It's cold and freezing

It’s cold and it’s freezing outside. -8°C.
Inside the house, it’s warm and cozy with family and friends dropping in for Christmas and New Year. But do you think that your precious guitar is having a good time too? Well, think again.

Wood is a living organism, even if the tree it originated from was harvested 20 years ago. It will keep on expanding and shrinking as the relative humidity of the air it is surrounded with changes. In the summer, when relative humidity levels are sometimes as high as 70%, the overall size of a piece of wood will be greater than in the winter when the freezing cold and the cranked up heating of our living room has squeezed out the last bit of moisture in the air. Although the changes affect the length, width, height and weight of a piece of wood, the differences are most notable along the width (across the grain) and have a greater affect on slab-sawn wood than on quarter-sawn wood (figure 1). That is one of the reasons why luthiers insist on using quarter-sawn tops and backs.

Once the wood has been thoroughly air-dried (over 20 years), the expansion and contraction is minimal. Unfortunately, most of the time we have to do with kiln-dried wood. Kiln-drying is nothing more than blowing heated air over stacked wood for 2 or 3 days. The moisture is sucked out of the wood by the dry, hot air. Wood that is kiln-dried is stable for use under normal circumstances and a fairly constant relative humidity of 45%-50%. Once the temperature and the relative humidity change, the wood absorbs or looses moisture again. Every luthier who knows what he/she’s doing, will always further air-dry kiln-dried wood before using it to minimize distortion of the assembled guitar.

So what happens to that beautiful guitar when you put in on the stand in your cozy living room? Well, everybody will admire it, especially when you start playing it. But after a while, you’ll notice that there’s a fret sticking out just a little bit, that the action has lowered and the tone of the guitar isn’t as full as it used to be. Or is it mere imagination? No, it’s not. Your guitar is suffering from the extreme dry air. It’s drying out. And if you don’t act promptly, you’ll end up with a guitar with a cracked top or back.

Check the following illustration. Notice all the small red arrows indicating the contraction movement when drying-out. In the end, something’s going to give or crack.

The first thing you’ll notice is the flat, not-as-sparkling sound of your guitar. Sadly enough, that is something that most people will try to ignore. Next thing you’ll notice (and believe me, you will !) are the protruding frets you cut your fingers on when performing a rather unprofessional slide. By this time, the relative humidity of your room has dropped to 35%. Next thing, the strings will start to buzz on several frets. That’s because the top has shrunk (it’s almost flat now) and the sunken saddle is now positioned several mm lower than before. The guitar constantly plays out of tune, whatever you do. The top has also pulled the fretboard end downwards so you’ll notice a hump at the 14th fret. With a relative humidity of 27% and still dropping, it will take only another week before your guitar top or back cracks.

There are several things you can do to avoid all this misery. First, always store your guitar in its case during the winter (and leave it outside the case during the hot, damp summer). Second, use a humidifier inside the case. They don’t cost a fortune but if you can’t afford one, use a damp sponge wrapped in a plastic bag so it doesn’t stain the guitar – and re-damp it often. Third, buy yourself an analogue or digital humidity reader so you don’t have to rely on the state of your dried-out lips or skin.
And if money is not a problem, buy yourself an air-conditioning system that is also capable of controlling relative humidity levels.

If your guitar is already in a dried-out state, you can still resurrect it by putting it in its closed case and restore the humidity level. Refill the humidifier every day or re-damp the sponge every other day and it will get all-right. It will take some time though, at least a week, probably more.
If the guitar top has cracked, act just the same before taking it to a luthier. We can fix the guitar by cutting the crack open, putting a shim of the same wood type in the crack and glue some shims to the underside of the top to prevent further cracking. But I hope that after reading this article, you’ll be smart enough not to let your beauty dry-out completely.

Sadly and controversially enough, only the expensive, solid wood guitars suffer that much of the fluctuations in humidity levels. It will take much longer before you’ll notice deformation on cheap plywood guitars. The expansion/contraction movement of each of the thin sheets of wood is counteracted by its counterpart that is glued across the grain (turned at a 90° angle). In terms of stability it’s a much better deal. If you have to travel a lot in extreme and uncontrollable conditions, you’d better leave your beauty at home in its case and take a cheap plywood guitar with you. And let someone refill the humidifier every week while you’re away.