Synopsis: Temperature extremes can threaten your guitar's structural integrity. Find out how to prevent this.

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Many guitarists are familiar with a guitar's response to small temperature changes.   For instance, a guitar taken out of a cool case comes alive to a player's touch as it absorbs warmth from his body and hands.  Less well known are the effects of temperature extremes on a guitar's health and well-being.  Certain materials used in guitar construction function well only within a temperature range that is reasonably tolerable to humans.  As temperatures exceed this range, these materials may react so as to damage a cherished instrument.

Wood is a remarkably hardy material with respect to temperature change alone.  It is almost always the changes in relative humidity which accompany temperature extremes that injure the woods in a guitar.  Modern glues and resins, on the other hand, respond directly to temperature changes, independent of any changes in humidity.

The thermoplastic resins in modern glues and lacquer finishes shrink and harden as the temperature goes down, and the result can sometimes be disastrous.  I have seen the bridge and a good deal of soundboard wood ripped right off a classical guitar as a result of exposure to sub-zero temperatures.  As the temperature goes down, the nylon in the strings shrinks, increasing the tension on the bridge.  At the same time, the glue holding the bridge to the sound- board hardens, which increases its grip.  The result is a tug-of-war in which the soundboard invariably loses.

The tension of the strings on a classical or other type of nylon string guitar should always be thoroughly lowered if there is any chance that the guitar may be subjected to unusually cold temperatures.  If you expect temperatures much below zero, it is a good idea to do this to a steel string guitar as well.

The other common casualty of extreme cold is lacquer finish.  This material also shrinks when the - temperature goes down.  However, the wood it covers shrinks minimally, if at all.  Thus we have yet another tug-of-war at the interface between wood and lacquer;  lacquer never wins.  Instead, it may relieve the stress by breaking down into a spider web network of hairline cracks:  This is called "cold checking."  Lacquers vary in their vulnerability to cold checking; some are quite resistant to it. (The new polyurethane and other catalyzed polymer finishes have excellent cold check, as well as abrasion, resistance.  However, the thick, hard finishes of these materials make them less acoustically desirable for high quality guitars.)  Do not assume that a given lacquer finish will withstand . any amount of cold.  To be on the safe side, get your guitar into a normal room temperature environment if the outside temperature goes below 20�F.

The effects of extreme heat on a guitar-say, over 110�F-are sometimes exactly the opposite of those of extreme cold.  However, high temperatures can also produce some unique and unexpected disasters.  Extreme heat, like extreme cold, has virtually no effect on wood by itself.  However, whenever the temperature of a given sub-environment goes higher than the ambient outside temperature, the relative humidity goes down.  Then any or all of the consequences de- scribed in "Humidity and Your Guitar's Health" can happen, particularly tonewood cracking.

All modern thermoplastic glues, the kind most commonly used in guitar construction today, will soften, expand and weaken when the temperature rises.  When the temperature tops 110�F, glue joints may begin to weaken; if the temperature rises to 130�F or over, failures in stressed joints are certain.  Such conditions encourage excessive forward bow in the neck.  This is caused by a phenomenon called "glue creep,"--the tendency of soft, or heat-softened, glue to "give" in the direction of a stress vector.  All glues will creep given enough time.  However, the harder the glue is, the slower it will creep.  At high temperatures, especially over 130�F, glue creep can occur very quickly because of softening.  The neck on a guitar tuned to concert pitch tends to warp in the forward- bow direction because of the string tension.  If the glue at the neck-fingerboard interface is heat-softened, the resulting accelerated glue creep resolves the stress, creating a permanent excessive forward bow.

The most common damage caused by high- temperature glue creep, however, is at the bridge, where the glue joint to the soundboard is under very high torque stress due to the tension of the strings.  Most cases of bridge/soundboard glue joint separation I have run into are caused by extreme heat.  I can always tell by the tendrils of glue that look like stretched taffy which are visible under the bridge where it has separated. In some cases the bridge may pull off completely.  This may be a merciful outcome if it relieves stress which might have added risk to a soundboard already endangered by extremely low humidity.

Do not assume that only the bridge or neck are vulnerable to extreme heat.  The wood shrinkage caused by the humidity drop which accompanies high temperatures creates a whole complex of new stresses which can threaten any number of heat-weakened glue joints. A guitar can literally fall apart in extreme heat.

It belabors the obvious to point out that any of the `above tragedies can be avoided by keeping a guitar away from extreme heat or cold.  However, people sometimes leave guitars for storage or safe-keeping in once temperate places, forgetting that these locations may later get very hot, or very cold, or both.  Attics and unheated garages rare to be avoided.  Guitar enemy number one, however, is without a doubt the parked car, especially the trunk, where temperatures can sometimes top 150�F or go down to the extremes of winter temperatures.

(This article first appeared in Guitar Review, Fall 1988.)