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Your guitar is made of thin wood which is easily affected by temperature and humidity. This combination is the most important single part of your guitar’s surroundings. Martin Guitar keeps their factory at a constant 45-55 percent humidity and 72-77 degrees Fahrenheit. If either humidity or temperature get far away from these factory conditions, your guitar is in danger. A rapid change in temperature or exposure to cold can cause small cracks in the finish. These are lacquer checks. We recommend the use of a hygrometer/ thermometer to measure the relative humidity and temperature surrounding your guitar.
As humidity increases, moisture content of wood goes up rapidly, causing it to expand and swell. A gradual increase in humidity won’t generally do permanent damage to your instrument. When very high humidity is combined with high temperature, glue joints could possibly become weakened and may even open slightly. If your guitar is exposed to high temperature or humidity for any length of time, the glue under the bridge could weaken causing the bridge to pull off.
Rapid changes in local humidity are what you want to guard against. If, for instance, you place your guitar near a source of dry heat, the humidity around it will drop much faster than it would naturally, although a sudden dry spell can have the same effect. If the moisture content of wood is forced down in a hurry, portions of it shrink faster than others, causing cracks and open joints. Don’t set your instrument next to a source of heat or hang it on a wall where it will dry out. At all costs, avoid hanging your guitar on an outside wall during winter months. The wall will be cooler than the inside air. The result is a conflict between the temperature of the top and back, with potential damage as a result.
Should the guitar be exposed to freezing temperatures, let it warm to room temperature while still in its case. This lets it come up to room temperature more slowly, decreasing the possibility of wood and finish cracks.
Caution should be taken if you choose to use a humidifier to combat low humidity. Moisture in direct contact with the guitar could cause damage, as can the rubber or vinyl parts of a humidifier.
We recommend storing your guitar in its case when not in use. Humidity is easier to control in a smaller space. Don’t bother loosening the strings when putting your guitar away unless it won’t be used again for several months. Constantly tightening and loosening strings quickly ruins their sound. A good hard case supports the neck and body of your guitar as evenly as possible. It’s important that you don’t let anything lie under the head (the tuning machine end), as this could damage the neck and body. | 
