Some feel it is necessary to have a humidifier on the inside of a guitar to keep it from cracking, but the opposite could also be true.

When I bought my Eastman archtop guitar, the store included a Dampit! in the deal. After only 6 days of use, I had a crack, and some damaged varnish on a $2600 guitar. Picture 1| Picture 2

The people at the store stated that it is important to keep the guitar at the right humidity. Of course that is true, but the correct humidity is to keep it in the same environment as it was constructed. For example if the guitar was constructed in 40% humidity, you should keep it in or near 40% humidity, or gradually allow its humidity to increase/decrease until an internal humidifier is not necessary.

Do not precisely follow the advice that comes with your humidifier. Mine says to always strive to keep the guitar at 50% humidity. In order to for this to happen, it would require me to always have the humidifer fully charged and inside the body of the instrument. I tried that method, and I still couldn't get the humidity above 30% even in the summer (measured inside guitar case). If I were to take the humidifer out, it would allow the humidity to go back down to less than 10% in a day and make the guitar crack. During my attempts to get the humidity up to what the instruction pamphlet said, the Dampit! humidifer leaked, causing a side to expand, and crack, then some extra moisture leaked through the crack, and got under the varnish. These humidity readings were taken from the inside of the guitar case. Picture 3 | Picture 4

Irving Sloane says in Guitar Repair, "the absorption of atmospheric moisture--is a less serious hazard than shrinkage...sudden drying out of wooden instruments is the major cause of cracks and seam separation". Do not allow the guitar to suddenly dry out. Use a humidifier to let its humidity decrease slowly (about 2 months). If the guitar is already stable, it is more important to maintain the current humidity, whatever it is, than to bring it and keep it in one that is difficult to attain. Also, if you require changing the humidity, do it gradually with your humidifer.

The makers try to assemble guitars in 45% to 55% relative humidity. By the time a guitar gets to a store in your neighbourhood (from China, or Korea), it is probably acclimatized to your environment. And if it has no cracks in the store, it won't have cracks when you get it home, unless your house has a sauna, or indoor swimming pool.

Another thing that helps keep guitars from cracking is the selection of quarter-sawn wood. Since a tree can change size without cracking, its circumference will change 3.14 (Π) times as much as the diameter. If a board is cut perpendicular to the growth rings, (the growth rings represent the circumference) it is quarter-sawn. The board width will change size at the rate of the tree's diameter. If the board is cut conventionally, it will change size 3.14 times as much, and warp, because it is parallel with the circumference (tangential).

It is my opinion that an instrument humidifer is a tool to allow the gradual change of relative humidity inside the instrument rather than keep it at the same level forever. The key is to gradually change the humidity.

Winter has just arrived, so I am going to charge up my humidifier, but not too much.