We really cannot have a discussion about making somewhat fragile stringed instruments without talking about the careful control of humidity in the luthier shop. Wood has the ability to take on moisture and shed moisture due to the environment that in which it currently exists.
If the shop has a humidity that is too high, which would usually be a problem in the summer or warm/humid months or too low, which is a problem during the winter or cold weather months. There has to be some sort of mechanical control and humidification/de-humidification system put in place.
Exposure of the end grain of the wood is the most critical as this is the hollow cell structure of the tree and therefore it absorbs moisture most easily, causes the most movement, and consequently causes the most damage in acoustic instruments. Have you ever looked at top and back plate sets that have wax on the ends? This is to prevent moisture from being absorbed into the end grain and is usually a practice of high quality instrument suppliers.
Many luthier shops keep the relative humidity at a constant level of about 45%. This proves to be about the best median humidity as it allows the guitar to resist short-time exposure to either higher or lower humidity levels. If the guitar is built in a 45% relative humidity shop, it should be able to exist in an environment of between 35% and 55%, when relocated. It is even possible that the guitar could be exposed to humidity levels between 25% and 65% for short periods of time, but I would not recommend it as at these lower and higher humidity levels cracking can occur.
When new lumber or new top and back/side sets or other guitar specific lumber is delivered to your shop, you should let it acclimate to your shop atmosphere for several weeks or a month before you intend to use it. This allows the wood to equalize to your shop humidity and will be much more stable and make a better instrument.
I like to buy my wood well ahead of time and let it age as well. Often it is good practice to let the wood age in your shop for a 2 year time period, so you need to be on your toes and know what to buy. I’m still using top tonewood that I bought 20 years ago! That stuff sounds better and better the longer it ages!
You need three pieces of equipment to establish a constant level of humidity. You have to have an accurate way to measure the humidity, you have to have a way to take humidity out of the atmosphere and you have to have a way to add humidity to the atmosphere.
There are a lot of great digital humidity measuring devices (also called hydrometers). One of my favorites is Extech 445814 Stationary Hydro Thermometer, which is very accurate, gives you temperature and high and low point memory. You should keep a constant eye on this instrument while in the shop, and thanks to a large LCD screen you can see it from across the room.
If you invest in a central heating system or air handling system, this is the easiest and best way to handle both high and low humidity levels. Add humidity by placing a duct-mounted humidifier on the return air duct at the furnace, similar to the Aprilaire
400 . These units have humidistats that control you humidity levels that are introduced into the air. During the summer, humidity is easily taken out with air-conditioning. You may have to supplement this with a dehumidifier, especially since it is likely you won’t air-condition the space when unoccupied.
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