Drying Wood for Bass Lutherie
By Robert G. McIntosh
The following article was published in the ISB Magazine, Bass World, Volume 33 #3, in 2010.
Bass players and luthiers alike, we are all sensitive to the effects of moisture and dryness on our basses. We use the humidifier in winter and the de-humidifier in summer. We adjust the height of the bridge from season to season. Some of us have a summer soundpost and a winter soundpost. Just like the planks of a wood floor, or a door that sticks in the summer, our basses move and change with the weather. Why? What is happening here?
Wood starts out as the living tissue of trees. It has vessels that transport water and nutrients vertically to and from the roots and the leaves, and horizontally from the inside of the tree to the outermost layer under the bark. When a tree is harvested and sawn into lumber the wood is very wet. In his book Understanding Wood, wood technologist R. Bruce Hoadley compares freshly cut wood to a wet sponge. Let’s say our sponge is 4 inches wide, 6 inches long and 1 inch thick. Wring out the sponge. It’s still the same size. It’s not dripping any more but it’s damp and flexible. As the water evaporates, the sponge will shrink and become lighter and stiffer. Wood behaves similarly. If we put a piece of freshly cut spruce in a vise and squeeze it, water will drip out of it. What drips out is called free water. After the free water has dried from our sample and no water can be “wrung out,” it is still flexible and damp because the cells of the wood contain water which is chemically attached. Wood in this state is at its fiber saturation point. This remaining water is called bound water. If our piece of quarter-sawn spruce started out 4 x 6 x 1 inches when green (freshly harvested from the tree), after proper “seasoning” it will shrink to approximately 3.87 x 6 x 0.93 inches for reasons I will explain below. A slab-sawn piece will shrink to 3.75 x 6 x 0.97 inches. The “grain” of wood runs parallel to the vertical axis of the tree. Shrinkage along the grain of the wood is negligible
Quarter-sawn (A) and plain-sawn (B) boards cut from a log.
Characteristic shrinkage and distortion of flat, square, and round pieces as affected by the direction of growth rings. Tangential shrinkage is about twice as great as radial.
Bound water will evaporate from the tissue of the wood into the surrounding air. As bound water leaves the wood, the wood will shrink. It will continue to dry and shrink until it reaches a balance point with the air around it. The amount of bound water in the wood at this balance point is called the the equilibrium moisture content (EMC). If the air becomes more humid the process is reversed: water from the air becomes bound again in the tissue of the wood. The wood adsorbs water vapor (and swells) until it reaches a new EMC. (Wood scientists use the word ‘adsorb’ when wood is taking on water vapor, and ‘absorb’ when the wood is taking on liquid water.) “Hygroscopic” is the term that describes this quality of wood. Wood is always hygroscopic. It responds to changes in atmospheric humidity, losing bound water (and shrinking) as the relative humidity (RH) drops, regaining bound water (and swelling) as the the RH increases. EMC is expressed as a percentage and represents the ratio of the weight of water in a given sample of wood to the weight of that sample after it has been completely dried in an oven. Wood that is air-dried outdoors in the New York City area will have an EMC of about 12%. If a New York City apartment has a humidifier that keeps the RH around 40% (a recommended minimum), by the end of the winter everything in the apartment that’s made of wood will have an EMC of 7%.
Water vapor is always present in the air around us. The amount of water the air is capable of holding depends on the temperature of the air: the warmer the air, the more water vapor it can hold, and conversely, the colder the air, the less water vapor it can hold. If air is cooled enough it will become unable to hold the water as vapor, and some of it will become liquid. The temperature at which water condenses is called the dew point or saturation point. The condensed water is called dew, rain, or condensation. Relative humidity (RH) is the water vapor content of the air relative to its content at saturation. Saturated air has a RH of 100%. The RH of outdoor air rarely falls below 30%, but when cool outdoor air is heated to “room temperature” its ability to hold moisture is substantially increased and the relative humidity decreases. For example, if we took a mass of 34° F air on a rainy day (34º F at 100% RH) and heated it to 72º F, the RH would fall to about 21%. An environment like this will cause wood to dry to about 5% EMC, dry enough to cause a flat-back bass to strain at its braces and seams. When choosing a place to store a bass, we should consider this: in the winter a cool room will have a higher RH than a warm room, and in summer a warm room will have a lower RH than a cool room, except where an air conditioner is simultaneously removing heat and water vapor.
Indoor climates in the Midwest and northeastern United States have extreme fluctuations. There are summer days when the temperature is 90º F and the RH is 90%. There are winter days when the temperature outside is -20ºF and the RH is 25%. Most homes have a chimney and many have vents for kitchen and bathroom fans, all of which create a draft which draws outdoor air in through every crack. This constant exchange of air replenishes the oxygen supply but also removes moisture. If it wasn’t for the many sources of water in an average home (plants, people taking showers, pets breathing, water from last summer stored in the woodwork, etc.), the RH could fall to 1% or less! A practice room in an old university building can have a RH in the low teens by the end of winter break when nobody is around.
A good indicator of relative humidity is your bow. Hair is dependably hygroscopic. As a matter of fact, the best hygrometers (the instruments used to measure relative humidity) use a human hair as the sensor. Picture a night club in New York in the middle of winter: the heat is on, and the air is dry. As the music starts and the place fills up with people drinking, talking, and sweating, the bass player will have to tighten his/her bow as the hair lengthens with the increased humidity. Thank the person who rehairs your bow for being conscious of relative humidity. If the bow hair is made too short in the summer or too long in the winter there may not be enough travel in the frog to accommodate different conditions in different venues and seasons.
Bass makers need to be equally conscious of relative humidity. We’re dealing with wood, which is hygroscopic, and we live in a climate where the relative humidity changes radically with the seasons, from 25% in winter to 90% in summer. According to the charts in the Wood Handbook published by the USDA Forest Products Laboratory, the EMC of a piece of spruce will vary from 5.4% at 25% RH to 20.5% at 90% RH. The bottom bout of an average bass measures 26” wide. Theoretically, an unrestrained piece of quarter-sawn spruce 26” wide can change dimension by as much as 5/8 inch if exposed to these extremes. The change would be double that for slab-sawn spruce. Thankfully there are mitigating factors, but the message to us bassists should be loud and clear: We’re all weathermen now.
The main factor that mitigates these dimensional changes is that it takes time for moisture to move in or out of wood, making the RH averaged over a few days a more meaningful number. Also, wood is elastic to a degree and will deform under stress before it cracks. Other factors include:
(a) The fact that most instruments are made with wood that has been quarter-sawn (radial cut), which is more stable than slab-sawn (aka flat-sawn, face-sawn, plain-sawn, tangential cut). The dimensional change in quarter-sawn wood is about half that of slab-sawn wood.
(b) The varnish, which inhibits the movement of water vapor.
(c) The fact that as wood ages it responds less and less to variations in RH.
(d) Our own efforts to modify the climate, such as Dampits, humidifiers, dehumidifiers, etc.
We want to keep the moisture content of our basses as constant as possible for two reasons: to reduce stress in the wood that might hinder tone production and to prevent the wood from becoming so dry that it shrinks away from itself, opening a seam, or worse, cracking the wood. Plywood (laminated) basses are exempt from these worries except for the neck which, because it is often a solid piece of slab-sawn wood, can be a problem.
The luthiers of old Europe never paid much heed to EMC and RH because the climate in Europe isn’t as severe as it is in the northeastern U.S., and because houses weren’t heated to the comfort levels we now take for granted. But in the last 60 years central heat has become the norm, and that has changed everything. In the Manual of Housekeeping, a publication of The National Trust of Great Britain for the Conservation of Historic Structures, the authors state: “In this country central heating has become perhaps the largest single factor in causing damage to the contents and even to the structures of our houses.” Nobody anticipated the heartbreaking results of round-the-clock heat in a historic house full of hand-carved panelling, furniture, inlays, and musical instruments.
One might think, “If we humidify the air the cracks will close.” But wood doesn’t quite behave that way, and herein lies the nugget of new information that may surprise you. There is a phenomenon called sorption hysteresis, which is described in the Wood Handbook published by the USDA Forest Products Laboratory, where “…the amount of water adsorbed from a dry condition to equilibrium with any relative humidity (EMC) is always less than the amount retained in the process of drying from a wetter condition to equilibrium with that same relative humidity. The ratio of adsorption EMC to desorption EMC is constant at 0.85. Furthermore, EMC in the initial desorption (that is, from the initial green condition of the tree) is always higher than any subsequent desorptions.”
In plain English it means that once wood is over-dried, when it is placed back in a normal atmosphere at, say, 45% relative humidity, it will never completely swell back to the dimension it had before. To an antique that has never experienced central heat, the damage is permanent.
Sorption Hysteresis. This has profound meaning for us because we know that there will come a cold winter day when the instruments we make will be stored in a room with central heat. “But,” you say, “my wood has been drying for 30 years.” This is good, but age is only one of the mitigating factors. Wood is always hygroscopic and if it has never been dried in conditions comparable to a warm room in the winter in Wisconsin or Boston, then when the bass is brought home to Milwawkee the wood will lose bound water until it is in equilibrium with the ambient air. The wood will shrink. A flatback with a full-width cross brace will become concave, the “potato chip” effect that is so commonly seen. Come summer it will flatten out to a degree, but because “the ratio of adsorption EMC to desorption EMC is constant at 0.85,” it will never regain its original shape. Here is the definition again: The amount of water adsorbed from a dry condition to equilibrium with a given relative humidity is always less than the amount retained in the process of drying from a wetter condition to equilibrium with that same relative humidity.
What this means to bassists is that your bass will eventually come into equilibrium with its environment. When buying an instrument, new or old, or when traveling or moving to a different climate, you should anticipate what effect the weather will have. (See Table 1 at the end of this article) Keeping a hygrometer near your bass will help you decide whether the environment is hostile. I recommend 40% RH as a minimum. Your bass case provides an effective short-term barrier against harsh conditions.
What this means to luthiers in the Midwest and northeastern U.S. is that, if, before assembling an instrument, we subject our wood to a “preview” of the dryest conditions it might encounter we can benefit from the 85% rule and eliminate some of the shrinkage that might otherwise cause problems later. Some guitar builders go to the extreme of baking their spruce in an oven before gluing braces. A common practice of importers of “in the white” basses is to open the seams except at the top and bottom blocks and hang the instrument in a dry space for a winter. At the Pollmann workshops there is a dry room where the rough-carved plates of new instruments are hung for several years while the wood dries.
Whatever the method, it requires some forethought and patience to give the wood time to do its worst shrinking before the final assembly. We can create an environment where the relative humidity is, say, 25%, which translates to an EMC of 5.4% (see Table 2). Even in the middle of summer we can create a “dead-of-winter” condition by adding a small amount of heat. I’m not talking about a dry-kiln, and we’re dealing with wood that has already been properly air-dried for several years. The temperatures required would rarely exceed body temperature. Thin pieces of wood will dry faster than, say, a neck block. Neck blocks require long range planning because it takes much longer for a thick piece of wood to dry, and hastening the process can crack the wood or make it prone to warping. So our artificial “winter” must be brought on gently and be carefully monitored. After thorough drying, the wood must be allowed to re-adjust to the normal climate of your workshop again before building, a process that can take a matter of days for thin wood, weeks or months for thicker wood. Rough-shaping the wood to near its final dimension will reduce the thickness and the time needed to acclimatize. It’s very important when drying wood that the end grain of every piece be sealed with wax, paint, glue, or a commercial end-grain sealer. Moisture will leave through the pores of the end grain six times faster than through the side grain, causing cracks and checks that will travel into the board, sometimes invisibly.
Luthiers in the southwestern U.S. must make allowances for the opposite effect. In Las Vegas and Phoenix the relative humidity will drop to 20% or less for long stretches in the spring and summer, drying the wood to 4% moisture content. When an instrument made with wood this dry is transplanted to, say, Seattle, the wood will adsorb moisture and will expand. The result is that the plates will grow in width, pop the seams, and increase the overhang at the edges. One remedy is to humidify the workspace and store the wood overnight in a humidified box. Whether the climate in your workshop is dry or humid, your instrument should be built to be in the mid-point between climatic extremes. It is impossible to predict what hardships will befall your instrument, but knowing the history and condition of your wood will help you make intelligent decisions as you build. Inevitably, the wood will swell and shrink. Due to the inherent flexibility of wood our instruments will tolerate moderate fluctuations in climate. To prevent damage from extreme conditions it is wise to use weaker glue where you want the seams to open. In general, it’s safer to build with wood that’s too dry rather than too wet.
It’s easy to build a wood dryer. You will need to buy: an indoor-outdoor thermometer; a hygrometer; a sling psychrometer; a small fan; a small heater; and a thermostat. Incandescent light bulbs are an adequate heat source for our purposes. Build an insulated closet large enough to hold your rough-carved plates and neck, and heat it until the RH is low enough. For example, if the RH in your shop is 50% at 70°F, we know from Table 2 that the EMC of your wood will be 9.2%. I suggest a goal of no more than 5.5% EMC as a worst case. Table 2 tells us that to achieve that we need to lower the RH inside the closet to about 25%. Table 3 tells us that if we start with 50% RH we need to increase the temperature by 18°F, so the thermostat will be set at 88°F (70° + 18°). For our purposes here, “inside” refers to inside the closet, and “outside” refers to the air in your workshop. Use the fan to keep the air in the closet from layering. The hygrometer will tell you what the RH is wherever you put it. The sling psychrometer will tell you more accurately what the RH is, and you can use it to calibrate an inexpensive hygrometer.
Monitoring your progress can be done by weighing a small sample on a gram scale. The sample should be as thick as your thickest piece and sealed on the ends. Equilibrium is reached when the weight stops changing. A moisture meter is handy, but some, especially the pinless type, can give false readings. Monitoring isn’t as important if you have enough time. For example, a year of storage in a very dry environment followed by a year of storage in what you consider a “normal” environment should be adequate. Add an extra year or two for neck blocks and thick flitches for plates. Use your hygrometer to confirm your opinions as to what is dry and what is normal.
In a nutshell the lesson is: know your wood and keep an eye on the weather, both indoors and out. Use a crayon and date every board that comes into your shop. Beware of the salesperson who assures you that the wood is “ready to use”. When in doubt treat the wood as though it has been only air-dried, and take pains to prevent surprises.
Table 1
Table 2
Table 3
The tables and drawings used in this article were taken from the Wood Handbook which is available from the USDA Forest Products Laboratory website:
You can download the entire Wood Handbook for more technical information. Chapters 3 and 12 contain information most pertinent to this discussion.
Robert G. McIntosh had been a cabinetmaker for 20 years when he began his apprenticeship with Louis DiLeone in 1993. He now makes and repairs double basses in his workshop in Cambridge, New York
The author wishes to thank two contributors to this article:
Double bass luthier Arnold Schnitzer of Brewster, New York. http://cellobass.com/
Wood engineer Craig DeWitt, PE of Clemson, South Carolina. https://www.rlcengineering.com