Instruments and humidity

In the life of every harpsichord maker, sooner or later some desperate musician will always appears, begging to examine and possibly “fix” the soundboard of some instrument built by some other maker. The explanations given by owners to motivate the “disaster” they’re hoping you will fix, are several: poor materials, poor craftsmanship, inadequate glues, poor understanding of historical models and so on.

With this article I want to shed some light on this matter; and I can only begin by saying once for all that a soundboard with a few small cracks, fortunately, is not “broken” . In fact, it can continue to perform its function for decades without any problem, even if it looks visually hopeless. Fixing, replacing or just leaving it as it is, is a choice that must be made on a case-by-case basis.

Secondly, I also want to say clearly that while constructive deficiencies can sometimes have a role, just as often the causes for cracks are to be traced somewhere else. Unfortunately, miracles are not of this world, but problems are; and it must be clearly said that even using the right materials, working conscientiously, with appropriate glues and models, it can happen that a soundboard will crack, if the instrument is not properly preserved. Many fine historical harpsichord are a proof for this fact.

Incidentally, as of today (Januar 2022) none of the instruments that have left my workshop have got cracks yet: so far the precautions I used have been sufficient. But I don’t delude myself: it is only a matter of time before I’ll receive an email from some gentle customer, where he will explain me that his beautiful new harpsichord, which he had placed with so much care in the corner near the radiator, strangely developed some cracks in the soundboard as soon as winter came.
And I also know that when I explain to him that it is basically his fault and that I have no intention of encouraging such a negligence by changing his soundboard for free, there will be a good chance that he will start cursing me, my work and my female ancestors.

I therefore want to write this article in advance, so that my clients and other conscientious musicians can read it, hoping that they will find it useful and that it might avoid us all as many desperate Emails as possible.
I like to think of my customers as clever and curious people; for this reason, instead of just giving them the usual idiot-proof rules, which lately became so fashionable in modern times (do not insert your instrument in the microwave; do not play it under the shower; if you ingest jacks and strings, contact your physician…), I prefer to give the complete picture, explaining how the materials works, making them autonomous and responsible. The reading will certainly be much longer and more complex, but as the ancient Chinese proverb says:

“Give a man a fish and you will feed him for a day. Teach him to fish and you will feed him for life ”.


The task of building an instrument begins with someone cutting down a tree. Beyond the bucolic reflections on the fact that a silent living being gives his life to an inanimate sound object ( “Viva fui in sylvis / Sum dira excissa securi / Dum vixi tacui / Mortua dulce cano” ), it is good to know that wood is a porous and hygroscopic material, which continuously exchanges water molecules with the surrounding air; and also that this process never stops.
At the time of cutting, about half or more of the weight of the thrunk is made up by water and it will continue to lose moisture until it has reached a situation of equilibrium with the surrounding environment.
During the first part of the process, as long as the wood still contains 30% moisture, there will be no changes in the volume of the log. Instead, below this limit shrinking begins.
A fresh to dried wood loses about 10-20% of its volume, which is already bad enough. What is worse is that it shrinks differently in each direction of the grain: this inevitably causes tremendous tensions that eventually will open cracks in the whole trunks; these normally radiate from the center outwards; to reduce this tendency logs are cut into boards before seasoning.
In the longitudinal direction, wood does not appreciably change its dimensions while drying and for all practical purposes its length can be considered fixed.
In the radial direction (usually called quarter sawn) , i.e. from the center outwards through the growth rings, the wood shrinks by about 3-6% depending on the wood species.
In the tangential direction (usually called slab sawn), i.e. along the same ring, the wood shrinks by about 6-12%, therefore about double as much as in radial direction. For this reason, with rare exceptions, for soundboards it is important to use wood cut in the radial direction, which is much more stable.

Once the wood has been dried to the point that its moisture content is in equilibrium with the surrounding air (for instruments this would mean with the air in the house, not just on the outside!), it can be worked with. Unfortunately this does not mean that the wood has reached its final dimensions, but only a situation of equilibrium with the environment’s humidity. Each time this will increases, the wood will want to swell and each time humidity will decreases, it will want to shrink. Amen.
This is the fundamental and immutable law to which every wood craftsman must adapt. On the basis of this rules over the centuries the best construction techniques, which are the basis of all beautiful, stable and durable wood manufacts, have been developed.
Whoever acts according to these principles will have good results; who does not, will surely witness many unpleasant surprises. It is as simple as that.

The trouble for people is that this expansion and contraction process is slow enough to be invisible to the eye, so most of us end up noticing it only when something has gone wrong. The forces that are created inside the material are also difficult to imagine; but you should consider that Romans used wooden wedges, which they wet, to split marble slabs. For this reason during the construction of an instrument it is essential to use the correct grain for each piece or a structural failure can happen.

Since it is also difficult to get an idea of ​​the amount of movement that can take place, I built a kind of very “analogic” hygrometer to make it visible to everyone visiting me. This simple instrument consists of a long fixed vertical structure on which a strip of soundboard wood is nailed, acting like a pointer. This has the growth rings arranged perpendicular to the fixed structure behind and the end pointing down is not fixed, but left free to move.

The strip measures exactly 1000mm when the (relative) humidity of the air is 50%, which is the normal value for my area for most of the year. When the humidity in late winter drops to 30%, the strip becomes only 997mm; during summer storms, when the humidity exceeds 80%, it can reach even 1006mm.
These several millimeters of difference are in practice the harpsichord maker’s cross! In fact, if that strip of soundboard were to be glued on both ends, as it is the case in a real instrument, by gluing it when it is 1000mm long, once it goes down to 997mm, it would recover the shrinkage by splitting; while when it goes up to 1006, it would bulge out creating a belly in the center. The variation over the year for a 1 meter broad soundboard is almost a centimeter in an unheated room and it can probably be even more at home.

Faced with this physical phenomenon, a humble harpsichord builder cannot work miracles: he can only choose an optimal humidity range based on his experience and prediction, hoping that the instrument won’t be exposed to extreme conditions. Despite all the precautions that a good craftsman can take, there are no solutions that work in every climate condition and therefore the owner’s active collaboration is required to keep the instrument at its best.


Once understood how wood behaves with moisture, it remains to be briefly explained how the other fundamental element behaves, i.e the air that surrounds our manufacts. In nature, above -40 ° C air always contains a certain amount of water vapor; unlike what one might think, however, it is not important to know the actual quantity of water (g / mc) to determine whether it’s humid or dry in a certain room, because air’s ability to absorb humidity varies according to the temperature: cold air can only hold a fraction of what hot air can hold.
What interests us to understand its behavior is how much humidity air contains in relation to how much it could take at its temperature: this ratio, expressed as a percentage, is called relative humidity and is what we mean when we discuss environmental humidity. If the air contains little water in proportion to how much it could potentially take, it is called dry; if it contains a high percentage of it, it is called humid.

From what has just been said, another fact with important practical consequences emerges: when cold air is heated, its relative humidity drops and will try to reintegrate it, taking it away from the house’s furniture, plants, from your body and from your instrument. On the other hand, when the warm air cools, the relative humidity rises; when this approaches 100% condensation will form: this is the reason why you will always find some water drops in the refrigerator or air conditioner.

Environmental humidity, is divided into low (below 40%), normal (40-60%) and high (over 60%).
This classification is conventional but also takes into account human needs: air that is too dry causes dehydration, tearing and irritation of the respiratory tract; it also favors the circulation of fine dust and viruses. It is therefore not only good for instruments but also better for your well-being to avoid humidity to drop below 30%; a quite rare situation in nature, which has became frequent due to artificial heating.
On the other hand, when the air is very humid, over 80%, bacteria can proliferate and mold can form in the darker and less ventilated corners of your house: these are also all health risks.
As you see there are several good reason to keep humidity under control.


Now that the general principles have been explained, we can derive some practical indications together:

The first step is to buy (or build) a hygrometer. There are devices of several types and prices: it is the typical example of how spending just a few Euros, can save you thousands later. The ideal thing would be to check the environment for a whole year already before the purchase of an instrument. In this way it will be possible to give valid indications to the manufacturer on the place that will host the instrument.

Unless otherwise specified, my instruments assume a humidity range from 30 to 80%. What does this mean in practice? That if the instrument is exposed for a sufficient time to humidity below the minimum threshold, the soundboard seriously risks developing cracks in the weakest areas. On the other hand, by leaving it for a prolonged period above the maximum threshold, it will develop an anomalous belly and small mechanical problems could occur, such as hangers, sticking keys and such; in the most serious cases the soundboard could even touch the strings. However, once the humidity has returned to normal levels, the instrument will gradually return to its initial state; unless something has cracked or broken: these will stay as they are …

It should also be noted that the whole structure changes slightly, so beyond actual damage, the tuning and all other adjustments of the instrument will also be affected to some extent. This is why harpsichords like a place where conditions are as stable as possible. Since these are adjustments that require precision to a tenth of a millimeter on a material such as wood, we should not be surprised that every now and then some small corrections are needed.  These are little things that are a matter of course, but keeping the situation under control is important to avoid worse. In the long run it is also really useful to learn how to carry out some small adjustment and maintenance operations yourself . Even if the manufacturer is willing to carry out these small interventions for you, you might have to wait and it is not always convenient to transport the instrument to his laboratory.

Speaking of stability, it must be said that harpsichords, at least theoretically, the larger they are, the more they suffer from humidity changes. This is because a wide soundboard shrinks and swells more than a narrow one. The worst are those in which the grain is at an angle to the spine, because this inclination has a similar effect to that of increasing the width.
Those who are offering a “busy” life for their instrument, should perhaps reflect on this aspect. Unfortunately many virginals, in my experience, despite the favorable dimensions of the soundboard, are often not very stable due to other structural characteristics. Clavichords, on the other hand, are not that sensitive and are by far the most sturdy and stable of all; this decreases the risk of problems and almost eliminates maintenance over time. All in all they are the keyboards that make most sense as travel instruments.

Heat sources are not uniformly arranged in the house and can therefore create dangerous spots of very low humidity: the radiator, the fireplace and other heat sources, however picturesque, should be avoided AT ALL COSTS, except when you are absolutely certain that they will always remain off.

Direct sunlight is also to be avoided; in fact it heats the soundboard, drying it. Sometimes no problem comes with it, but if the indoor humidity is already low, this can induce a crack. In addition, sunlight darkens the wood over time (but this is more a matter of taste). The lid is certainly less sensitive and is better to keep it closed while you are not playing.
Remember that the same can happen if you leave the instrument for a few hours in the car under the sun, with potentially even worse consequences. In the case of outdoor concerts, you really want to have some kind of protection above the instruments, because not only the sun, but also some raindrops, insects or the souvenir of some bird could damage your instrument.

When the ambient humidity is temporarily very low throughout the house, a solution must be found to milder the situation at least near the instrument, waiting for the weather to get better. The first thing is to turn off the heating in that room closing also door and lid. Humidifiers, plants, drying racks with damp cloths and so on, can help improve the situation locally in case of emergency.

Windows near the instrument are to be kept under control; the weather can change while you are away and in summer it is not all that uncommon to experience water pouring down from a semi-open window during a rough thunderstorm.
Some prefer to drink something while they practice and it may be a good idea to stay hydrated. But whether you prefer water, Coca-Cola or a good Chianti, just carefully avoid situations in which the glass can end up spilling into the instrument. The same is true if you just keep it closed and place your things on the lid; anything liquid can be a risk. Avoid cleaning instruments with damp cloths, even on the outside, unless you are absolutely sure they have been treated with a waterproof finish.

The case of the instrument contains within it a certain volume of air which changes very slowly; therefore it will take some time for it to adapt to a new climatic situation. For this reason, rapid changes towards extreme conditions should be avoided: these only create imbalances between the most exposed parts and those that undergo more gradual change.
In the case of a concert it is advisable to calculate at least one day of settling in the new environment before the instrument is stable, this rule is particularly true if you have no clue about the conditions you can expect. If you are lucky, on a good instrument it will be enough to tune only at the beginning of the rehearsals and just check it before the concert. But if either the instrument or the environment are not ideal, with at least one day of settling the situation will improve a lot.

The lid is a good mean to mitigate and slow down the changes that can occur during transportation and when not being played. And in general it is a good protection from light, dust, insects and from anything else that might come into contact with your soundboard. If you are not playing, it is a good practice to keep it closed. In an emergency you can use its own transporting bag (clavichords) or a blanket.

There are humidifiers of all shapes and prices on the market. You might consider, checking with the hygrometer, if it is worth buying one. My general advice is that if you are living in temperate zones, especially in Europe, you probably won’t need it: keeping an eye on the situation and perhaps putting the instrument in an unheated room might be all you need. If, on the other hand, you know that you will be faced with extreme climatic situations, then the investment could be justified and save you some nasty surprise later.


In conclusion, I just wanted to say that by reading these instructions, which describe a long series of problematic situations, one might be led to think that maintaining a harpsichord or clavichord is damned hard work and that the disaster is always only around the corner.
Well, quite the opposite is true in fact: damage is only an exceptional event, which might occurs only under particular conditions. Humidity here in Europe is between 30% and 80% during most of the year and with a little care you can avoid all other risks as well.
If we are well used to “cuddle” objects made of metal such as cars or motorcycles and none of us expects that ignoring basic maintenance they can last over time, why should we then expect that instrument, made of wood, should not require any care? After all, you will discover that a little bit of attention and a hygrometer next to the instrument is pretty much all what is needed to guarantee yourself many years of enjoyment.