How to Keep Your Harp or Hammered Dulcimer Safe from Cracking in the Winter Months
You’ve probably heard at some point that bad things might happen if a wooden musical instrument gets too dry, but what does that actually mean? How dry is too dry? Is it the inside air or the outside air that matters? How do you measure it? What’s actually at stake if you don’t pay attention to the humidity? Is there any reason to panic?
As manufacturers and purveyors of wooden musical instruments, humidity and dryness are constantly on our minds, but we can’t control what happens to our instruments once they leave our shop, and that’s where you come in! Winter can be a particularly dangerous season for wooden instruments, so the more you know about the risks and how to prevent them, the better.
We’ll start with the basic terminology and an overview of the essential points, so if you don’t have the time for all the gritty details, you won’t have to read the whole thing!
Relative Humidity (RH): A percentage expressing the amount of moisture in the air as compared to the maximum amount of moisture the air could hold at that temperature. Warm air can hold more moisture than cold air, and there can be a huge difference between the outdoor RH and the indoor RH. Ultimately, it’s the RH in your instrument’s immediate environment that counts.
Moisture Content (MC): This is a measure of how dry a piece of wood is, expressed as a percentage comparing the weight of the water it contains to the weight of the wood if completely dry. When a piece of wood is no longer gaining or losing moisture, it has reached equilibrium moisture content (EMC) with the air around it. EMC is very closely related to RH, but refers to the wood, not the air.
Hygrometer: A device that measures relative humidity in the air.
Moisture Meter: A device that measures the moisture content of wood.
Humidifier: A device that releases moisture into the air.
The Essential Points
Here are the basics of what you need to know to keep your instrument safe during the winter.
- Wood gains and loses moisture until it’s in equilibrium with the air around it. When the air is humid, a piece of wood will swell as it gains moisture. When the air is dry, the wood will shrink as it loses moisture. (Think of a kitchen sponge that shrivels up when it dries out.) This process happens fairly rapidly with thin pieces of wood, such as harp and hammered dulcimer soundboards and backs, and if they get too dry, they can crack.
- Winter is a particularly risky time of year because cold outside air can’t hold much moisture, and when it’s brought inside your living space and heated, the relative humidity drops - sometimes to dangerously low levels. Most dryness cracks happen during the winter.
- The widely accepted safe range for wooden musical instruments is between about 40% and 60% relative humidity. The only way to know if you’re in that range is to measure it by keeping a hygrometer near your instrument.
- If your instrument’s environment is too dry, you can either humidify the whole space with a room humidifier that emits mist into the air, or you can keep your instrument in its case and use some type of case humidifier to keep it moist.
- It’s important to regularly monitor your hygrometer to make sure you are humidifying enough!
And now, more details about the relationship between wood and moisture
We’ve already covered the basic fact that wood will expand and contract when it gains and loses moisture, but it can be surprising how much movement there actually is. Virtually all of the movement happens across the grain, which means a harp or dulcimer soundboard wants to shrink from top to bottom, whereas a harp back wants to shrink from side to side. Also, the wider a piece of wood is, the greater the total distance it wants to move.
Let’s use as an example a hammered dulcimer soundboard, 18” from top to bottom and made of quartersawn sapele wood. In a 70 degree environment at 50% relative humidity, the soundboard’s equilibrium moisture content will be about 9%. If it spends enough time in a drier environment, say 20% RH, the moisture content drops to about 4.5%, and the soundboard can shrink by an eighth of an inch in width.
Before it’s glued into an instrument, this soundboard is free to expand and contract as much as it wants to without causing problems. But picture it once it’s part of a hammered dulcimer, securely glued on all four sides to a sturdy frame. It still wants to shrink by 1/8”, but it can’t move. This creates an incredible amount of stress, and if that particular piece of wood can’t handle the stress, it will crack to relieve the tension.
The longer an instrument spends in a particular environment, the closer it gets to equilibrium moisture content with the surrounding air. The drier the environment is, the lower the EMC, and the more stress the wood is under. Thin pieces of wood like soundboards and backs reach EMC fairly quickly – it can happen in a matter of days.
A note about laminated wood
Laminated wood, or plywood, is made of multiple layers of wood glued together, and each layer is oriented with its grain direction perpendicular to that of its neighbors. This makes laminated wood much less vulnerable than solid wood to shrinking and expanding, and a great choice for those who live in particularly extreme environments, who are unable to control the humidity, or who simply don’t want to worry about it as much. Although they may not have the unique visual character and tonal depth of a solid wood instrument, laminated instruments (such as our Ravenna harps or Overture hammered dulcimers) can still look and sound fantastic!
So why should I worry about the winter?
Most cracks happen during the winter months in colder climates. The air outside is cold and can’t hold much moisture to begin with, and when you bring that air into your home and heat it up, the relative humidity drops drastically. For example, let’s take a 39 degree day where the outside air is a nice and humid 65% RH. That same air, when warmed up to a comfortable 68 degrees indoors, drops to an uncomfortably low 24% relative humidity. In many places, winter conditions are much drier and colder than this, and wooden musical instruments will be under a lot of stress unless properly humidified.
What about too much humidity?
This can be a problem! In North America, we tend to focus on the risks of dryness because that’s by far the most common problem. But there are certainly other parts of the world (and even some parts of our own continent) where excess humidity is the bigger issue. If a soundboard gets too moist and can’t expand as much as it wants to, it could react to that pressure by warping.
In our experience, this tends to affect hammered dulcimers more than it does harps. The reason is that a harp soundboard is already bowed upwards under the pull of the strings, and it can expand further in this direction with little consequence. Hammered dulcimer soundboards, on the other hand, are under different sorts of pressure in different areas due to bridge and brace locations, and can react to expansion by dipping in certain places and bellying up in other places. In really severe cases, this can cause incurable tonal or intonation issues and basically render the instrument unplayable.
Both humidity and dryness factor into the following discussion of our building methods, but then we’ll get back to the main focus of protecting instruments from dryness cracks.
What do instrument builders do to protect their instruments?
There is no way to build a solid wood instrument that isn’t vulnerable to cracking or warping in extreme conditions, so instrument builders try to achieve a balance by drying the wood to a time-tested standard moisture content before gluing up their instruments. You could theoretically protect against any dryness cracks by building with extremely dry wood – pre-shrunk wood, in effect. Or you could protect against warping by building with fairly moist wood. But to do both is impossible, so instrument builders aim for a sweet spot in the middle that will protect the instrument from cracking or warping in the most common environments.
This is where the 40-60% RH safe range comes in. It’s a widely accepted standard in the musical instrument industry that an instrument built with properly-conditioned wood is unlikely to crack or warp if kept in that range. This means that it’s up to the instrument’s caretaker to keep it safe. The further outside that range and the longer it spends there, the greater the risks.
Let’s use our dulcimer soundboards again as a specific example of how we condition our wood at Dusty Strings.
The kiln dried sapele boards we buy arrive with a moisture content between about 6% and 10%. This is verified using a moisture meter and recorded for each board. After re-sawing, gluing and sanding our soundboards, we use a moisture meter again to measure the moisture content of each soundboard and record it in pencil on the edge.
The soundboards are placed in a drying cabinet, which is controlled at 44% relative humidity, and are left there for a minimum of two weeks to reach equilibrium moisture content with that environment.
Drying cabinet for hammered dulcimer soundboards and backs
When one of our craftspeople is ready to glue a soundboard onto the frame of a dulcimer, he or she takes one out of the cabinet and measures the moisture content yet again. If the wood has reached equilibrium with the air in the drying cabinet, the moisture content should be about 8%. If it’s very far off from this, the soundboard is returned to the cabinet and another one is chosen. Then the date, starting and ending moisture content, cabinet temperature and relative humidity are recorded in a log, and the instrument is built and hopefully finds a happy home!
The data in this log is what we’ve used to arrive at our very specific conditioning specifications. In the early days of dulcimer building, we didn’t have lots of experience to go on, so if a dulcimer ever came back to us with warping or cracking problems, we would use the log to look up the recorded RH of our drying cabinet when the instrument was built. We’d compare these findings with data we’d collected about other warped or cracked instruments, and we’d adjust our target RH accordingly. Over the years, with vigilant record keeping and numerous small adjustments, we honed in on a drying cabinet RH specification that resulted in the fewest problems out in the real world.
And speaking of the real world…
Mathematical formulas can tell us a lot about how wood behaves and what an ideal moisture content might be, but what really matters is what happens to actual instruments in the real world.
For example, Taylor Guitars sticks to 47% RH when conditioning their guitar tops. They have found that this level of dryness allows their guitars to exist happily in a median range of environments. We dry our dulcimer soundboards in 44% RH, which achieves the target moisture content we’ve arrived at over 35+ years of record-keeping and analysis. And we actually dry our harp soundboards and backs in 30% RH (to about 6% moisture content), which might seem crazy low to someone from Taylor, but which works for our harps in the real world.
Drying cabinet for harp soundboards and backs
Hygrometer and dehumidifier control
The reason the harps are different has to do with a couple of things. First, as we explained in the section on excess humidity, harp soundboards are simply less vulnerable to warping when they absorb moisture. This is because they are already effectively warped due to the pull of the strings, and a little extra doesn’t make much difference. Second, harp soundboards are really long, which means that they shrink a greater distance than dulcimer soundboards when dry, and are therefore more vulnerable to cracking. Because of these factors, we worry less about warping and lean more towards protecting our harps from cracking, which means starting with drier wood. And like the dulcimers, the scientific theory has been adjusted and honed over many years of practical experience and the data we’ve collected from actual harps.
The Game of Chance
When someone suffers the trauma of a cracked instrument, they often want to know why that one instrument cracked but not any of the other instruments in their music room. What was wrong with that one? The answer (which we know is not an easy thing to accept) is that nothing was wrong with it. Assuming all of those instruments were built with wood that was dried to the proper moisture content (as any reputable builder will have done), none of them would have cracked from dryness if they were kept at or above 40% relative humidity. But below that level, you’re basically rolling the dice.
The risks increase as an instrument gets more dry, but because each piece of wood is different, there is no way to predict what will or won’t happen to any particular instrument. You might live in a dry environment, never humidify, and yet never have any problems with any of your instruments. You might find that one instrument cracks and the rest do not. Or you might have an instrument that survives ten dry winters with no problems and suddenly cracks in the 11th winter. All you can know for sure is that below 40% RH, all solid-wood instruments will be under stress.
What can I do and why shouldn’t I panic?
Winter is the time to pay special attention, but even if you live in a really dry environment, you can still safely own a solid wood instrument. With a little care and regular attention, the average person is more than capable of safeguarding their instrument from dryness cracks! As with most things, the first step is to find out if you actually have an issue that needs addressing.
Invest in a hygrometer and keep it where your instrument lives - either in the room with the instrument, or in the case if you keep the instrument packed up. This is the only way to know what the relative humidity actually is. Here’s an example of an inexpensive hygrometer. (Amazing as it seems, there is a fair amount of variability between different brands of hygrometer, and this was the most accurate of the various ones we tested.)
If it’s too dry (below 40% RH), there are a couple of different ways you can go about introducing moisture into the air. One is to use a room humidifier (the kind that plugs into the wall and emits water vapor). For this to be successful, you’ll probably need to keep the door to that room closed. Otherwise, the humidifier is going to be working on your entire living space, and it probably won’t be able to keep up.
The other way is to use a case humidifier and keep your instrument in its case when you’re not playing. Case humidifiers can take a variety of forms, but the basic idea is a porous, sponge-type object that holds water and slowly releases it through evaporation. They are commonly available for guitars, but can generally be used for any instrument. Here’s one example. This type of humidifier is too small to have any effect on an entire room, but it can be very effective on the smaller pocket of air inside the instrument’s case.
Example of a case humidifier
No matter what method you choose, it is crucial to keep an eye on your hygrometer to see if you are successfully humidifying enough. Both types of humidifiers need to be refilled periodically, and how often will simply depend on how dry it is and what volume of air you’re working with. You may also find that it takes more than one to do an adequate job. The amount of water needed to keep the RH in a safe range can be surprisingly large!
If regularly refilling a humidifier sounds like a chore, there is a different type of case humidifier that doesn’t require adding water. It actually works in both directions, using specially-formulated packets of salt crystals to pull moisture out of the air or release it into the air in order to keep the humidity constant between 45% and 50%. The packets do wear out and need to be replaced every few months, and you do still need to monitor your hygrometer to make sure they’re working, but there is less to worry about on a day-to-day basis. One thing to note is that if your instrument is fairly dry to start with, you may need to “condition” it first using a sponge-type humidifier, and then switch to the packets for ongoing maintenance. More information is available here.
If you’re feeling worried about all these dire warnings, cheer up! You don’t need to worry about taking your instrument out to play. Several hours in a dry environment will not be enough to cause any problems. Just be sure you’re storing it in a properly humidified environment when you’re not playing, and all will be well!
Lastly, a few myths we’d like to clear up
Placing a bowl of water next to my instrument or on the radiator will keep the instrument humidified.
This is a common belief, but you have to consider the whole space. If you release moisture into the air right next to your harp, it will very quickly distribute itself evenly throughout the whole room (or whole house, if the room is open). So unless you have the instrument and the bowl of water in a small enclosed space (the size of an instrument case, for example), the slow evaporation will simply not be enough to have a noticeable effect on the relative humidity in the room or to provide any appreciable moisture to your instrument.
The bottom line is that the only way to know for sure if you are humidifying enough is to keep a hygrometer close to your instrument and monitor it regularly.
If I have many musical instruments in the same room but only one of them cracked, that one must be defective.
As we said above, all wooden instruments will be under stress if they spend much time below 40% RH, but whether any particular piece of wood will or won’t crack from the stress is up to chance. Properly drying the wood before building the instrument is the most a manufacturer can do to protect against dryness cracks. Once the instrument is out of their hands, it is up to its caretaker to keep it in a safe environment or risk it cracking.
If the wood is finished, it won’t gain or lose moisture.
It is true that some types of finish can slow the loss of moisture from a piece of wood, but most finishes are not a perfect vapor barrier. Even a finished piece of wood will eventually reach equilibrium with the air around it. (Theoretically you could use something like epoxy resin to create a perfect seal around the whole instrument, but this is costly, messy and impractical in an instrument manufacturing environment.)
If I live in a dry place, I can’t have a solid wood instrument.
If you want a solid wood instrument, you can make it happen! Living in an extreme environment just means you’ll need to take more care to keep the humidity in a safe range, but there are definitely ways that you can do this without breaking the bank or keeping yourself up at night worrying. (See the previous section for details.) And even if you slip up and your instrument suffers a crack, chances are quite good that it can be repaired. So if you’re dreaming of the bold tone of a bubinga harp or the richness of a D670 hammered dulcimer, don’t let dryness scare you away!
- "Drying and Control of Moisture Content and Dimensional Changes" from Wood handbook: wood as an engineering material, by William T. Simpson of the US Forest Service
- Woodweb's wood shrinkage calculator
- Article from the Wood Database with a chart showing how EMC and RH are related