[ Guitars by Cumpiano ] [ Classes by Cumpiano ] [ Newsletter archive ] [ Articles by Cumpiano ] [ Book by Cumpiano ]
Mother Nature--at your peril!
© William R. Cumpiano 1998, All Rights Reserved
Guitars can be described as hellish contraptions structures with very high entropy, that is, with a pronounced capacity to come apart even if just left alone. In order to perform properly, guitars must be made of thin sheets of brittle material which have the troublesome habit of changing shape in response to the moisture in the air. Oh, and besides, this same fiendish device is required to present a strictly stable platform to its strings: a rise or fall of the string array due to distortion of its structure or movement of its plates amounting to barely a few thousands of an inch can convert the thing into a wall-hanging. Now, do you REALLY want to make guitars?
Definitely, a recipe for a nervous breakdown. Indeed, my experience has shown that guitarmaking is a profession where far more people apply than are called. Of those who are called, even fewer endure. These givens are to a large extent, the reason why. The few blokes that endure are the patient and steady ones who finally come to intimate terms with woods innate habits and characteristics: its strengths, weaknesses, limits and propensities.. Those that ignore them must sooner or later cash in their chips in a game which is too difficult to play.
WOOD: THE REMAINS OF A LIVING ORGANISM
Then come the demolition & salvage men: the timber companies. They sever the trunk from the roots, chop off the crown, and carry off the commercially useful part the trunk, the mass of sap-filled pipes and watery, gooey protoplasm-filled pipe walls to the saw mill. The log arrives at the sawmill, one fifth of it cellulose (the fiber) and lignin (the natural glue holding it all together)--mixed with a chemical warehouse of sugars, minerals, and chemical compounds. The rest, the other four-fifths if it, are swollen with water. Its more like a huge, fibrous elongated watermelon.
CUTTING IT ALL UP
The best of the best, that is, the most valuable hardwoods are sawn into flitches, that is its sliced lengthwise like you might do with a boiled-egg cutter, and then the slices are kept together in the same order as they were cut. That way the slices are all as similar as can beand expensive. Instrument makers like flitches of course because selection becomes far easier.
AIR SEASONING VS KILN DRYING
The distinction between the two is important. The free water, the largest portion, is just held in the tubes by capillary force. Indeed, some of it will actually ooze out one end if you stand the plank upright. Also, if you put the plank in a huge centrifuge, you could force the water out just by spinning it out. But the rest, the bound water, is locked tightly inside the once-living tissue of the cell walls. To remove that water, you must wait a long, long time, and eventually let the surrounding air evaporate it out, at a rate of about an inch of plank thickness per year. If you dont want to wait that long, you got to boil it out in a big steam oven, called a kiln. Then, it will only take several weeks, instead of years, to turn the green wood into useable lumber. When kiln-dried, the wood is blasted with heat and steam for weeks, while when it is air-dried it just gently dries slowly and leisurely in the air. Which is better? The wood technologists will swear that the end product is the same; but most of us earthy-crunchy luthiers prefer "natures way" better and will swear that the air-dried end product, untraumatized, is far better. We like to speculate endlessly about resins that "crystallize" and stuff like that.
OKAY, FINALLY YOU GOT THE STUFF IN YOUR SHOP. IS IT SAFE? IS IT SAFE?
Wood, even the well-seasoned, mummified, once-living cellular material that it is, will still take in and lose moisture just sitting in your studio, and grow in size when the humidity in the air happens to go up (as it does on a rainy day or even during a steamy summers day) or shrink in size when the humidity happens to go down (as it does when you turn on all the 200-watt lamps in your shop all day long or the heat radiators switch on in your shop in late fall). By the way, this behaviour is often romanticized by calling wood a "living" material--which justifies a lot of dumb and destructive behavior, such as the need for "nourishing" or "feeding" it with oils and polishes.
Often, you dont know the environment in which the wood was stored before it arrived in your shop. Wherever it was stored before, it certainly is going to have to get used to a different temperature and humidity now that its going to live in your shop. Inevitably, it will slowly acclimate itself to its new home, and inevitably will change size as it does. So the last thing you want to do is cut it up and glue it down while its still moving! So you should store it for as long as you can, before using it, preferably in the room with the most constant environment in your shop. A good, dry closet, or a room far away from windows or furnaces is a good place for it. If nothing else, storing it for several months helps makes sure that it wont be moving when you use finally it (provided the ambience of the rest of the shop remains constant) and also that you dont have to rely on the sellers word that the wood is well seasoneda long stay in your closet should insure that its useable, no matter what the seller said.
By the way, a good but rule-of-thumb (literally) test is to put your palm flat on the wood. If the wood feels noticeably cooler than the air in your shop, its moisture content is probably higher than it should be. If your hand cant tell much difference, the piece is likely to be in pretty good shape.
EXPANSION AND CONTRACTION OF WELL-SEASONED WOOD
So, weve got the become pretty familiar with this piece of wood before we put it into the guitar. We also have to realize that just because its become used to our indoor environment, its still going to swell and shrink slightly, responding with hydraulic certainty to the incremental changes of humidity and temperature that occur in the shop. So its incumbent on us to keep our ambient air fairly equalized throughout the year, so the parts that weve stacked up are the same size we left them when we actually get to use them. And whats most important, weve got to close up our soundboxes at a humidity level which is HALFWAY between the extremes to which the guitar is going to be subjected during its working life. And we have to expect the worst for the guitar, unfortunately. People dont treat their guitars like fine museum furniture. They take them outdoors in freezing weather, they play them while sitting just several feet in front of raging wood stove. They leave them inside of locked car trunks in the summer. So it stands to reason that we should permanently lock the plates into a box right while the fibers are configured to an average humidity, so when the guitar is exposed to either extreme, these fibers will only have to strain half as much as they would if the box had been enclosed closer to one or the other extreme. So if one extreme is 5 percent humidity and the other extreme is 90 percent humidity, the guitar is far safer if the soundbox box is glued shut, that is, enclosed, at between 40-45 percent humidity. Then, the swelling and shrinking is moderate in any direction. (If during my New England winter, I make a guitar which is going to go to the tropics, I will increase the humidity in my shop closer to 50-60% during the week before and after I enclosed the box by gluing the back on.
These practices have worked for me, in conjunction with my practice of arching all the braces of the guitar, which allows the plates to rise and fall slightly as the plates expand and contract...rather than crack. In twenty years, I havent had any of my guitars crack from temperature and humidity shifts, so I must be doing something right. But I better not get too smug, I must be ever watchful. Because you cant fool with Mother Nature!
PROPER CARE GUIDELINES