By Paul McGill
In instrument repair shops everywhere, there is a constant every year when the weather first gets cold and the heat goes on. Musicians come in with their beloved instruments, not quite sure how dry weather damage could have happened to their prized possession. "I got it out the other day and noticed that there was a crack in the top along the edge of the fretboard." "There’s a crack in the back somewhere." "The end of the fretboard is split." Or, "The action suddenly changed." Why does this happen? Why are guitars like this?
Guitars are not the only instruments to have problems with the weather. However, they are particularly sensitive to it due to their inherent structure (plywood backed or topped guitars excepted). As many know, wood expands and contracts laterally across its grain and also with the grain, although much less. This is due to the amount of moisture that it absorbs from its surrounding environment, which is called Wood Moisture Content (WMC). WMC is the amount of water as compared to the total mass of the material. Because woods can be composed of differing amounts and types of resins, oils, metals (yes!), minerals, and, of course, cellulose fiber, different species of materials have greater or lesser rates of expansion which cause some guitars to be more sensitive to changes in WMC. Regulated by the environment, the wood moisture content, as it changes, will put the many parts of your guitar under stress.
In the guitar making process, WMC is controlled in order to limit the possible amount of stress in the joined parts of your guitar. Some of you may have talked to knowledgeable advisors who have mentioned building guitars for different climates. Wood becomes saturated at 27% WMC. If a guitar is made with a WMC of 8% (approx. 45% humidity at 70 degrees), it is 29.6% saturated. I use this 8% WMC in my own work because in general it is a good, moderate humidity in which to build an instrument. One percent WMC doesn’t sound like much, however, it is actually 3.7% of the amount of water that the wood will absorb.
If the guitar is exposed to climates of extreme humidity, the material will expand. Likewise, if it exposed to dryness, it will contract. It is possible for a guitar to gain or lose as much as 20% of its possible WMC (approx. change between 6% and 12% WMC). Making a guitar at a moderate WMC, however, reduces the stress on the joints if they are ever exposed to either extreme. For example, if the instrument was made at 6% WMC and was exposed to very humid conditions, or if it were made at 10% WMC and exposed to very dry conditions, the stresses on the components would be greater at the opposite extreme from which it was constructed. If you were going to keep a guitar in the arctic, you would do better to have a guitar made at a very dry WMC. If you were going to live in Hawaii, you would do better to make the instrument at a relatively high wood moisture content. The effect on the sound is significant as well.
Most sensitive to extreme stress from dry or damp environments is the dynamic interaction between the braces and the top or the back where they adjoin inside your guitar. Because wood moves the most across the grain, the top and the back move much more relative to the braces, whose grain direction is placed in the opposite way. So the braces hardly move at all in the direction that the top and back are moving, which causes the braces to bear all the force of the top and back wood as it expands and contracts. To further explain, the brace is glued on one side to an expanding or contracting surface (the top or back). As that surface expands, it pushes the ends of the brace further apart as it stretches the brace along the glue line. As it contracts, it pulls the ends of the brace towards the center and compresses the brace along the glue line. The resultant stress created between these parts will cause the top or back to bow and become convex in high humidity and concave in dry conditions. Sometimes guitars can become so wet that the expansion will actually push the edge bindings around the guitar out of their channels. Of course, dryness will result in cracks. The constant forces back and forth can also weaken the adhesion of the glue and cause the brace to come loose and to rattle or vibrate while you are playing. It’s easy to see how these stresses can impede the vibrations of the top and back.
The ebony used for fret boards expands and contracts more than most woods. The end grain on many fretboards is not sealed, which allows the very end of the board to lose moisture faster than the rest of the board in dry conditions. The stress can be so great that the fretboard can crack and even split the top as well, usually along the edge of the fretboard. Or the shrunken board will leave the fret ends sticking out where your hands can feel their sharp corners.
Other neck movement from weather changes is something most players have experienced. A guy I worked with here in Nashville a few years back played a lot of gigs in Memphis. It was a good thing he was a skilled technician as well as guitarist, because every time he went there to play, the necks on his Strats would backbow. It’s only 200 hundred milesaway, but the Mississippi is a mighty river! The result was that his fretboards would acquire enough water along the way to cause the unfinished playing surface of the fret board to expand. That slight expansion that wood mades in the direction of the grain caused enough stress on the neck to force it into a different alignment.
So what do we do? What is the best way to keep your guitar stable? The first thing you need to know is under what kind of conditions the instrument was constructed. Some manufactures don’t have a lot of time to cure wood because of the expense involved in stocking materials. Most try to stock up a year in advance while others use quick-dry techniques to cure wood right away. The Yamaha factory in Japan many years ago had a process of saturating and kiln-drying their woods over a period of time to force any cracks to appear before ever using the material. This would also tend to eliminate neck material that would later twist and generally de-stress the wood as well. If a manufacturer recommends the guitar be kept at a humidity above 40%, that suggests that the guitar was constructed at a somewhat higher humidity. There is no standard for this, and given the conditions of any given day, the WMC could be very different even in the same manufacturing location.
Next, you need a good hygrometer. This will tell you what humidity you are keeping your guitars in so you can know if you have a problem which needs attention. For example, if your guitar requires 40% and you are down to 25%, you have a problem, or if you are going to spend three months in Southeast Asia during monsoon season you might also have a problem. There are several methods of controlling you instruments WMC.
In dry conditions, many people like to use devices such as dampits which hold a small amount of water in a sponge and release it into the interior of your guitar through a breathable container such as a plastic bag with holes in it or a small film canister similarly modified. These devices may work for some environments, but they can only do so much. The guitar surfaces, although sealed in finish, are water permeable and over time do allow penetration of moisture. By placing moisture inside your guitar and maybe even sealing the sound hole you are adding some moisture. However, the exterior is losing moisture as well. If you are putting a dampit in the case pocket, the guitar is competing for moisture with the case fabric and other absorbent materials in the case. I had many customers come to my shop in Wisconsin with a new crack and a dampit stuck in the sound hole. It just wasn’t enough for that harsh, dry winter weather. The other shortcoming of dampits is that they offer little, if any, help for controlling the stability of the neck and fretboard. All that said, if the dampit seems to be working for you, you probably have all the protection you need.
A better solution, if possible, is a humidifier. Many people would find one beneficial for their own health as well as their guitar’s. If you put a humidifier in one room of your house or apartment and maintain it through the dry weather, you will overcome all the shortcomings of a dampit. I use a 12 gallon per day Toastmaster humidifier in my 800 sq. ft. shop. I purchased it at Wal-Mart for around $100. I can keep my humidity above 45% even through the driest of conditions of the year with the forced air heat going full blast.
A less thought-of consideration is wet weather. In very humid times, guitars will act like sponges, expanding and getting soggy. Nylon strings also absorb moisture, which impedes their output. The great Brazilian composer Augustine Barrios played his guitar with metal treble strings. Gut strings, common in his time, take on so much water in his humid homeland that they sound dead and refuse to tune. You may find that silica gel packs, which come in many Asian guitars, will act as a desiccant. Or you can rig a light bulb in a closet with a dimmer switch and use its heat to dry out your guitar in wet conditions.
It may also be helpful to obtain a copy of the US Forest Products Research Laboratory’s handbook. The lab is located in Madison, Wisconsin, or you may find a copy in your local library. In it you will find a table for wood moisture content at different temperatures and humidity. It is also important to note that according to their research, wood will lose moisture ten times faster than it will acquire it. That means that if you go to the rain forest, you can remove ten days of acquired moisture in one day of dry exposure. It also means that when the weather gets dry, the potential for damage can come on very quickly. I have seen materials shrink up in hours laying around in the shop. I recently made a guitar for a Panamanian gentleman who spends time in both the New York area and in Panama. It’s hard to get any more extreme than that with a guitar. While in Panama, he used my light bulb drying technique, a closet with a 200 watt light bulb controlled by a dimmer switch, and in New York during the dry months, a humidifier.
Those of us who make guitars spend a great deal of time contemplating problems related to the stability of our guitars. Once they are out of our hands, it takes an informed public to keep them in good condition. So the more we all do to protect them the more enjoyment they can give us in return. Keeping ahead of the repairman requires a little forethought, and you will win every time.
