I have generally kept out of this discussion, but there are some things that seem to need saying, all in one place. I can't guarantee that everything I'm saying here is new information (blame low-quality short-term memory if it isn't.)
Humidity is a function of water-content and temperature when we're talking about air. This means that any discussion of the effects of humidity on wood needs some basic qualification right from the start. Where you live (i.e., your climate) has a gigantic effect. If you live in New England, humidity control is a given. If you live in Arizona, humidifying an instrument may be utterly fruitless. This is because the climate of New England varies, seasonally and day-to-day, but Arizona is dry pretty much year-round. One thing that attests to this is the story of a master piano technician who showed me some of the ropes: he'd lived all his life in Arizona, and tuned each piano about once a year. When he came to Connecticut, he found himself tuning similar pianos (there's no such thing as two identical pianos) as many as four times a year, once for each season. In New England, the air tends to have a large amount of water in it, year round, but in the heating season (which wraps from early in Fall to late in Spring), cold moist air, drawn into a house and heated, drops in relative humidity. Relative humidity is important because that is what wood responds to. In the piano trade, we have the Dampp-chaser. This is a name-brand humidity control system designed for pianos (which grew from an idea the inventer came up with to keep the insides of radios he maintained in Africa from corroding, and later applied to cedar closets in Florida.) The system consists of a heater, to drive rel humidity down, a warm-moist humidifier, to drive rel humidity up, and a humidistat, to monitor the relative humidity and ensure that it is maintained within the affected space to a 5%-20% range, regardless of the room humidity. Up here in CT, adding one of these systems to a humidity-ravaged piano is like magic: pinblocks tighten up, improving tuning stability, seasonally-affected-actions even out and can be adjusted once-and-for-all (all meaning, in this case, for as long as the owner keeps the system plugged in and water in the reservoir!) One technician who was in a position to make some tests reported that in dead winter, with a pair of humidifiers running in the piano room, it took about a minute after the humidifiers shut off for the relative humidity in the room to drop from ~45% to lower than his meter could measure (around 5%). That is because there is nothing, in a "normal" room, to stop communication between the air in the room and the air of the rest of the house. I know I was surprised to find out how quickly the humidity of a treated space changes when the area around the enclosure has access to it. For upright pianos, the insides are sufficiently enclosed that, as long as the top is closed, the insides can be regulated well over the entire space. For grand pianos, with the soundboard exposed at the bottom side, the space (as much as five inches) between the sound board and pin block, and the fact that the "box" is habitually open if the desk is up, it's almost hopeless, but some good comes from closing off the bottom of the piano with a dense-gauze cloth cover. How does this relate to lutes? Well, first of all, the surface of a lute can be exposed for short periods of time (as much as days) to various humidity levels without the wood automatically assuming that level. If the lute is exposed to a humidity level significantly different from the starting level in the wood for "enough time", then the wood will shrink or swell, and if the changing dimension is anchored at the ends, will split, crack, bow or warp. I don't know what relative humidity level is maintained in any specific luthier's shop, but it'd be good to find out, and take steps to maintain a lute at that relative humidity. Getting it back to that atmosphere will return the lute very closely to its original condition if it hasn't been too dried out or over-humidified (i.e., if the lute hasn't sustained damage that requires repair.) The wood in a lute is cut to a shape, bonded into the lute, and most often, further shaped. If done well, the result is a balanced system with stresses worked out of it. As long as nothing happens to change the dimensions of an individual part of the lute faster than other parts, things should stay as it was built. The thing that changes wood with the longest-lasting effect is humidity, and the general rule is that cold will make things change but returning the instrument to the original temperature will bring the parts back quickly to their 'original' sizes, but a humidity change affects different parts of an instrument differently (based on composition, shape, etc.) and just restoring the instrument to the same humidity level will not immediately restore everything. Humidity generally gets into wood faster than it gets out. Either way, once a lute enters a new climate, it will change from its initial condition, with stresses being formed where different parts react differently to moisture. When these stresses exceed the bond strength of the adhesives, parts will break loose. Where it doesn't, but does exceed the wood's internal strength, cracks will happen. Where stresses don't rise to the breaking point, they can still cause bulging, etc. This is why _not_ taking efforts to humidify a case in a dry climate can lead to wood cracking and splitting, and why _going too far_ to humidify a lute can cause splitting and cracking. I do not know of a controlled humidification system for any instrument cases. It would be hard to make, hard to get to work well (because there is so little room in the closed case for air to be shoved around to get a good homogeneous atmosphere throughout the case), and horribly expensive. The heater part would actually be pretty easy to do, especially with adhesive-backed sheet heaters), but the warm-humidity generator would be a real bother. The electronics would seem to most an unwelcome intrusion, along with the extra weight of the system, and the water reservoir... well, just try flying with a case like that! Anyway, if you are worried about the fate of your lute, in case or on wall, this is what I'd suggest: First, ask the luthier (if they are still alive) what humidity they keep their wood stored in. Then: in a case, have a battery-operated humidity sensor, and look at it every time you open the case. That'll be as close to the steady-state closed case humidity as you can get (short of the remote-sensing approch Anthony Hind mentions). Figure that the meter is guaranteed to be off by +/-3% if it's expensive, +/-5% otherwise. The absolute measurement isn't really so important as keeping the level the same. So, if you open the case and it's 15%, use a humidifier (snake, pumice-inna-snuffbox with holes, damp washcloth in a plastic bag in the accessory bin, etc.) If it is 50% or higher, consider leaving it open in a room with a dehumidifier, where the doors are closed, and monitor the room. If you hang the lute on the wall, put the humidity meter very close to it (as close as aesthetics will allow, or closer if you're a _real_ aesthete!) and keep an eye on the room humidity that way. Again, if it is low, humidify. If it is high, dehumidify. In either case, use the meter to monitor humidity _near the lute_, because humidity over by the de/humidifier isn't going to be the same. I hope this is helpful! ray To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
