Excess heat is one of the most unrecognized and destructive forces that act on stringed instruments. I'm talking about the kind of heat that builds up in parked cars. It's the kind of heat that kills the family dog in short order.
It's the kind of heat that drives out moisture and causes instruments to crack. It's the kind of heat that causes glue to melt. It's the kind of heat that causes necks to warp.
How hot is it?
My car can make it to 175 degrees Fahrenheit in only 15 minutes on a hot day in the sun. At that temperature, it's not long before the inside of an instrument case gets to be hot, too. At around 140 degrees the glue that holds modern instruments together begins to turn to liquid. LITERALLY.
Let me show you some of the really common heat stress problems that I see regularly.
This Martin D-18 was left in a hot car:
The glue lost all its strength and the bridge simply came loose completely. I know that's what happened because I can see little "strings" of glue underneath, showing that the glue stretched as the bridge was lifted off the face of the guitar. This can ONLY happen in high heat.
Because the strings anchor under the face of the guitar on the bridge plate, the bridge can't fly off all at once. A classical guitar bridge usually does, because the strings are tied on and don't go through the face of the guitar.
This guitar was lucky. It's owner saw the problem right away and loosened the strings before the top suffered more damage.
Here's a D-28 that wasn't so lucky. The glue melted in the heat and the bridge came loose and was subsequently reglued by a well-meaning repairman. Apparently no one noticed the severe wrinkles in the top, extending from the corners of the bridge toward the end of the guitar:
I hope the photo is clear enough. By the way, that's a round desk lamp and the reflection is distorted by the wrinkled spruce top. When I see a wrinkle this severe in this location, I can tell that the brace has been pulled loose inside the guitar:
See that light gap right at the back corner of the bridge shadow? That baby's really loose! With any luck, I can reglue it and restore much of the strength of the top brace. This is serious stuff, because regluing a brace inside means adding new glue to old glue, and the strength of the bond will be compromised. Usually that's not a problem the first time. . .When I go to vintage guitar shows, I'm always amazed at how many old guitars I see with this telltale top damage, yet I've never seen either a dealer or customer looking inside a guitar to check for problems
Hey, take a look at this guitar neck:
Plastic parts tend to shrink with age as they lose volatile chemical components. In a hot car, this shrinkage can happen much faster:
This Martin was repeatedly left in the car at festivals. It never got hot enough to melt glue, but it did get hot enough to accelerate the aging process. Here the pickguard shrank and curled, taking the top with it. Notice the two cracks that are starting to shred the top at the edge of the pickguard.
In addition, the entire instrument was covered with a gummy haze all over the finish. This haze is a result of repeated overheating. In the heat the moisture is driven out of the case lining and interior of the guitar, condensing on the finish as it cools. The gummy residue causes no problem and can be wiped or buffed off, but it is a clear symptom of overheating.
The fingerboard binding on this Martin D-35 shrank prematurely in the heat of a parked car, causing it to crack and separate at its weakest point where the edge dot was inlaid:
Here's a fingerboard that shrank in the heat. The moisture was driven out and the wood shrank enough that the fret ends stick out like little spears:
These guys are sharp enough to cut your hand. Not just annoying, they too are a symptom of overheating. Once the wood shrinks, no amount of rehumidifying will cause it to regain its full original size. The sharp fret ends are easily repaired by filing level with the fingerboard.
What's not easily repaired is the other damage caused when the fingerboard shrank:
This is the same classical guitar fingerboard. It was originally flat, but is now cupped and concave through its entire length. Unfortunately, because of the heat the frets are also slightly loose as well. The only way to make this guitar play really well is to take out all the frets, level the fingerboard and install new frets. A big job that would not have been necessary otherwise.
And, just to show how bad it can get here's one of the most dramatic ones we've seen. It's a Martin D-45, and it was left in a parked car in the sun during at least one hot summer day. The wood shrank as a result of catastrophic drying, and it cracked alongside the fingerboard where the wood had been cut for the original abalone inlay. The heat was great enough to soften the glue that held the interior braces, allowing the string tension to pull the neck forward, displacing the soundhole inlay as well:
Please watch out for parked cars. Your car won't get hot while you're driving, so when you stop, just think of your instrument as being a living thing inside the car. If the heat would kill your dog, it could also kill your guitar. The same thing goes for other sources of high heat: direct sunlight, heaters, fires, etc.
Now, if you're travelling, you may have your sleeping bag with you. You can protect your instruments in the car trunk by covering them with LOTS of insulation. After all, if your sleeping bag will keep you warm, it will also keep your guitar cool. Insulated and reflective case covers help, too. Only, don't count on them for long term protection. Better to avoid heat altogether.