Isn't it time for Meadowood to nag about humidity - Mar 2003
Every year, Meadowood News includes an article in “Repair & Care Tips” about humidity (or the lack of humidity) in a Winter issue. We do it because instrument dehydration, though damaging, is gradual & unnoticeable until it’s well advanced. We beat the drum because it’s really that important.
Instrument tone wood - though aged in a controlled environment to make it stable - still swells & shrinks with moisture content. For this reason, instrument builders keep humidity in their workshops steady at about 50% so that the wood with which they work doesn’t continually change size & shape as it absorbs & loses moisture. Even after the instrument is complete, relative swelling & shrinking of the wood creates stresses in the instrument.
Spruce & cedar, used in the tops of acoustic instruments, are more prone to moisture exchange than harder tonewoods used for backs & sides (mahogany, koa, maple, walnut, rosewood...). Spruce & cedar are also weaker than woods used in the back & sides (which, when curved & glued onto bracing, are even more rigid). The greater tendency to expand & contract, coupled with lower strength, make instrument tops the most likely place for damage from humidity problems. Small, gradual humidity fluctuations should not cause concern. But large, sudden changes & prolonged dryness represent a grave danger to instruments.
In humidity above 60%, the unfinished wood on the inside of your instrument absorbs water & the wood swells. As an instrument absorbs moisture, the rigid sides hold the edges of the weak top in place and the swollen top puffs outward a little. The swelling tends to raise the strings higher from the fingerboard. Also, swelling compresses the wood & can muffle the instrument’s tone.
In humidity below 40%, wood loses moisture & shrinks. Trying to shrink away from the rigid sides, the top is in tension and can crack. When humidity drops below 40% for an extended period of time or quickly drops a significant amount, cracks can suddenly appear in the top.
Because cracking in dry conditions is so likely, & the problem is preventable, no instrument builder warrantees against it. Therefore, the repair cost for dehydration cracking comes out of your pocket. Depending on the size, location & character of the crack (or cracks), the repair can be pretty pricey – often hundreds of dollars. To add insult to injury, the cracks are sometimes very difficult to fix, & evidence of the crack is often visible after repair – even with a properly done repair. The risk of visible evidence of a crack increases dramatically if the repair is not done immediately & the wood has time to distort or dirt has collected on the crack surface.
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.