... than that can cause swelling.
http://www.martinguitar.com/guitar-care/6-main-site/guitar-care/213-temperaturehumidity.html
Martin keeps the 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.
http://www.premierguitar.com/articles/Fighting_the_Humidity_Battle
What is the best humidity level for my guitar? Most experts say 40-50 percent. At this level, a guitar will sound and play its best. A great way to control humidity is to use a humidifier. Think of it as an inexpensive insurance policy to protect you from very expensive repairs.
A guitar humidifier is easy to use and very effective. Some guitar humidifiers are suspended between the 3rd and 4th strings and contain a damp sponge that needs remoistened every two or three days. This type of product works okay, but it’s not consistent. The humidity will spike at first, then slowly diminish as the sponge dries out.
The more modern guitar humidifiers use a gel that not only emits humidity, but also absorbs it if the humidity gets too high. This technology was first developed for cigar humidors, and now it’s available for guitars. Planet Waves makes a great humidity control system called the Humidipak that uses this technology. I’d also recommend using a hygrometer to measure the humidity.
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