A Luthier’s Guide to the Moisture Content of Tonewood

Handcrafted wooden musical instruments are works of art. And like art, they need special care and attention to keep their beautiful sound and stand the test of time. But there’s one factor above every other factor that must be guarded against.

“Dampness is the enemy of all stringed instruments, particularly the guitar,” says Irving Sloane, a luthier who has written books on building guitars.

He would know. He understands the effort and energy that goes into a beautiful guitar, violin, or other wooden instrument. To have that effort ruined by moisture is heartbreaking.

Thankfully, it’s a preventable problem. Read on to learn what you need to know about wooden musical instruments and moisture content, including:

• What is tonewood?
• Why does moisture content matter for wooden instruments?
• How to achieve the ideal moisture content for tonewood
• How to prevent moisture damage in wooden instruments

Let’s start with a definition.

What is tonewood?

Tonewood is the name given to wood used for making wooden instruments. It’s called this because of the tone it gives the instrument.

Here are some types used for different parts of instruments:

• Basswood, mahogany, and rosewood (for bodies of instruments)
• Spruce, koa, eastern redwood, cedar, and maple (for the soundboards or tops of stringed instruments)
• Walnut, cherry, maple, mahogany, rosewood, sapele, and koa (for backs and sides of instruments)
• Cocobolo, purple heart, ebony, maple, and rosewood (for fingerboards)

Luthiers and instrument makers may use other types of wood too.

Tonewood is typically quarter-sawn wood—wood obtained from a cut perpendicular to the tree’s annual growth rings. It is cut along the vertical grain. The benefit of quarter-sawn wood is that it is less affected by changes in moisture.

Let’s take a look at how moisture content plays into this.

Why does moisture content matter for wooden instruments?

Wood is an ever-changing, hygroscopic material—meaning that it’s affected by the moisture and humidity around it. Like a sponge, it expands as it takes on the moisture from its environment and shrinks as it dries out. Significant moisture content (MC) changes in tonewood can affect the structure, sound, and quality of a musical instrument.

Finding the ideal MC—the “sweet spot”—is essential. This number will be a happy medium—not too moist and not too dry.

If the wood is too wet, the instrument will shrink as it continues to dry, leading to the following problems:

• Shrinking fingerboards
• Cracking

And how about an instrument with wood that’s too dry? That can be an issue too! It will expand when exposed to moisture, causing:

• Glue coming apart
• Damaged finish
• Dull sound

The guitar may end up with a bulging fretboard that changes its action (the distance between the strings and frets). This action, in turn, affects the playability of the guitar.

By now, you should be convinced that you can’t ignore MC. So how do we determine the ideal number and avoid these problems?

What is the ideal moisture content for tonewood?

The ideal MC for tonewood will depend on how dry or humid your region is. The key is to determine the equilibrium moisture content (EMC) of the final destination of your instrument. The indoor environment of the instrument will play into this too.

But first of all, what is EMC? In short, it’s the MC that wood will eventually reach when its environment remains at a specific temperature and relative humidity.

The ideal MC for a piece of tonewood will be close to this number. In indoor environments, it will usually fall between 6 and 9%.

“I like to see my wood in the 6–8% range. I feel this is perfect for joinery and all aspects of guitar making,” says custom guitar builder Scott MacDonald, who keeps his shop at a relative humidity of 45–50% at all times.¹

You may wonder, how can I get my wood to that MC level?

How to achieve the ideal moisture content for tonewood

Getting your wood to the ideal MC starts with knowing where you’re at and where you need to go. Once you’ve figured out how much the wood needs to dry, you’ll have to decide whether to acclimate or kiln-dry it.

But first, use a moisture meter to figure out the current moisture level of your wood. A pinless moisture meter, like those sold by Bessemeter, will be your best option. Instead of using electrical probes that could damage the surface of the wood, pinless meters have an electromagnetic sensor that rests on the flat surface of the wood. You’ll get results in seconds.

Before purchasing a meter, you should also consider the thickness of the wood you work with and the depth at which the pinless meter takes moisture measurements. For most musical instruments, using either a shallow depth or dual depth moisture meter will be appropriate.

Then comes the drying process. Luthiers and instrument makers vary in their opinions on drying wood, but we’ll look at your two options: air drying (acclimation) and kiln drying.

Air drying (acclimation)

One way to dry wood to its ideal MC is to allow it to dry naturally. This process is called acclimation.

Stack the wood in a controlled and dry environment out of direct sunlight, placing spacers (stickers) between the boards to allow the air to circulate. You may want to use fans or dehumidifiers to help with this process. Then, periodically check the wood with a moisture meter.

But warning:

This process can be a long one—sometimes a year or two—though this can depend on the density of the wood. Denser wood will take longer to dry than less dense wood.

If you have the time for this process, great!

Here’s a pro tip:

Luthiers who use this method recommend using a sealer on the ends of the boards. The reason is that wood loses moisture more quickly from its end grains than from its center. If the ends dry more rapidly than the rest of the wood, the board may crack.

For those in a time crunch or those who can’t seem to get their wood down to that ideal 6–8%, kiln drying is another option.

Kiln drying

The kiln method of drying involves placing the wood in a special oven (or kiln). This kiln remains at a stable temperature and can dry the wood in a shorter amount of time than natural drying. The process takes around two to four weeks, depending on the type of wood.

Throughout the process, check the MC of the wood, being careful not to overdry it. Overdrying the wood can make it stiff and hard to work with.

Getting your wood to its ideal MC is key to succeeding in the construction of a musical instrument. But we’ll look at some other ways that you can prevent moisture damage during that process.

How to prevent moisture damage in wooden instruments

As previously discussed, moisture can make or break your musical instrument. And when you’re using expensive materials and investing many hours, you want to make sure you take every possible precaution.

Let’s walk through some tips for preventing moisture damage before, during, and after building your musical instrument.

Maintain a consistent environment in your workshop.

Most luthiers keep their shops at a specific relative humidity and temperature at all times—about 45–50% relative humidity and 60–70 degrees Fahrenheit (about 15–20 degrees Celsius).

To do this, you’ll need a hygrometer—a device that measures humidity levels.

In more humid months, use a dehumidifier or air conditioner to keep the air from becoming too moist, particularly if your shop is in your basement. Conversely, during drier months, you may need to run a humidifier.²

Determine the final destination of your musical instrument.

Because you know relative humidity (RH) will affect your instrument, you’ll want to consider where it’ll end up and build it with that environment in mind.

So figure out the highest and lowest RH levels for that area. Then, average those numbers. You won’t want to build the instrument in an environment with higher RH levels than that.³

Here’s why.

A wooden instrument can handle some expansion if placed in a more humid environment. But if it’s moved from a very humid region to a less humid region, it can shrink, causing cracks in the instrument.⁴

So say you’re building a guitar that’s going to end up in the dry climate of Denver, Colorado. The least humid month there is July, which has an average RH of 45%. The most humid month is December, with an average RH of 58%.

Averaging those two numbers would give us about 52%. Thus, you’d want to make sure that your workshop was under 52%.

Next, we’ll look at factors specific to the wood you use.

Use quarter-sawn wood.

As mentioned earlier, tonewood is often quarter-sawn, which means that the wood is cut perpendicular to the growth rings of a tree rather than parallel to it (as is the case with flat-sawn wood).

Because of the direction of its grain, quarter-sawn wood handles moisture changes much better than flat-sawn wood. This means it’s more stable and less likely to warp.

Measure the moisture content of your wood.

We can’t emphasize this point enough! You’ll want to measure the MC of your wood before purchasing it to decide whether it’s close enough to your ideal MC. Then, measure it again throughout the acclimation process so that you’ll know when it’s ready for use.

Here’s a quick tutorial for using a moisture meter.

Take a pinless wood moisture meter and input the correct species of wood into the device so that you’ll get an accurate reading. Firmly hold the meter on a flat surface of the wood to get the reading. Though the exact number will depend on the project’s final destination, you’re aiming for somewhere between 6 and 9%.

Make sure your wood is at the proper moisture level before building.

Do not begin building until your wood has reached the proper MC.

If you just purchased the wood and its MC is not too far from the ideal MC, then all you may have to do is allow it to adjust to the workshop. Irving Sloane suggests putting a weight on top of the wood and letting it sit for a few weeks in the workshop until it reaches its EMC.⁵

But as discussed earlier, some wood may need kiln drying or a longer process of acclimation. You’ll have to decide what to do based on your time constraints.

Store your musical instrument in a stable environment.

As with building the musical instrument, you’ll want to keep it somewhere that doesn’t experience major fluctuations in temperature and relative humidity. Any extreme shifts could change the way the instrument plays or cause distortions in the wood.⁶

Again, depending on the region, humidifiers or dehumidifiers/air conditioning may be necessary.

In a dry climate, consider getting a humidifier for your instrument. This could be a case humidifier (for smaller instruments) or a soundhole humidifier (for acoustic guitars). You can also buy larger humidifying systems for pianos.

And here are a few pointers for storing your instruments. Keep them:

• In their cases
• Out of direct sunlight (even when in their cases)
• Away from windows, doors, and vents
• Away from places where they could get wet from water spills, broken pipes, etc.⁷

Taking these precautions ensures that you’ll have an instrument that sounds rich and beautiful and lasts a lifetime.

Conclusion

Building a musical instrument is an investment of time, money, and effort—meaning that you’ll want to do everything possible to prevent damage to it.

To recap, you’ll want to

• Maintain a consistent environment in your workshop.
• Determine the destination of your instrument and its ideal MC there.
• Use quarter-sawn wood.
• Measure the MC of your wood.
• Make sure your wood reaches its ideal MC before you begin building.
• Store your musical instrument in a place with a consistent temperature and humidity level.

And all these steps relate to the moisture content of your wood. Having your tonewood at the proper moisture level will prevent irreparable damage, improve the sound of your instrument, and more.

But to get an accurate moisture reading, you’ll need a quality wood moisture meter.

1. MacDonald, Scott, “Tone Wood Moisture Content,” S.B. MacDonald: Custom Instruments, customguitars.com, Dec. 9, 2006. (↑)
2. Sloane, Irving, Classic Guitar Construction (New York, E.P. Dutton & Co, Inc., 1966), p. 18. (↑)
3. Ibid. (↑)
4. Ibid. (↑)
5. Ibid. (↑)
6. Kinkeead, Jonathan, Build Your Own Acoustic Guitar (Quarto Publishing, Inc., 2004), p. 151. (↑)
7. Sloane, p. 18. (↑)