Why does Meadowood harp on humidity? - Nov 2001
We nag because the damage that instruments suffer is gradual & unnoticeable until it is fairly well advanced. Last year, November's article was about humidity. This year, we beat the drum again. It is that important.
In humid conditions, the unfinished wood on the inside of your instrument absorbs water & the wood swells. Conversely, the wood loses water & shrinks in arid conditions. Small, gradual humidity fluctuations are not a cause for concern; sudden changes & prolonged dryness are.
Tone wood ages in a controlled environment to ensure that the wood is used in instruments is stable. And, because wood swells & shrinks with moisture content, instrument builders keep humidity in their workshops very stable, near 50%. This way, luthiers prevent the wood that they use in instruments from changing shape while they build them. Even after the instrument is complete, relative swelling & shrinking of the wood remains important.
Spruce & cedar, used in the tops of acoustic instruments, are softer & weaker than woods used for backs & sides (mahogany, koa, maple, walnut, rosewood...). In addition, the sides, are curved & glued onto bracing that makes them even more rigid. So, when the instrument absorbs moisture (humidity >60%), the rigid sides hold the edges of the weak top in place, & the swelling top puffs outward a little. While the instrument is swollen, the strings are higher from the fingerboard & the tone of the instrument is slightly muffled.
In dry conditions (humidity <40%), the sides still hold the top in place. Shrinking away from the sides, the top is in tension, and can crack. When humidity drops below 40% for an extended period of time or drops a large amount in a short period of time, the top of your instrument is at risk of cracking.
No one warrantees against cracks caused by lack of humidity. If your instrument dries out & cracks, the repair cost comes out of your pocket. And, depending on the size, location & character of the crack (or cracks), the repair can be pretty pricey sometimes hundreds of dollars. To add insult to injury, because the cracks are sometimes very difficult to fix, evidence of the crack is often visible after repair, even when the repair is properly done. The risk of having visible evidence of a crack increases dramatically if the wood has time to distort after cracking or if dirt collects in the crack before it's repaired.
What Can I Do?
In humid periods:
keep instruments in air-conditioning,
if air-conditioning isn't possible and you note undesirable changes, take instruments to a repair technician for seasonal adjustments.
In dry periods:
keep instruments away from heaters,
use room- or instrument- humidifiers where you store instruments.
Take instruments that crack for repair immediately to increase the possibility of complete repair.