Illustrating the Relationship Between Humidity and Wood

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Author: Michele Vanderlip

Whether you are a string player or luthier, whether consciously or not, much of your career involves reacting to changes in instruments because of atmospheric fluctuations, particularly those changes in the amount of moisture contained in the air — humidity. This is a theoretical explanation of only a few important physical properties of wood. Furthermore it is a compilation of evidence for why keeping your instrument in an atmospherically stable environment should be more than just a preference.

DEFINE CONCEPT 1: WOOD IS HYGROSCOPIC

Water and wood have a very dynamic relationship. Wood is hygroscopic meaning it takes in and releases moisture from its environment. When wood is still green, just harvested or unseasoned, it contains both free water and bound water. Free water is water in liquid form that flows through the cavities — cell lumens — of the wood’s cellular structure (see Figure 1.1). This free water travels primarily by a very specialized mechanism along wood’s cell walls called a bordered pit (see Figure 1.2).

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Figure 1.1.  Illustration of free water in the cell lumen (Peralta, NC State Lecture)
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Figure 1.2. (Left) Bordered pits along the longitudinal tracheid cells of a softwood. (Right) Bordered pit anatomical structure. (Hoadley)

Bound water is the water which is attached by hydrogen bonds to the woods cell walls. When a piece of wood has no more free water yet its cell walls are fully saturated and expanded because water has bonded at all possible connections, the wood has reached fiber saturation point or FSP.

Tonewood — and any wood used in a capacity by which it will need to withstand a stress, strain, or shear force — is seasoned well below this fiber saturation point, preferably dried to a moisture content that will be in balance with the surrounding atmosphere upon the wood’s end use. Moisture content is simply the mass of water present in the wood divided by the mass of dry wood, expressed as a percent. This point of balance between the water bonded to the wood and the water in the air is known as equilibrium moisture content or EMC. Temperature and relative humidity are the principal factors controlling EMC (see Figure 2.1 and 2.2).  A piece of wood may be at EMC at varying temperatures and relative humidities so this equilibrium is not static but rather dynamic; the rate of moisture gain is equal to the rate of moisture loss.

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Figure 2.1. Relationship between relative humidity, temperature, and equilibrium moisture content. (Hoadley)
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Figure 2.2 (Click on the photo to enlarge for a clearer view) Calculated values of equilibrium moisture contents for a given temperature and relative humidity. (FPL Wood Handbook)

For example, if the makers workshop is 70F with a relative humidity of 55% the tonewood will be at about 10% moisture content once it has equilibrated with it’s surroundings. During the winter months, if the temperature in the workshop remains the same, 70F,  yet because the indoor heat has been running to maintain the temperature the relative humidity will dip to 15% and that very same piece of wood will eventually equilibrate to about a 3.5% moisture content, a 6.5% difference from the previous conditions.

See “Appendix: Humidity Terminology” for more detail.

DEFINE CONCEPT 2: SHRINKAGE & SWELLING

So what does all this discussion of moisture in the atmosphere and it’s relationship with wood mean for the integrity of a string instrument? Moisture loss and uptake are accompanied by shrinkage and swelling of the wood. This shrinkage and swelling results in the movement of every single component of the string instrument.

Here are just a few symptoms of an instrument that may have shrunk or swollen due to atmospheric moisture changes:

  • Slipping or sticking pegs
  • Open seams (or worse yet, cracks!)
  • Buzzing, nasal or muted sound
  • Bow skating across the strings
  • Decreased projection or response
  • Change in neck projection

Several factors contribute to the rate at which an instrument will shrink or swell. These factors are:

  • Extent of moisture loss or gain
  • Wood species and specific gravity (density in relation to water)
  • Age of instrument
  • Grain orientation
  • Stresses and strains caused by joints
  • Treatments that may have been applied

Let’s take grain orientation for example. The amount of shrinkage or swelling of a given piece of wood will depend on which plane and direction you are observing. This is because wood is an anisotropic material meaning that it exhibits varying physical and mechanical properties in different directions.  We can look on an anatomical level (see Figure 3.1) at the structure of a softwood species and see that although softwoods only contain two types of cells — tracheids and rays — the organization of these cells make for a very different appearance on each plane of the wood.

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Figure 3.1. 3D Sample of softwood anatomy under 25x magnification (Peralta, NC State Lecture)

The following photos (Figures 3.2 -3.4) illustrate the three surfaces and directions that are used to describe wood and predict dimensional changes.

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Figure 3.2. (Click on the photo to enlarge for a clearer view.) Surfaces and directions (or planes) of reference as seen on a Spruce block of wood.
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Figure 3.3. (Click on the photo to enlarge for a clearer view.) Wood orientation or directions as seen on a violin top
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Figure 3.4. (Click on photo to enlarge for a clearer view.) Wood orientations or directions as seen on a dowel and the related shrinkage amounts (by Jeff Lefkowitz for Brian Boggs Chairmakers).

Shrinkage and swelling in the longitudinal direction is almost never a factor to consider because it is responsible for less than 0.1% of the change in most woods. However, dimensional changes in the tangential and radial directions are quite significant; tangential shrinkage is approximately but not always double that of radial shrinkage. The amount of change is of course also dependent on the amount of surface area being observed. In the context of evaluating a string instrument, changes in the radial direction are most observed and have a significant impact on the arch of the violin and its interacting parts.

APPLY CONCEPT 1 AND 2.

Let us now apply these two concepts to determine how the same wood from our earlier example will change after it equilibrates from a 10% moisture content to a 3.5% moisture content. For simplicity purposes we will use only the violin top, pretending it is a stand alone piece. Our violin top is made of Engelmann Spruce (Picea Engelmannii) which has an average green specific gravity of  0.33. It measures 353mm in length and 121mm across it’s width just above the bridge feet (see Figure 4.2 and 4.3). It is also given that the Engelmann Spruce has the following values for average Shrinkage (%) from green to Zero % moisture content: 3.8% radial direction, 7.1% tangential direction, and 11% volumetric.

What is the new dimension in the radial direction, the violin width measured at the bridge feet (see Figure4.1), of this top if it were to equilibrate to a 70F and 15% relative humidity atmosphere? We know that the equilbrium moisture content of wood, given this temperature and relative humidity, is approximately 3.5%. At the time of measurement, the violin top was in an environment with a temperature of 70F and relative humidity of 55% with known equilibrium moisture content of approximately 10%.

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Figure 4.1. Measuring the violin top width.
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Figure 4.1. Measuring the violin top length.
  1. First calculate the percent radial shrinkage from 10% moisture content to 3.5% moisture content using the following equation:

%s radial (MC1 -MC2) = %s radial (green to 0) (MC1 -MC2) / FSP

 

Given:

FSP (fiber saturation point) is a standard, given at 30%.

MC1 = 10

MC2 – 3.5

%s radial (green to 0) = 3.8

 

Solve:

%s radial (10-3.5) = 3.8 (10-3.5) / 30
%s radial (10-3.5) = .82%

  1.  Then, calculate the width at 3.5% moisture content using the percent radial shrinkage  from 10 to 3.5 percent found in Step 1 using the following equation:

%s radial (MC1 -MC2) = [ D radial (MC1) –  D radial (MC2) / D radial (MC1) ] x 100

 

Given:

MC1 = 10

MC2 – 3.5

%s radial (10-3.5) = .82%

D radial (10) = 121mm

 

Solve:

.82 % = [ 121mm – D radial (3.5) / 121mm ] x 100

D radial (3.5) = 121mm – (121mm)(.82/100)

D radial (3.5)  = 120.01mm

 

So, the violin top in a 70F room with 55% relative humidity had a width measurement of 121mm and during the winter months, when the heat kicked on, the violin top in the same room with a temperature of 70F and relative humidity of 15% is predicted to shrink by close to 1mm. This illustrates that relative humidity changes in the atmosphere do in fact have an affect on the violin.

Keep in mind that ultimately the violin’s top plate is connected to other components of the violin and these factors do play a role in determining the amount of dimensional change and where the change takes place within the instrument. However, the discussion of  mechanical forces at play and their role(s) must be discussed only after the concepts laid out in this discussion of wood and water’s relationship have been mastered.

 

COPE WITH THE CHANGES

Players will notice the changes illustrated in this example of the shrinking violin top by way of the symptoms previously mentioned. Seams may come apart, the neck projection may change, the sound may change and so on. The only thing we as makers, restorers, and players of string instruments can do to counter these effects is attempt to control the environment in which our instruments live. There are many products and resources available to combat dry air and overly hot and humid conditions. Speak with a professional at violin shop or trusted colleague to get a few recommendations and tips on the best methods and products available.

 

APPENDIX: HUMIDITY TERMINOLOGY

Humidity is the commonly spoken term used in general conversation to describe moisture in air. Absolute humidity is the quantity of moisture present in the air expressed in grains per cubic foot (1 grain = 1/7000 lb) or grams per cubic meter. Relative humidity, as the term ‘relative’ implies, relates moisture to temperature. It is expressed as a percentage of the maximum amount of water vapor the air can hold at a given temperature.

 

 REFERENCES:

Brian Bogg Chairmakers, http://www.brianboggschairmakers.com/grain-orientation-and-wood-movement/

Glass, Samuel V.; Zelinka, Samuel L.;Wood Handbook, Chapter 04: Moisture Relations and Physical Properties of Wood (General Technical Report FPL-GTR-190. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory: 4-1 – 4-19. Chapter 4.) 2010.

Perry Peralta, “Shrinkage and Swelling of Wood” Lecture, NC State Department of Forest Biomaterials, 2012.

R. Bruce Hoadley, Understanding Wood (The Taunton Press, Newtown, CT, 2000).

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