This is not going to be a "Tool De Jour" article but it might sound like it because there are some basic fundamentals about the materials from which our instruments are constructed that we would do well to remember.
As I have told you before, I am not a luthier but I do know about wood and woodworking and construction techniques. The two materials we need to understand are wood and iron. Rather interesting when one thinks about it; that is, two totally dissimilar substances which when brought together in a certain fashion create a beautiful instrument that produces beautiful sounds. [That was me waxing poetic.]
Iron is an element and wood is an organic compound called "cellulose". Let’s look at iron first. Most of us are playing instruments with "metal" strings. This metal can be any number of different combinations of iron, nickel, brass, copper, bronze, zinc and what have you. All of these metal strings have a unique property to some degree or another. They will expand and contract according to temperature. Heat makes metal expand and become more flexible. Cold makes metal contract and become more rigid. Ask any railroad person if this is not true. During the summer railroad tracks expand and buckle at the joints causing train wrecks. During the winter train tracks contract and even break under extreme cold, also causing train wrecks. This is one of the reasons your instrument goes out of tune if it is subjected to extremes in temperature.
Wood on the other hand, starts out as a living breathing plant. It must die in order to become lumber and it must become lumber before it can become your instrument. Before that piece of wood became a piece of wood it was a log. Logs have growth rings. Wood will always and continually expand and contract along the radial axis of its concentric growth rings. (I’ll bet you did not know that the wood in your instrument even had a radial axis of concentric growth rings.) Most people quickly forget that wood has grain. One can see these rings if they look at the end grain of a piece of wood. This movement is determined by the moisture or lack of moisture present or not present in the wood. Cracking or "checking" happens when the moisture leaves or returns in an uneven or sudden fashion. The fundamental principle about wood that every woodworker knows is that "wood moves!" Forget that fact and your woodworking project will fail. For instance, what happens to any cutting board that one might run through the dishwasher? It will invariably split. Wood will also move longitudinally but only in a negligible and insignificant amount.
When a log is cut, the wood holds quite a bit of water. The wood must be cured. This is usually done in a type of kiln or a drying yard where it is stacked and air allowed to circulate between the boards. When the moisture content drops to around 7-8%, the wood is then suitable for working. However, unless the wood is sealed it will re-absorb moisture from ambient humidity. The piece of wood will always try to stabilize close to the ambient humidity. That is why furniture you buy in one part of the country will sometimes creak when you move to another part of the country. Assuming; of course, that the furniture is made of real wood in the first place.
So what? Someone asked me the other day about leaving an instrument in a hot car. I gave them the short answer. I said, "Don’t do it!"
In such a case, the least that can happen is that the instrument will be out of tune; the worst thing that can happen is like a train wreck. Wood can easily split because of the change in moisture content brought on by heat. Hide glue liquefies at 140 degrees. PVA glue softens at 180 degrees. Both glues crack below zero. Since most of our instruments are constructed with one or the other of these glues; this is good information to know.
It may sound extreme, but treat your instrument like a child or a pet. If your car is to hot or cold for a child or pet, it is too hot or cold for your instrument. In the case of your instrument, disaster is not always inevitable but the odds are greatly increased if one where to subject their instrument to extremes of temperature or humidity. Why chance it?