There are a few
places in the world, like parts of the American
Southwest or other desert regions, where the normal
everyday humidity may be low enough to be hazardous
for a guitar. However, low relative humidity starts
to become a problem anywhere whenever buildings are
heated. If the temperature outdoors is 20 degrees
Fahrenheit, the relative humidity 50%, and we take
this air inside and heat it to 70 degrees, then the
relative humidity can drop to 30% or less. The
colder it gets outside, the more the relative
humidity drops inside.
The ideal humidity
level for a guitar varies from one instrument to
another, depending on the humidity conditions in the
factory or workshop where it was assembled. The
humidity during assembly establishes the basic
dimensions of the instrument. This dimensional
configuration is permanently locked into the total
structure when the guitar is assembled. Thereafter,
when humidity varies, the individual components will
shrink or expand unevenly, while the dimensions of
the total structure tend to remain proportionately
constant.
The humidity level
in a guitar workshop or factory is extremely
important. It must be measured objectively at
frequent intervals with an accurate, reliable tool
and controlled within a narrow range; especially
when assembling bodies. While it is possible for a
workshop or factory environment to be too dry, the
more typical problem is excessive humidity. If the
humidity is too high during assembly, a guitar may
suffer great damage when subjected to a North
American winter—severe
warping, multiple cracks, glue joint failure, etc.
In my workshop, humidity is kept within a range of
35% to 40% when assembling bodies and 30 to 45% at
other times during construction of instruments.
This assures that a guitar will perform optimally
and keep its structural integrity under the widest
possible range of humidity conditions.
There are several
ways to keep track of humidity in your guitar's
environment. A sling psychrometer, which I use as a
reference tool in my workshop, is a very accurate
device for measuring dewpoint/relative humidity but
is also rather expensive and cumbersome to use. For
many years the only other practical alternative for
a guitarist was a dial-type hygrometer. This type
of hygrometer, even the more expensive of them ($50
or higher), are not particularly accurate or
reliable and need frequent recalibration to be of
any use at all.
In recent times the
technology of digital hygrometers has improved
markedly. I currently offer a
Thermo-Hygrometer set,
consisting of two wireless-linked devices, one for
the home/studio and one for case mounting. I have
tested these devices rigorously with my sling
psychrometer under varying humidity conditions and
found them to be accurate to within ±0.5%. My new
guitars all now come equipped with this
thermo-hygrometer set; it is also available on this
website as a separate purchase item along with a few
parts to help perform a simple case installation.
It can also be
helpful simply to keep an eye on the guitar itself
in deciding whether to take steps to control
humidity. The best indicator on the guitar is the
back: when humidity drops, guitar backs always tend
to sink in, because of a shrinkage differential
between the back wood and the braces. Conversely,
backs develop arch when humidity rises. If a back
becomes dead flat, you should be introducing some
moisture or storing the instrument in a place where
humidity is higher. If the back develops a concave
dish, you should be adding moisture more
aggressively. On the other hand, if a back shows an
unusually high dome, you should try to get the
instrument into a dryer place.
You should be alert
to low humidity conditions whenever winter weather
keeps the temperature below freezing all day. If
the daily temperature range is 10 to 25 degrees, you
should add a little moisture to the guitar's
environment. If the range is as low as minus 15 to
10 above, you should take decisive action to
introduce lots of moisture if you hope to avoid
tonewood cracks or other structural problems.
However, the temperature conditions which point to a
need for more moisture will vary from guitar to
guitar, because of differences in humidity
conditions at the time when the guitar's body was
assembled. This is why you need to keep an eye on
the guitar, especially the back, as well as on the
weather.
Particularly in
winter, a guitar should be stored in its case, far
away from any heat source, not on a stand or hanging
on a wall. The case should be kept on the floor,
because indoor air in winter is quite a bit cooler
(and the humidity is therefore higher) in that area
of the room than closer to the ceiling. One of the
best ways to raise the humidity in winter is simply
to keep the room temperature lower. A drop in
inside temperature of only five degrees, from 70 to
65 for instance, can sometimes raise the humidity as
much as 10%.
During periods when
humidity is very high, the best remedy is to keep
the guitar in an air-conditioned area; air
conditioning dehumidifies the air in addition to
cooling it, usually to a level of 50 to 55%,
depending on outside humidity. If it is not hot
enough outside for air conditioning (during a rainy
spring, for instance), you should try to keep the
guitar in a relatively warm area, avoiding places
like cool basements. It may be advisable at such
times to keep the guitar in a room with a
condenser-type dehumidifier, especially if the
instrument is being stored in an area like a
basement recreation room. A dehumidifier running
intermittently can maintain a humidity level of 50%
to 60%, which is an acceptable level for storing
almost any guitar.
There are several
devices available to cope with excessive dryness.
If low humidity in your area occurs only in cold
weather, a good quality furnace-mounted humidifier
is most effective and hassle-free. Console
humidifiers with rotating belts or cascading-water
evaporators are also effective humidity sources for
any season in limited spaces. Another remedy for
low humidity is a "Dampit," a device which can be
placed inside a guitar to release moisture directly
inside the instrument. I prefer a plastic food
container with a damp sponge placed inside the
storage compartment of the case. Never mind the
closed compartment; the moisture will slowly get out
into the case and the guitar. These devices are
particularly useful when traveling. However, during
the times when added moisture is essential (severe
winter cold snaps), they need re-wetting at least
once a day.
Protecting a
valuable guitar from humidity extremes, if
approached intelligently, can be done with minimal
inconvenience. It also makes good economic sense.
The most expensive of the remedies suggested above
may end up costing considerably less than the
expense that can result from neglect, especially if
there is a permanent loss of instrument value
because of environmental damage.
(This article
originally appeared in Guitar Review,
Summer 1988. It has since been edited.)
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