The guitar is made of different kinds of wood: spruce or cedar for the sound board Indian or Brazilian rosewood, cypress or mahogany for the back and sides, ebony for the fret board, and Spanish cedar or mahogany for the neck. These woods are sensitive to changes in humidity and will expand or contract according to how much humidity is in the air around them.
The ideal humidity for wood instruments is between 40 and 70 percent. Above 70 percent the instrument usually begins to lose its brilliance of tone. Below 35 percent a guitar runs the risk of the body cracking and the neck warping. This is the more serious menace and occurs mostly during the cold winter months in temperate climates and in arid regions. A good indicator of whether or not your guitar is in need of additional humidity is the fret board. If the ends of the frets stick out it means the ebony has become too dry and has contracted. You can assume that the other woods have also shrunk and could crack under the stress. The reverse is also true, check to see if thefretboard has expanded, you can see this by looking at the ends of the metal frets, are they sitting inboard of the fretboard, if yes then the ebony has expanded.
There are a number of effective humidifying devices available to counteract this danger: The Dampit, the Kyser system, and the D'Addario system all provide a humid atmosphere inside the guitar body. When using them it is important that you leave them, inside the closed case when you take out the guitar to play it otherwise the case will become too dry.
A method of providing humidity to the neck and fret board is with a plastic travelling soap dish with a damp sponge inside and holes punched in the lid. If humidity is properly maintained your guitar should never suffer cracking or warping.
The D'Addario Two-Way Humidification system helps to maintain a constant 45%-50% relative humidity inside the instruments case.
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 keeps its 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.
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.
The 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.
Repairs to your instrument should be performed by an authorized repair person.