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William Cumpiano's
String Instrument
Newsletter #11

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Ignore 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 it’s 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 wood’s 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.

When it was part of the living tree, wood was an organism consisting of dozens of interacting systems that are each made up of specialized tissues, each one contributing in their marvelous way to the growth and survival of the whole. I like to look at a tree as a Grand Hotel or a skyscraper, with all its plumbing and conduit systems in the trunk and roots and all its rooms and offices in the leafy crown. But instead of its plumbing and piping made of iron, plastic or aluminum, the piping is itself composed of organic tissue: living, breathing pipes as it were. So wood is essentially a mass of bundled-up tubing, where the tubing itself is a living structure of specialized hollow, elongated cells.

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. It’s more like a huge, fibrous elongated watermelon.

The huge band saws or circular saws at the sawmill then divide up the log according to the intended use of the end product. More often than not, it’s ground up into chips for pulp, but a fraction of it, the best of it, is sawn into big long blocks, for furniture, building materials, or veneers. That’s how you get the most product yield out of it. If its spruce, or cedar, and its earmarked for musical instruments, it’s cut in a far more wasteful way, but it’s the way that yields the most vertical-grain product. In most cases, when the grain is straight and vertical, the material is at its most stable and strongest configuration. This is especially so for softwoods, but less so for hardwoods. But that’s the stuff we prefer. Usually, however, hardwoods are just cut up into planks and then we have to sort through jumbled piles of them to find the one or two out of the pile that happen to have originally radiated from the center of the log in a way that yields vertical, or near-vertical grain.

The best of the best, that is, the most valuable hardwoods are sawn into flitches, that is it’s 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 be—and expensive. Instrument makers like flitches of course because selection becomes far easier.

So there the plank sits, an oblong mass of cellular pipes or tubes. Four fifths of its mass is water. And of that water, about two thirds of it (called free water) resides in the sap which still remains in the tubes. The smaller fraction of it is bound up in the once-living protoplasm that makes up the cell walls of the tubes themselves, and is called bound water. In this state, the plank is of no commercial value: it is weak, slimy, and unstable dimensionally. You can’t really make anything of any quality out of it until you rid it of both its free and bound water.

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 don’t 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 "nature’s 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.

What will happen to the plank if we just let it sit there, and do nothing? Simple: it’ll slowly disintegrate. If it’s a plank of Brazilian rosewood, we obviously don’t want that to happen! A large chunk of freshly- cut wood will start losing its moisture by evaporation through all its exposed surfaces—but mostly and most quickly from its ends. That stands to reason: if wood is a bundle of organic pipes and all the pipe openings are exposed at the chopped-off ends of the plank, that is where most of the moisture is going to be lost first. So, all the exposed outer surfaces will start to dry out, and the outer surfaces will correspondingly begin to shrink, but at different rates, not equally. If the plank is thick enough, the dried-out portion will seal a portion of wet wood deep inside of itself, which will not be able to gain access the outside air. The plank is then said to have become CASE HARDENED. This is a very bad condition, the exterior of the plank is drying and shrinking around a wet core of green wood which has been effectively sealed off from drying. The outside starts to crack. The wood then slowly becomes useless, good only for firewood. To keep it from case hardening, then, you must plug the ends with paint or wax, helping to slow and equalize the drying process, and then you must stack the wood in a ventilated, shaded place so the sun won’t hurry the process..

Well, what if you don’t cut it into thick planks, but right away you cut the wet wood into thin sheets? That’s actually an excellent idea, if your end-product will be thin sheets of wood. In this configuration, you have a much greater amount of surface area for a given amount of volume. Case hardening doesn’t happen and the material dries safe and sound naturally in the air within a couple of months (sooner, if you blow air over it with a fan, providing you stack it very carefully so it doesn’t curl up in the process.

You’ve bought the flitch, or the planks, or even the sets, and they’ve just been delivered and they’re sitting on your shop table. Do you cut it up and start making guitars? No.

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 summer’s 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 don’t 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 it’s 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 won’t be moving when you use finally it (provided the ambience of the rest of the shop remains constant) and also that you don’t have to rely on the seller’s word that the wood is well seasoned—a long stay in your closet should insure that it’s 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, it’s moisture content is probably higher than it should be. If your hand can’t tell much difference, the piece is likely to be in pretty good shape.

Green wood just shrinks, because it’s moisture content is so much more saturated than the surrounding air.. Seasoned wood, however, both expands and contracts, always in an attempt to attain a balance, or put fancier, reach equilibrium with the ambient air. It will always expand and shrink in the direction which is perpendicular to the growth rings. So if you have a quarter-sawn, or vertical grain plate of wood, it will always shrink or expand in width only, that is, the spaces between the grain-lines will widen or narrow. And this will happen evenly in vertical grain wood, so the piece should remain flat as it widens or narrows in response to ambient changes (that’s one of the reasons we like it so much!)—provided all faces are equally exposed to the air. My students are often dismayed when they see a purportedly well-seasoned soundboard curl up while laying flat on a table. "It probably wasn’t well seasoned," they say.   They're wrong. I demonstrate by simply flipping it over, and in a couple of hours it has flattened nicely. The face down is being shielded from any erratic. localized ambient changes. I tell them that some luthiers store their plates by hanging them like bedsheets on clothespins. That way the air catches them on all sides and they stay flat. I don’t bother because they usually curl well within their natural resilience—the material is, after all elastic to a point (that’s what makes them good resonators), so I really don’t worry if they curl a bit before using them, since they will be eventually  gently constrained by the braces.

So, we’ve 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 it’s become used to our indoor environment, it’s 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 it’s incumbent on us to keep our ambient air fairly equalized throughout the year, so the parts that we’ve stacked up are the same size we left them when we actually get to use them. And what’s most important, we’ve 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 it’s working life. And we have to expect the worst for the guitar, unfortunately. People don’t 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 haven’t 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 can’t fool with Mother Nature!

By the way, I thought you all might be interested in reading my "Proper Care Guidelines" which I include with all my new-guitar paperwork. I express to my customers that after they buy my guitar and the guitar leaves my shop, I feel that the customer and I remain in a sort of "partnership" to insure the future well-being of the rather fragile and highly-optimized instrument. My role in the partnership is to cheerfully and promptly repair or refurbish any fault of mine in the construction or design of the instrument and the customer’s role is to strictly follow the "Proper Care Guidelines." If there is evidence that the customer has not followed the guidelines, I will charge for the repair. This system works well. I’ve made something in the order of 500 instruments during my career, and I’ve had to repair the flaws on five or six—not counting the dozen or so free neck resets that I did for free because the instrument required them after a time period that I considered premature.